The mirror arrived before anyone measured the bath
The mirror is 1200 by 700, backlit, with a demister pad on the back, a motion sensor and a shaver socket let into the frame. It has been sitting in its carton in the hall for a fortnight and it cost more than the basin. The wall it was bought for is the one at the tap end of a 1700 mm bath, and the centre of that mirror will finish about 700 mm from the bath rim. Nobody measured that before ordering, because nothing about buying a mirror suggests that a tape measure is involved.
That is the shape of almost every argument on a bathroom refit. In any other room a fitting's position is a matter of taste, sightlines and where the joists are. In this one, position is permission. A room containing a bath or a shower is treated as a special case by every wiring code worth the name, and what governs is not the wall, the ceiling or the room's dimensions but the distance from two objects: the bath and the shower. Move the bath 300 mm and a compliant light becomes a non-compliant one without anybody touching the light.
The sequencing makes it worse. First fix happens on a day when the sanitaryware is a line on a plan and the mirror is a page in a brochure, and the cable has to be in the wall before either of them exists. So the useful move is to settle the geometry on paper — the volumes, not the fittings — mark them on the studs, and then let the shopping list be checked against a drawing that already exists. That is a morning's work, and it is the difference between choosing fittings and returning them.
One rulebook draws a shape, the other writes a list
BS 7671 Requirements for Electrical Installations, in Section 701 — locations containing a bath or shower — takes the approach inherited from IEC 60364-7-701: it defines volumes. Zone 0, zone 1 and zone 2 are three nested regions measured off the bath or shower, and every question about ingress protection, about what accessories are allowed and about how a cable may be run gets answered by first establishing which zone the point is in. AS/NZS 3000 Wiring Rules works the same way in its Section 6 on damp situations, with its own dimensions.
NFPA 70 National Electrical Code does not have a zone system, and looking for one is the fastest way to misread it. What it has instead is a set of separate prohibitions, each living in the article that governs the thing being prohibited: receptacles in 406.9, luminaires and ceiling-suspended fans in 410.10(D), ground-fault protection in 210.8, the branch circuit in 210.11(C)(3), the basin receptacle in 210.52(D), and hydromassage tubs in Part VII of Article 680. CSA C22.1 Canadian Electrical Code likewise draws its own exclusion distances around a tub or shower. The practical consequence is that under BS 7671 you can answer "may this go here" once, from one drawing, for everything; under the NEC you have to ask the question separately for each class of equipment, and the answer for a light is not the answer for a socket. An electrician who learned one framework and applies its habits to the other will get a defensible-sounding wrong answer.
The zones, measured off the rim rather than off the wall
Zone 0 is the inside of the bath tub or the shower basin — the volume that holds water. Nothing ordinary goes in it, and the only equipment that has any business being there is fixed, SELV, and specifically suitable for immersion. In practice it is a definition you use to bound zone 1 rather than one you install anything in.
Zone 1 is the volume standing on zone 0, bounded by the finished floor and a horizontal plane 2.25 m above it, and limited horizontally by the vertical surface of the bath or basin. Where a fixed water outlet is higher than 2.25 m, zone 1 rises to meet it, which catches the drencher head on a wall arm in a high-ceilinged Victorian bathroom. The space under a bath is part of zone 1 if it can be reached without a tool, and outside the zones entirely if the panel is screwed on — so the choice between a magnetic catch and four screws is quietly an electrical decision, and it wants taking before anything is fixed under there.
Zone 2 extends 0.6 m horizontally beyond the boundary of zone 1, to the same 2.25 m height. Two things about it catch people out. The first is that BS 7671 has had no zone 3 since the 2008 edition; anyone still working to a fourth volume is working from a book two amendments out of date. The second is that Section 701 applies to a location containing a bath or a shower, and a room with only a washbasin in it is not one — a downstairs cloakroom with a WC and a basin has no zones at all, and the fittings that would be forbidden three metres from a shower are unremarkable there.
