Running Power to a Detached Garage

What a Workshop Draws, and When It Draws It

Add up the nameplates in a working garage and you get a number nobody would pay to bury. A welder, a compressor, a table saw, a dust extractor, a car lift, a heater and a kettle will total well past a hundred amps on paper, and they cannot all be running because there is one person in the building and two hands on the job. Feeder sizing has always dealt with this through demand and diversity — Article 220 in the NEC, the diversity allowances in the IET On-Site Guide to BS 7671, the maximum demand tables of AS/NZS 3000 Electrical Installations — and the outbuilding case is where those allowances earn their keep hardest.

The exceptions are the loads that sit at full value for hours, because diversity is an argument about coincidence and those loads do not care. Electric vehicle supply equipment is the obvious one: it is a continuous load, it is sized at 125 percent of its rating in NEC practice, and it is the single change most likely to arrive after the trench is closed. Resistive space heating in a workshop is the same shape of problem — nobody switches a garage heater on for ten minutes in February. Where those two exist or are plausible, they get added at full value on top of a diversified figure for everything else, and that is usually what decides between a sixty amp feed and a hundred.

Motors are the other special case, and they are special in a way that is easy to get backwards. Conductor sizing for a motor branch circuit works from the tabulated full-load current for its horsepower and voltage rather than from the amps printed on the plate, and the tables exist precisely because plate values vary between manufacturers for the same nominal machine. A compressor or a two-post lift at the end of a long soft feeder is also the load that will expose every millivolt of drop, because its starting current is several times its running current and the drop scales with the current of the moment.

Then there is the question the client will not ask and you should. A trench is dug once. The marginal cost of the next conductor size up, while the ground is open and the duct is being bought anyway, is copper and nothing else; the cost of finding out in three years that the feeder cannot take a charger is the whole garden again. Size the feeder for the building the garage is going to become, write down what you assumed, and let the client decline it in writing if they want the smaller number.

The Lug Decides Which Column You May Read

Conductor insulation is rated for a temperature and so is every terminal it lands in, and NEC 110.14(C) settles the argument in favour of the lowest-rated part of the connection. In practice that means equipment rated 100 amperes or less, or marked for conductors 14 AWG through 1 AWG, is treated as a 60 degree Celsius termination unless the device or its instructions are marked otherwise. Most modern breakers and panelboard lugs are marked for 75 degrees; plenty of older ones are not, and an unmarked lug is a 60 degree lug. The 90 degree column exists chiefly so you have something to apply correction and adjustment factors to on the way down — it is almost never the number you finish on.

That distinction is worth a size on a garage feeder. Six AWG copper is 55 amperes in the 60 degree column and 65 in the 75 degree column, so the same cable is either short of a sixty amp device or comfortably over it depending on a marking you can only read by taking the dead front off and looking. Check both ends. The breaker at the house and the main lug or main breaker in the garage board are separate answers, and the lower one governs the whole run. UL 486A-486B Standard for Wire Connectors is the document behind the connector side of that relationship, and the torque value in the equipment instructions is part of the listing rather than a suggestion — a lug tightened by feel is the failure that shows up as a browned insulation tail a decade later.

The wiring method carries its own constraint. Everything inside a buried raceway is a wet location from the first winter, whatever the weather was on the day you pulled it, so the conductors have to be listed for wet locations: the dual-rated THHN/THWN-2 that most suppliers stock, or XHHW-2, or a direct-burial cable such as USE-2 where the design is to bury the cable rather than a duct. Plain THHN is a dry-location rating and has no business in a duct that runs under a lawn. In the United Kingdom the equivalent decision is usually made in favour of steel wire armoured cable buried directly, and its ratings come from Appendix 4 of BS 7671 with an installation-method correction that a duct in the ground and a cable in free air do not share.

