Getting the Incoming Electricity Supply Upgraded

Three Jobs Share One Name

The hob has already been delivered. The heat pump has a deposit on it, the charger is booked for a Tuesday, and somebody has told the customer that the supply needs upgrading to a hundred amps — a sentence that describes four completely different pieces of work, costing anywhere from a call-out fee to the price of a small extension, on lead times separated by an order of magnitude.

Sorted by what has to be dug up, they are these. A larger fuse link in the existing cut-out, where the service cable behind it was always adequate and only the link was conservative. A replacement service cable from the property to the main already lying in the street. A change in the number of phases delivered, which is a new service and a new meter and often a new intake position. And reinforcement, where the main itself, or the transformer feeding it, has no capacity left to give and the work moves out of the garden and into the road.

Which of the four you are in is not your decision and cannot be inferred from anything visible at the property. It turns on the size of the buried service cable, the spare capacity in the low-voltage main, the loading on the transformer, and — increasingly the deciding factor — how many neighbours have asked the same question in the last three years. What you control is the quality of what you send in: an accurate description of what is there, and a capacity figure you can defend when somebody queries it.

The Fuse Is the Distributor's, and It Is Not a Capacity

Start at the intake with a torch and a camera. On a British service you are reading a moulded carrier with a rating on it, a fuse link inside that carrier with its own rating and standard marking, and a wire-and-lead seal across the assembly. Domestic service cut-out links are made to BS 88-3, which superseded the BS 1361 Type II links still sitting in plenty of older intakes; sixty, eighty and one hundred amperes are what you will normally meet. On a North American service the equivalent reading is the utility transformer and service conductors on one side and the rating of the service disconnecting means on the other, the latter governed by NFPA 70 Article 230.

The carrier and the link are separate ratings and they disagree more often than people expect — a hundred-ampere carrier will happily hold a sixty-ampere link, and on many designs the link rating cannot be read at all without withdrawing the carrier. Do not withdraw it. That assembly is the distributor's property, the seal is there for a reason, and in Great Britain the whole arrangement sits under the Electricity Safety, Quality and Continuity Regulations 2002 and the distributor's own access policy. A booked fuse pull costs very little set against what happens to an installer found working inside sealed equipment.

The more useful thing to understand is what the fuse is for. It is protecting the service cable against fault current, not rationing the household. Its time-current characteristic is drawn to clear faults, which means a sustained modest overload can persist for a very long time without operating anything — so a house that has been fine on sixty amperes for thirty years has told you nothing whatever about whether that service cable has been running warm for a decade. The absence of a blown fuse is not evidence.

The reverse error is commoner and more expensive. A hundred-ampere fuse is not a promise of a hundred amperes. Behind it sit a service cable of unknown size, a main under the pavement shared with the rest of the street, and a transformer, and every one of those was planned on an after-diversity maximum demand — a per-property figure of a few kilowatts, set out in each distributor's own network design standard and its licence-mandated long term development statement, and ask for the current one rather than quoting a number you remember. Nothing in that planning chain assumed your customer would draw the twenty-three kilovolt-amperes a hundred-ampere single-phase service could theoretically deliver, and the whole street's worth of that assumption is what a wave of heat pumps is currently testing.

Record all of it before you leave. You will be asked for every line of it, a second visit is unpaid, and each of these readings is silent about something people routinely assume it covers.

A Better Supply Raises the Fault Level

The thing that makes an upgraded service good is exactly the thing that makes it more dangerous at the origin. A larger conductor, a shorter run, a nearer or larger transformer — every one of those lowers the source impedance, and lower source impedance means a higher prospective short-circuit current at the cut-out. The consumer unit downstream, its main switch, and the conditional short-circuit rating of the devices inside it were all selected against whatever the old service could deliver.

So measure the prospective fault current and the external loop impedance after the changeover rather than reusing the survey figures, and record both. Distributors in Great Britain declare a maximum prospective fault current for a domestic service — sixteen kilo-amperes is the figure most commonly quoted at the origin — and an assembly whose rating was justified by a comfortable measurement taken on the old supply is a non-compliance that nothing in the installation's behaviour will ever reveal.

The Earthing Arrangement Comes With the Offer

A new service can arrive with a different earthing arrangement from the one it replaced, and this is not always flagged. A property that never had a distributor earth may be offered one; a property that had a combined neutral and earth arrangement may find it withdrawn, particularly where the replacement service comes in overhead or where the distributor's policy for that class of connection has moved on. Either change lands on the installation designer, not on the distributor.