A shower with no basin — a wet room, or a tray-less enclosure over a tanked and graded floor — is measured differently, because there is no rim to work from. Zone 1 is then taken as the volume within 1.2 m horizontally of the fixed water outlet, up to the same height. That radius is generous, and in a small wet room it swallows most of the walls — which is why wet rooms end up with a SELV downlight scheme, a pull cord and no accessories at all inside the door. The layout that makes a wet room work electrically is not a variation on the layout for a room with a tray.
- Chalk the bath and shower footprints on the floor, and mark the fixed water outlet position on the wall.
- Strike the 2.25 m plane round the room, then check it against any outlet mounted above that height.
- Project zone 1 up off the footprint and set the 0.6 m zone 2 offset out from that boundary.
- For a shower with no basin, swing 1.2 m off the fixed outlet instead and mark the arc.
- Settle the bath panel fixing, since it answers whether the void beneath is in zone 1.
- Photograph the marked walls with a tape in shot before the boarding date.
What is allowed to stand in each of them
Once a point has a zone, ingress protection follows. The IP code itself is defined in BS EN 60529 Degrees of Protection Provided by Enclosures, and the digit that matters here is the second one, for water. Section 701 asks for IPX7 in zone 0, and IPX4 in zones 1 and 2, rising to IPX5 where water jets are likely to be used for cleaning — a commercial shower block or a communal changing room rather than a family bathroom. The rating belongs to the fitting as installed, complete with its gasket, the correct bezel and the lamp the manufacturer tested it with, so a downlight that loses its seal when a fire hood is squeezed over it is no longer the fitting on the data sheet. And a stated IP is a floor rather than a specification: nothing stops an IP65 luminaire going in zone 2, and in a room where somebody will one day hold a shower head at the ceiling to clean it, that is the sensible over-specification.
The rule that surprises people most is the horizontal one. Regulation 701.512.3 keeps socket-outlets away from the wet end of the room entirely: other than SELV outlets and shaver supply units complying with BS EN 61558-2-5, a socket-outlet has to sit at least 3 m horizontally from the boundary of zone 1. That is measured from the zone boundary and not from the bath, so it is comfortably more than three metres from the water, and in the large majority of British bathrooms there is simply nowhere in the room that satisfies it. This is why a UK bathroom has no thirteen-amp socket in it, and why the answer is a shaver supply unit rather than an argument. It is also a genuinely separate question from whether the work has to be notified to building control, which is settled by a different test on a different volume and is covered on the notifiable-work guide.
| Where the fitting is going | BS 7671 Section 701 | NFPA 70 equivalent |
|---|---|---|
| Inside the tub or shower basin | Zone 0 — fixed SELV equipment suitable for immersion only, IPX7 | No receptacles; luminaires within the tub or shower zone must be listed for the location |
| Directly over the bath, at ceiling height | Zone 1 if the ceiling is at or below 2.25 m — IPX4 and 30 mA RCD | Inside the 410.10(D) zone: no pendant, track, cord-connected fitting or paddle fan |
| The wall at the tap end, 400 mm off the rim | Zone 2 — IPX4, no socket-outlet other than SELV or a shaver supply unit | Governed by 406.9 for receptacles; check the adopted edition, the wording changed |
| The far wall of a large family bathroom | Outside the zones, but still inside the 3 m rule for socket-outlets | A receptacle is required within 900 mm of each basin under 210.52(D) |
| The void under the bath | Zone 1 if the panel comes off without a tool; outside the zones if it is screwed | Access to a hydromassage motor is governed by Part VII of Article 680 |
| A wet room with no tray | Zone 1 taken as 1.2 m horizontally from the fixed water outlet | The tub-rim and threshold datum does not exist; the shower stall boundary governs |
The only socket a bathroom gets, and the one it is not allowed
In the UK the shaver supply unit does all the work the 3 m rule forbids anything else to do. It is a socket fed through an isolating transformer to BS EN 61558-2-5, which is what makes it acceptable where an ordinary outlet is not, and it may be installed in zone 2 provided it is unlikely to be reached by direct spray. The transformer is the whole point of the product, so check the two-pin outlet on a mirror cabinet against that standard before it goes on the wall: a cabinet with an unisolated socket in it does not become compliant by being sold with a mirror attached.