One Volt Budget, Spent Twice

Voltage drop on this job is not a single calculation, it is a budget with two claimants. The feeder spends some of it getting to the garage, and every branch circuit inside the garage spends the rest getting from the garage board to a socket at the far wall. The familiar three percent for a branch circuit and five percent overall is informative guidance in the NEC rather than an enforceable rule; BS 7671 sets limits in its appendices that must actually be met — three percent for lighting and five percent for other uses on a supply taken from a public distribution network. Either way the arithmetic is the same, and either way a feeder that eats four and a half percent has left nothing for the circuits it exists to supply.

So set the feeder a target rather than a limit. Somewhere around two percent leaves the inside of the garage a workable allowance, and it also leaves room for the load to grow without the drop growing past what the equipment tolerates. It is a design decision, not a code number, and it is the single most useful thing to fix before pricing conductor.

Measure the path, not the lawn. The line somebody paced off is the shortest distance between two buildings and the conductor will not travel it. Add the climb from the house board down to the point where the duct leaves the wall, the offsets around a soil stack and a bay window, the sweeps at each end of the trench, the slack you leave in the duct so the cable is not stretched, and the rise inside the garage to a board that is mounted at chest height. Forty metres of string line lands somewhere near fifty-five metres of conductor on most gardens, and drop is linear in exactly that number.

Drop is also linear in current, which means the worst case is not the average day. It is the moment the welder strikes while the charger is running and the compressor kicks in, and the voltage at the garage board falls furthest exactly when three things want it most. Motors respond to low voltage by drawing more current and running hotter, which makes the drop worse, which is a loop that ends in a tripped overload or a shortened motor.

When the calculation says upsize, the upsize has consequences beyond the price of copper. The protective conductor is increased in proportion under NEC 250.122(B) where the ungrounded conductors are enlarged for voltage drop, which changes the cable you are buying rather than just one core of it. Larger conductors need a larger duct, a longer bending radius inside both enclosures, and lugs that will physically accept them — check that the garage board's main lug takes the size you have arrived at before it is on a reel in the van. Upsizing for drop is the most common reason a feeder outgrows the enclosure that was picked for it.

A Fault at the Far End Has to Get Home

Here is the constraint that separates a competent outbuilding feed from a dangerous one. When a live conductor touches the metal frame of a machine in the garage, the current that operates the protective device has to return to the source through a conductor, and the earth beneath the garden is not that conductor. NEC 250.4(A)(5) says so explicitly: the earth shall not be considered an effective ground-fault current path. The arithmetic is Ohm's law and it is brutal. A driven rod that achieves 25 ohms — a respectable figure in ordinary soil — offers 240 volts divided by 25 ohms, which is under ten amperes. A sixty ampere breaker will sit and watch that indefinitely while the machine frame stays live.

Which is why a feeder to a detached structure runs with an equipment grounding conductor pulled alongside the live conductors and the neutral, and why the neutral is not re-bonded to earth at the garage. NEC editions from 2008 onward removed the old permission to use the grounded conductor as the fault path at a separate building, so the modern arrangement is four wires to a three-wire supply: two ungrounded, one grounded, one equipment grounding conductor, with the bonding screw or strap taken out of the garage board and the neutral bar isolated from the enclosure. The garage still gets a grounding electrode of its own under NEC 250.32 — a rod, or the reinforcement of its slab where that qualifies as a concrete-encased electrode — but that electrode is a reference and a surge path, not a fault path, and it never substitutes for the conductor.

The British framing gets to the same place by a different route and with a number attached. Automatic disconnection of supply requires the earth fault loop impedance at the point of the fault to be low enough that the device operates within the disconnection time — commonly 0.4 seconds for a final circuit and 5 seconds for a distribution circuit on a TN system. A Type B miniature circuit breaker needs somewhere between three and five times its rated current to trip magnetically, and a Type C between five and ten times, so a 40 ampere Type B wants roughly 200 amperes of fault current before it will operate quickly. Fifty-five metres of feeder with an undersized protective conductor adds resistance to both legs of that loop, and the adiabatic check in Section 543 of BS 7671 then has to confirm that the protective conductor survives the current for as long as it takes to clear.