The consequences run through the whole protective system. Main protective bonding conductor sizes are set by the earthing arrangement and the size of the supply neutral, not by the load — the requirements where a protective multiple earthing terminal is provided are larger, and BS 7671 Chapter 54 read alongside Energy Networks Association Engineering Recommendation G12 is where that comes from; NFPA 70 Article 250 and its grounding electrode conductor table are the counterpart on the other side of the Atlantic. A new service therefore means going back to the gas and water bonds and the structural steel, which in a house of any age means finding a single undersized conductor clamped somewhere convenient rather than where the service enters.

Where the distributor declines to provide an earth at all, the installation needs its own electrode, and that is a redesign rather than an adjustment: earth fault loop impedance is now dominated by the electrode resistance, every final circuit depends on residual current protection for disconnection, and the soil the electrode is going into is a variable nobody chose. Price that possibility before the offer is accepted, because discovering it afterwards makes it the customer's surprise instead of a line in the quotation.

Trench, Meter Box, and Who Digs What

Almost every offer splits the physical work at a point of connection: the distributor works up to it, and everything on the customer's side is the customer's to provide. Excavation, ducting, backfill, reinstatement and the meter enclosure are the usual customer-side items, and taking the self-dig option is normally the single largest saving available on the whole job — provided the groundwork is done to their specification and not to a general idea of what a trench looks like.

That specification is prescriptive and it is worth having in writing before anyone hires a machine. Depth of cover, bedding material, warning tape, duct diameter and colour, minimum bend radius, a draw rope left in place, and jointing pits wherever the route changes direction enough to make the pull unreasonable. Have the open trench inspected before it is closed. A trench backfilled early on the assumption that it was obviously fine is a trench that gets reopened, and the second excavation is nobody's contractual obligation but the customer's.

Land the customer does not own stops jobs dead. A service crossing a neighbour's garden, a shared drive, a farm track or an access strip requires a wayleave or an easement, and that is a legal instrument on a legal timescale measured in months rather than weeks. On rural connections it is the commonest reason a scheme stalls after the offer has been signed and the money paid.

Settle the intake position early, because it decides more than it looks like it decides. Moving a meter from an internal cupboard to an external box is frequently bundled into the upgrade, and where the intake ends up sets the tail lengths, the board position, the route of the main protective bonding, and whether the consumer unit finishes somewhere with the clear working space the rules in force require in front of it.

1. Walk the route with the distributor's engineer before anybody prices the groundwork.
2. Establish the point of connection on the ground and confirm in writing who owns each side of it.
3. Get the duct, depth, bedding and reinstatement specification in writing and read it before hiring plant.
4. Establish ownership of every metre of the route, and start the wayleave or easement the same week if any of it is not the customer's.
5. Hand-dig at every crossing with an existing service and record the depth and separation you found.
6. Have the trench inspected and photographed open, dimensioned from two fixed points on the building.
7. Leave the duct roped, sealed at both ends and drawn through, and fit the enclosure before the energisation date.

Three Organisations, One Energisation

The connection involves three separate parties and only one of them is the distributor. The distributor owns the service cable and the cut-out. The supplier holds the customer's account. The meter operator, appointed by the supplier rather than by the customer, owns and fits the meter. Nothing coordinates the three by default, and the customer is the only person with a contractual relationship with all of them.

The failure this produces is predictable enough to plan around: a new service energised and sealed, and no meter, because the metering appointment was never raised. A change in phase count almost always means a new meter, sometimes a new supply number, and always a fresh appointment booked through the supplier — who will ask the customer for it and not you. Raise it at the same time the offer is accepted, not when the distributor has finished.

Sequence your own work to be genuinely ready. Tails of the right size and length, an isolator where one is needed, the board fitted, the earthing and bonding complete and tested, and the installation certificate available. The energisation visit is short and it is not a fault-finding visit; an installation still open when the engineer arrives becomes a second appointment at a lead time nobody quoted for.

What the Next Person Needs From You

Leave a record that survives you. The offer reference and the capacity requested; the new fuse rating with its standard marking; the earthing arrangement actually provided and whether it changed; the prospective fault current and external loop impedance measured after energisation; the duct route dimensioned from two fixed points; and every configured setting — the hob's power limit, the heat pump's backup element, the charge point's output — that the demand assessment quietly depends on. Whoever adds a battery to this property in three years is either building on that or repeating all of it.

Then be straight with the customer about what has and has not been bought. A reserved capacity is a commitment about this connection, not a statement about the street. If the network was already tight, the next few applications on the same main are the ones that get the conditional answers, and if the property is on the end of a rural spur then voltage at the far end — the range every distributor must hold under its own statutory obligation — may govern long before capacity does. Saying so on the day is a better conversation than having it after a complaint.