North America inverts the problem. Rather than keeping receptacles out, NFPA 70 requires one in: 210.52(D) puts a receptacle within 900 mm of the outside edge of each basin, and 210.8(A)(1) requires ground-fault circuit-interrupter protection for it. What the code excludes is the space over the bathing fixture itself. Section 406.9 has historically said that a receptacle may not be within or directly over a bathtub or shower stall, and the 2023 edition restated that as an all-encompassing zone measured horizontally and vertically off the tub rim or shower threshold. Those two wordings give different answers for a basin tucked in beside a tub, so confirm which edition the authority having jurisdiction has actually adopted before you set the box out — the version you learned is not necessarily the version being inspected.
The circuit itself carries a restriction that is easy to read past. Section 210.11(C)(3) requires at least one 20 A branch circuit to supply bathroom receptacle outlets, and that circuit is not allowed to pick up other outlets — with the single relaxation that where it serves only one bathroom, other equipment within that same bathroom may share it. So the choice is a real one: one circuit serving the receptacles of every bathroom in the house and nothing else, or one circuit per bathroom that may also take that room's fan and light. Deciding which before first fix is what stops the second bathroom being fed off the landing lighting circuit because it was the nearest cable.
Worth running as a sanity check rather than as the governing rule, because in a bathroom the binding constraint is 210.11(C)(3)'s restriction on what else may share the circuit, not the per-receptacle planning count. Read the answer as the number of general-use positions the circuit is nominally good for, then ask how many bathrooms you are proposing to hang off it — and remember that a hair dryer takes most of a 20 A circuit on its own, which no count based on a per-outlet allowance will tell you.
The circuit's breaker rating.
General-use receptacle circuits are standard 120V.
Estimated maximum outlets
10 outlets (estimated maximum)
This is a general planning guideline based on the NEC's standard per-receptacle load value, not a strict maximum-outlet-count rule — actual safe count depends on what's actually plugged in and used simultaneously, which can be far less than this theoretical maximum.
- Safe continuous circuit capacity
- 1,920 VA
They open the calculator with your figures already in it
Outlets Per Circuit Calculator: 10 outlets (estimated maximum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- IN A DWELLING THIS IS A RULE OF THUMB, NOT A CODE LIMIT. The NEC assigns no load to a general-purpose receptacle outlet in a dwelling unit — lighting and receptacles are covered together by a volt-amps-per-square-foot figure — so there is no code maximum number of outlets on a house circuit. The 180 VA per outlet this uses is the NON-DWELLING rule, and applying it to a house gives a sensible answer for the wrong reason.
- Counts general-purpose outlets only. Kitchen small-appliance circuits, bathroom, laundry and any dedicated appliance circuit are required to be separate and are not part of this count.
- A single high-draw appliance can exceed the whole circuit on its own, so the count is meaningless once anything substantial is plugged in. A space heater is most of a 15 A circuit by itself.
- Says nothing about conductor size, voltage drop over the run, or the arc-fault and ground-fault protection the location requires.
- Local amendments are common on this subject and can impose a hard maximum where the model code has none.
A hole in the ceiling over a bath is three problems at once
The first is water. A downlight in the ceiling directly above a bath sits in zone 1 where that ceiling is at or below 2.25 m, and outside the zones where it is higher — so the identical fitting in the identical position is a mandatory IPX4 under the 2.2 m ceiling of a converted attic bathroom and merely a sensible one under the 2.4 m ceiling downstairs. Two hundred millimetres of ceiling height decides it, and nobody measures the ceiling before ordering lights. Specify IPX4 for the whole run anyway. The alternative is a ceiling where two of the six fittings are visibly different from the other four, and a mixed order that somebody will get wrong on a Friday.