The earthing arrangement itself is a separate decision in the United Kingdom and one worth taking advice on rather than assuming. Exporting a PME earth to an outbuilding is constrained by the distributor's conditions and by the Electricity Safety, Quality and Continuity Regulations 2002, and the common alternative is to earth the garage as a TT island with its own electrode and residual current protection, which changes the disconnection argument from impedance to residual current. Buried cable in a garden attracts a 30 milliampere residual current device in any case, and the NEC reaches the same protective outcome for people through the ground-fault circuit-interrupter requirements that cover garage and outdoor receptacles.

The Trench, and What Is Already In It

Before anything else, find out what is already under the lawn. A garden between a house and a garage is a good place to find a gas service, a water main, a soil drain, a telecom duct and an irrigation line, none of which are on a drawing and at least one of which is shallower than anybody expects. HSE guidance HSG47, Avoiding Danger from Underground Services, sets out the sequence — plans, then a locator, then careful hand digging near anything found — and it is the right sequence whether or not you are working under UK law. Where a locate service exists, use it, and note that a private garden run between two buildings is exactly the case such services do not have records for.

Depth is set by the wiring method and by what is on top, not by preference. NEC Table 300.5 gives different minimum cover for different methods and locations: direct-buried cable deepest at 600 mm, rigid nonmetallic conduit at 450 mm, rigid metal conduit shallowest at 150 mm, with a reduced row for a residential 120 volt branch circuit at 20 amperes with residual protection, and different figures again under a driveway or a building. Read the row for your method in the edition your jurisdiction has actually adopted. BS 7671 prescribes no single depth in Section 522, requiring instead that a buried cable be of a type suitable for the duty, at sufficient depth to avoid damage, and marked — with NJUG Volume 4 giving the positioning and colour coding practice the utilities themselves work to. Frost depth, tree roots and the route of a future patio all argue for going deeper than the minimum rather than exactly to it.

Trench safety scales with depth faster than people expect, and the point at which a garden trench becomes an excavation with rules attached is lower than the point at which it looks dangerous. OSHA 29 CFR 1926 Subpart P, Excavations, governs that in the United States: spoil set back from the edge, a means of getting out, and a protective system where the depth demands one. A collapse in a metre and a half of wet clay is enough to kill someone kneeling in it, and a domestic job is exactly where the discipline slips.

1. Establish and mark the existing services across the whole route before the first spit is turned, then hand dig within the safe distance of anything located.
2. Set the route to avoid tree root plates, soakaways and the footprint of any structure the client has mentioned wanting, and record where you moved it to.
3. Excavate to a consistent formation level, keeping spoil back from the edge and off the reinstatement material.
4. Bed the duct on screened fill, lay it with long sweeps rather than elbows, and leave a draw rope in it whether or not you intend to pull that day.
5. Bring the surround up over the duct by hand, lay the warning tape well above it rather than on it, and photograph the open trench with a tape measure in shot.
6. Backfill in lifts and compact each one, then reinstate the surface proud of finished level to allow for the settlement that is coming anyway.

Pulling It Through

A long duct pull fails at the bends, not along the straights. Tension builds through friction on every straight section and then multiplies through each change of direction, so two ninety degree elbows in an otherwise easy run can turn a comfortable pull into one that stretches the conductor or strips the jacket. Use long sweeps, keep the total angular content of the run down, and put a pit or a pull point in wherever the geometry demands it rather than trusting to lubricant and effort. Cable-pulling lubricant is not optional on a long feeder, and the manufacturer's guidance — the Southwire Power Cable Installation Guide is the usual reference for the tension and sidewall pressure arithmetic — states a maximum tension for the conductor and a maximum sidewall bearing pressure at a bend, both of which are exceedable by hand with a rope and two willing people.

Feed from the end with the awkward bends and pull from the end with the room. Let the conductors come off the reel in the direction the reel wants to turn, keep them off the ground and out of the spoil, and pull all of the conductors at once including the protective one — the equipment grounding conductor belongs in the same raceway as the conductors it protects, and pulling it separately later is both a nuisance and a mistake if it ends up in another route. Leave enough tail at each end to dress the conductors properly into the boards, because a feeder cut short at the garage end is a joint in a box you did not plan.