The second is fire. The ceiling of a ground-floor bathroom with a bedroom above it is the underside of a fire-resisting floor, and its resistance came from a tested assembly — BS 476-21 or BS EN 1365-2 in the UK, and Approved Document B expects the construction to hold the period it was designed for. Cutting six 75 mm apertures through the membrane is not something the test allowed for, so the aperture is closed again with a fire hood tested with that construction, or the luminaire is one tested as a rated assembly in its own right. The two are not interchangeable and neither is a generic tub of intumescent. In an NEC jurisdiction the parallel concern is thermal rather than compartmental: Article 410's recessed luminaire provisions keep thermal insulation clear of a luminaire enclosure by 76 mm unless the fitting is identified for insulation contact, which is what the Type IC marking exists to say.
The third is air, and it is the one that comes back as a complaint about mould. A bathroom ceiling with six unsealed apertures in it is a chimney feeding warm, saturated air straight into a cold roof space, and a fan on the other side of the room cannot compete with it. Sealed, gasketed fittings with the insulation dressed back to the clearance the hood requires answer the leak and the fire question together; insulation piled over an uncertified fitting answers neither.
What a downlight over a bath has to get through
- Joists and floor deck — decides which of the marked light positions is actually available before anything is set out on the ceiling plan Ceiling Joist Spanning Lineal Lumber Aggregator
- Loft insulation, dressed back — held off the fitting by the clearance the hood or the luminaire listing requires, which is a gap somebody has to be told to leave Insulation Batt Calculator
- Fire hood over the aperture — restores the membrane the cut-out removed, and only where it was tested with this construction rather than bought as a generic cap
- Plasterboard ceiling — the membrane whose fire resistance the tested assembly assumed, bought by the sheet against the ceiling area rather than the room floor Ceiling Plasterboard (Gypsum Board) Sheet Calculator
- Sealed luminaire body and driver — must carry IPX4 anywhere in zone 1 or 2, and it is the driver's input wattage rather than the lamp equivalence that loads the circuit Lighting Circuit Connected Load Calculator
- Bezel and gasket — the part that makes the stated ingress rating true, and the part a fire hood forced over the fitting can quietly compromise
What a bathroom hangs off its lighting circuit
A bathroom lighting circuit is not a lighting circuit. Between the ceiling and the mirror wall it typically ends up carrying the downlight run, the mirror's LED perimeter, the demister pad glued to the back of that mirror, the isolating transformer in the shaver supply unit, and the extract fan with its overrun timer — because the fan is conventionally fed from the lighting circuit so that it runs when the light is switched on. Only two of those five are lights.
Count them at the driver, not at the lamp. An integrated LED downlight sold as a sixty-watt equivalent is a marketing statement about brightness; the number that loads the circuit is the driver's input wattage on the label, usually somewhere between six and twelve. The demister pad is the opposite trap — a resistive heating element of thirty to sixty watts that appears on no lighting schedule and runs whenever the light is on. Add them honestly, then hold the total against the breaker already protecting lighting on that floor rather than against the bathroom in isolation, since the whole upstairs is usually on one device.
Enter the driver input wattage from the label rather than the equivalence on the carton, and count everything the bathroom is adding to that circuit — the downlights, the mirror, the demister pad and the fan — not just the fittings that make light. Then enter the breaker that already protects lighting on that storey, because the bathroom is joining an existing load rather than starting a fresh one.
The total number of light fixtures on this circuit.
The rated wattage of each fixture (or lamp/driver combination).
The nominal voltage supplying the circuit.
The rating of the protective device on this circuit.
Total connected lighting load
720 W
The circuit's current draw is 6.0 A, at or below 16.0 A — 80% of the breaker rating, which NEC 210.19 and 210.20 set for a continuously-loaded circuit. Under BS 7671 the design check is instead Ib ≤ In ≤ Iz — load under device, device under cable capacity — with no 80% derate for lighting, so this comparison is on the cautious side of the British rule rather than a statement of it. Being inside the rating on this one check settles nothing about the rest of the circuit — the conductors, the overcurrent device, and the work as installed are all outside it.
- Circuit current draw
- 6 A
- Maximum continuous load (80% of breaker)
- 16 A
They open the calculator with your figures already in it
Lighting Circuit Connected Load Calculator: 720 W — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 720 W — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Watts divided by volts is the current only at unity power factor, and lighting is not at unity. LED drivers and ballasts pull current out of phase with the voltage, so the real figure is watts divided by volts times power factor — at 0.9 that is 11% more current than shown here, and uncorrected budget drivers sit nearer 0.5, which roughly doubles it. The breaker responds to current, not to watts.
- Steady-state load says nothing about the moment of switch-on. Electronic drivers charge their input capacitors in the first milliseconds at many times running current, and enough drivers on one circuit will trip a B-curve MCB or a thermal-magnetic breaker every time the lights are turned on, on a circuit that clears this check with room to spare. That is a driver-count and breaker-curve question, and it is decided nowhere on this page.
The fan needs a position, a rating and a way to be switched off
A fan in the ceiling over a shower is in zone 1 and has to be suitable for it, which for a mains-voltage unit means an IPX4 housing and 30 mA residual current protection like everything else in the room. Where a fan is genuinely inside the shower enclosure, the answer is a SELV unit with its transformer sited outside the zones. The third option is the one most often overlooked at design stage: move the motor entirely, put an inline fan in the roof space or a boxed void, and leave nothing in the room but a grille. That solves the zone question and the noise question in one move, and it costs a duct run rather than a different fitting.
Isolation is where fan installations most often fall down. BS 7671 requires a means of isolating every circuit and every item of fixed equipment, and a fan with an overrun timer takes three conductors plus earth — switched live, permanent live and neutral — so the isolator has to break all of them. That is what a triple-pole fan isolator is for, and it goes outside the room or on a ceiling pull cord inside it. A wall switch by the door does not satisfy it, because the permanent live stays live after the switch is off, and the person who finds that out is the one who takes the cover off a running fan.
Everything downstream of the housing belongs to a different question and is covered on the extractor-fan ducting guide: the duty, the bore, the fall, the lagging across a cold roof space and why water arrives back at the grille. Two points are worth stating here only so they are not assumed. The rate is set by Approved Document F or ANSI/ASHRAE 62.2 depending on where the work is, not by the fan's carton. And a powered roof ventilator is not a fix for a loft made wet by an extract terminating in it — a separate machine with a separate duty, which that guide takes apart.
Where the heating mat is not allowed to be
An electric underfloor mat is sized by the floor it can actually cover, and in a bathroom that is a much smaller number than the room. Manufacturers universally prohibit laying mat under a bath, a shower tray, a WC pan, a vanity plinth or any other fixed enclosure, because a heating element with no route for the heat to leave runs hot against its own insulation. In a typical 3.9 m² bathroom the fixtures take a third of the floor before you start, and a mat cannot be cut to fit — only the mesh between the runs can — so an over-ordered mat is waste rather than adjustment. Set the floor probe while you are at it: in a conduit, in the adhesive midway between two cable runs, reachable from the thermostat. A probe laid across a cable reads the cable rather than the floor.
The installation itself carries its own requirement. Section 701 of BS 7671 requires a floor heating unit in this location to have either an earthed metallic sheath or an earthed metallic grid over it, connected to the protective conductor of the circuit — which is a property of the product, so it is a purchasing decision and not something that can be added on site. NFPA 70 reaches a similar place through Article 424, which covers fixed electric space-heating equipment and requires ground-fault protection for heating cable installed in the floor of a bathroom. In both regimes the mat wants its own circuit and its own protective device, not a spur off whatever was nearest.
Set the coverage figure from the fixture footprints you chalked on the floor rather than from a default — bath, tray, pan and vanity subtracted — because that fraction is what decides both the mat you order and the wattage the circuit has to carry. The output figure is the load to take into the next section.
The total floor area of the room to be heated.
The share of the room's floor area that will actually be covered by heating mat.
The heating mat product's rated power output per square meter.
Total heating wattage
959 W
Confirm your specific mat product's rated output per square metre or square foot and your home's electrical circuit capacity before purchasing — radiant floor heating circuits typically require a dedicated GFCI-protected circuit.
- Heated mat area
- 68.8 ft²
They open the calculator with your figures already in it
Heated Wire Floor Mat Coverage & Wattage Calculator: 959 W — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 959 W — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Mats are not cut to fit. The heating wire cannot be cut anywhere along its length — only the mesh it is stapled to — so a floor is tiled out of standard factory mat sizes and the purchased area lands above the usable area computed here, sometimes well above in a room broken up by a vanity, a door swing and an angled wall. Lay the standard sizes onto the floor plan before ordering to the square meter.
- Wattage is not warmth. Whether this output holds a room at temperature depends on what sits above and below the mat: thick stone or porcelain responds far more slowly than vinyl or engineered timber, and without insulation board underneath, a large share of the output heats the slab rather than the room. At 150 W/m² over 80 percent of the floor this is a floor-warming figure — a room with big glazing or an exposed floor below often still needs another heat source to reach setpoint on a cold morning.
- The number has to fit the thermostat, not just the panel. A floor-heating thermostat typically switches around 15 or 16 A — roughly 1,800 W at 120 V and about 3,500 W at 230 V — so a floor above that needs two thermostats or a contactor, and each thermostat wants its own floor sensor run in the screed between wire runs rather than across one.
Whether the house can carry what the room now wants
A bathroom that used to hold one light and a pull cord is being asked, after the refit, to hold a heating mat, a heated towel rail element, a fan, a mirror and in a great many British houses an electric shower. That last one alone is usually the largest single load in the house: an 8.5 kW instantaneous unit draws about 37 A at 230 V, which is over half of what an older 60 A supply was ever sized to deliver, and it is not a load that shares a circuit with anything. The refit is therefore a supply question before it is a bathroom question, and it is worth asking on the survey visit rather than on the day the second fix is booked.
Take each new item as its own circuit and check it, then take the room's total to whoever is doing the real calculation. What that means by "real" matters: a full assessment applies diversity across the installation, accounts for the existing load and answers to the adopted code, and it is the electrician's work rather than an article's. The check below is one circuit's headroom against the continuous-load limit — useful for settling whether the towel rail can join the lighting circuit, or whether the mat needs its own way, and no substitute for the sizing exercise the branch-circuit guide sets out.
Run it once per proposed circuit rather than once for the room: the mat at the wattage the previous section produced, the towel rail element at its nameplate, the lighting and fan group at their driver inputs. A shower belongs on its own way and its cable size is a separate exercise, so use this to confirm nothing else has been quietly added to it.
The amp rating printed on the breaker in your panel.
Most household outlets in North America are 120V; large appliances (dryers, ranges, EV chargers) are often 240V.
The sum of the wattage of everything plugged into this circuit at once.
Current draw
10 A
- Total connected load
- 1,200 W
- Safe continuous limit (80% rule)
- 16 A
- Safe continuous limit
- 1,920 W
- % of safe continuous capacity used
- 62.5 %
They open the calculator with your figures already in it
Electrical Circuit Load Calculator: 10 A — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 10 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- ONE circuit, not the panel. Whether the service and the panel can carry everything together is a separate calculation under the code's demand-factor rules, and a house full of individually compliant circuits can still overload its supply.
- Says nothing about the WIRE. Conductor ampacity is set by the conductor, its insulation, the ambient temperature and how many current-carrying conductors share a raceway — a breaker rating does not guarantee the cable behind it, and a correctly sized breaker on undersized cable is the dangerous combination.
- Voltage drop over the run is not checked here. A circuit inside its current limit can still deliver too little voltage at the far end, which is a separate calculation.
- Motor and compressor loads draw several times their running current at start-up. Nameplate watts describe the running condition and understate what the breaker sees on a cold morning.
- The 80% figure applies to CONTINUOUS loads — three hours or more at full draw. A load that is genuinely intermittent may use more of the breaker's rating, and which of the two a given appliance is can be a judgement.
Code thresholds this tool can check
Code thresholds this tool can check
Checked for United States. Each check below names the body that published the limit it uses. Switching market re-runs them. This is not a code review and has no official standing.
These checks cover only the specific numeric limits listed below. They are not a complete code review: fire separation, egress, structural capacity and accessibility provisions are outside their scope, and only the handful of local amendments offered in the selector are modelled — your municipality may have others. Passing every check here does not make a design compliant. Final approval rests with your local building authority.
Bonding, and the argument you can now sometimes win
For decades a UK bathroom meant supplementary equipotential bonding: green-and-yellow conductors clamped to hot and cold pipes, the towel rail, the bath and any metal waste, tied together and back to the earthing terminal. Section 701 now permits that supplementary bonding to be omitted where a set of conditions is met together — all circuits of the location having 30 mA residual current protection, all circuits satisfying the disconnection times required for automatic disconnection of supply, and every extraneous-conductive-part of the location being effectively connected to the installation's protective equipotential bonding. All of them, not the convenient ones.
The last condition is the one worth testing rather than assuming, and the answer has changed in most houses. A copper tail rising out of a plastic riser to serve a basin is not an extraneous-conductive-part at all — it introduces no potential from anywhere — and clamping it to earth achieves nothing except a reassuring green label. Whether the pipework in a given bathroom qualifies is a measurement with a low-resistance ohmmeter between the part and the main earthing terminal, taken on that installation, and it belongs in the certification rather than in an assumption. The NEC has no general counterpart to any of this for an ordinary bathroom; the nearest thing is the bonding required around a hydromassage tub in Part VII of Article 680, which is a rule about one appliance rather than about the room.
The order the marks go on
None of the above is difficult. All of it is sequence-dependent, and the failures are almost always a decision taken in the wrong week rather than a rule misread. The zones come off the sanitaryware drawing, the fitting schedule is checked against the zones, and the circuits are settled before the first cable is pulled — because the cheapest version of every problem on this page is a pencil line on a stud, and the most expensive is a chase through a tiled wall in a finished room.
- Fix the bath and shower positions on paper before the fitting schedule is priced, and treat any later move as a re-check.
- Check every proposed fitting against the zone it lands in, client purchases included, and reject on ingress rating before delivery.
- Settle the socket question early — shaver supply unit, or basin receptacle and its dedicated circuit — because it decides the circuit topology.
- Confirm the ceiling's fire resistance and order hoods or rated luminaires against it, rather than deciding at boarding.
- Run three conductors and earth to the fan, and put the triple-pole isolator where somebody can reach it with the fan running.
- Keep the mat, the towel rail and any shower off shared ways, and check each against the continuous-load limit before the consumer unit is specified.
- Measure the pipework before deciding whether supplementary bonding may be omitted, and put the reading on the certificate.
What the zones turn into on an order sheet
Every line below is settled by a distance rather than by a preference, so work them off the marked walls in one pass and the fitting schedule stops being a series of returns.
- Luminaires by zone, with the ingress rating stated per position — Count the positions inside zone 1 and zone 2 separately from those outside, then specify one rating across the whole run so a mixed order cannot go wrong on site.
- Fire hoods or rated luminaires, counted against the ceiling below a habitable room — One per aperture through a fire-resisting ceiling, tested with that construction. A luminaire tested as a rated assembly replaces the hood rather than joining it.
- Shaver supply unit, or basin receptacle and its dedicated circuit — One or the other depending on the regime, never both reasoning applied at once. In a UK room check the 3 m horizontal clearance from the zone 1 boundary before choosing the wall.
- Fan, its isolator and three-core-and-earth to the position — Triple-pole isolation for an overrun timer, sited outside the room or on a pull cord. Add the SELV transformer position if the fan is going inside the enclosure.
- Heating mat measured against the free floor, not the room — Fixture footprints subtracted first. Include the floor probe, its conduit and a spare conduit run to the thermostat while the floor is open.
- Protective devices, and the bonding decision with its test reading — 30 mA protection for every circuit of the location; a dedicated way for the mat, the towel rail and any shower. Record the continuity reading that justified omitting supplementary bonding.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.