SWA Cable Calculator

How to use this calculator

1. Enter the protective device ratingThe cable must be protected by the breaker as well as carry the load, so the design current is the larger of the two. A 32 A breaker means the cable must handle 32 A regardless of the connected load.
2. Measure the actual cable routeNot the straight-line distance. Round the corners, up the wall, along the ceiling — a 20 metre garden can easily be a 35 metre cable run.
3. Use a tighter drop limit for a submainThe 5% allowance covers the whole installation. A submain feeding final circuits should use much less, because the drops add together.
4. Read which test bindsIf voltage drop is limiting, a longer run needs a bigger cable regardless of load. If current is limiting, the run is short and the load is high.

Two tests, and length usually decides

A cable has to satisfy two entirely separate requirements, and passing one says nothing about the other.

The first is thermal. Current flowing through a conductor generates heat, and beyond a certain point the insulation degrades. Every cable size has a tabulated current-carrying capacity, and that capacity depends on how the cable is installed — a cable buried in the ground, bunched with others or run through insulation cannot shed heat as well and carries less.

The second is voltage drop. Cable has resistance, and current flowing through resistance loses voltage. By the time the supply reaches the far end of a long run there is less of it than there was at the origin, and equipment designed for 230 V does not work properly at 210 V.

BS 7671 permits 3% drop for lighting circuits and 5% for everything else, measured from the origin of the installation to the point of use.

Here is why length dominates. Current capacity does not care how long the cable is — a 6 mm² cable carries the same current at 5 metres as at 100. Voltage drop is directly proportional to length, so doubling the run doubles the drop.

The result is that on short runs the current rating governs, and on anything over about 20 or 30 metres the voltage drop takes over and forces a larger cable than the current alone would need.

That is exactly the situation with a garage, a shed or a garden office, which is why the cable someone recommends from the current rating alone is so often too small. A 32 A supply needs 6 mm² on current capacity; over 30 metres it needs 10 mm², and over 60 metres it needs 16 mm².

The cumulative point is important and frequently missed. If a submain drops 4% getting to a garage, the final circuits inside that garage start from 4% down and have only 1% left before breaching the limit. A submain should therefore be designed to use a small fraction of the allowance — 1% or 2% is a sensible target — not the whole 5%.

Correction factors reduce the capacity further. Grouping several cables together, high ambient temperature, and running through thermal insulation all derate a cable, sometimes substantially. A cable passing through 300 mm of loft insulation can lose half its rating.

Worked example: a 7 kW garage supply at 30 metres

A 7,000 W single-phase load at 230 V with a power factor of 1 draws 30.43 A.

Protected by a 32 A breaker, the design current becomes 32 A — the larger of the two — because the cable must carry whatever the breaker will allow through before it trips.

On current capacity alone, 2.5 mm² two-core SWA buried in the ground carries 33 A and passes. That is the answer the thermal test gives, and on its own it would be dangerously misleading.

Now the voltage drop test. The 5% limit on 230 V is 11.5 V.

At 4 mm² the figure is 11 mV/A/m. Over 30 metres at 32 A: 11 × 32 × 30 ÷ 1000 = 10.56 V, which is 4.59%.

So 4 mm² is the smallest size that passes both tests, and voltage drop is what forced it up from 2.5 mm². The maximum run at 4 mm² is 32.7 metres — barely more than this job.

But 4.59% is almost the entire allowance spent on the submain alone, leaving 0.41% for every circuit inside the garage. That is not a workable design, even though it is technically compliant.

Change the drop limit to 2%, which is a realistic target for a submain, and the answer becomes <b>10 mm²</b> with a drop of 1.837%. That leaves over 3% for the final circuits, which is the position you actually want.

This is the single most important thing the calculator does. The minimum compliant cable and the right cable are different, and the difference is two sizes.

Now double the run to 60 metres at the full 5% allowance. The minimum becomes 10 mm² rather than 4 — the drop scales directly with length while the current rating does not move at all.

One more consideration: a 7 kW load is close to what a domestic supply can spare. Before specifying anything, the existing main fuse rating and the total load on the property need checking. A 60 A or 80 A main fuse feeding a house with an electric shower and a hob may not have 32 A of headroom, and a load assessment comes before cable sizing.

Cable size by load and run length

Two-core SWA buried in the ground, single phase at 230 V, spending the <b>full 5%</b> drop allowance with no derating. Read along a row and the effect of length is obvious — the same 32 A load needs 4 mm² at 20 metres and 10 mm² at 60.

These are minimum compliant sizes, not recommendations. A submain feeding further circuits has to leave room for their drop too, so set the limit to 2% in the calculator and expect to go up one or two sizes from this table. On the 32 A at 30 m row that takes you from 4 mm² to 10 mm².

Armour, burial and getting it certified

Steel wire armour serves two purposes: mechanical protection, which is why the cable can be buried directly, and a potential circuit protective conductor.

Whether the armour can be used as the earth depends on its cross-sectional area being adequate for the fault current and disconnection time. For smaller sizes it usually is, which is why two-core SWA is common for a single-phase submain — line, neutral, and earth through the armour.

That only works if the armour is properly terminated at both ends with the correct glands, banjos and earth tags, providing a continuous low-resistance path. A gland fitted without an earth tag looks identical and provides no earth at all, which is a genuinely dangerous failure mode because it is invisible.

Where the armour is not adequate, three-core is used with the third core as the earth, and the armour bonded as well.

On burial: 600 mm depth under a garden and 750 mm under a driveway is the usual guidance, with warning tape laid above the cable so anyone digging later finds the tape before the cable. Direct burial is permitted, but running the cable through a duct makes future replacement possible without excavation — worth the small extra cost.

The cable must be protected where it emerges from the ground and rises up a wall, and it must not be run where it could be damaged by ground movement or vehicle loading without additional protection.

Earthing arrangements for an outbuilding are a genuine complication. Where the property has a TN-C-S supply, exporting that earth to a detached building with extraneous conductive parts — a metal gate, a water pipe, a garden tap — can create a hazard if the supply neutral fails. The usual answer is a separate TT earth electrode at the outbuilding with its own RCD protection, and that decision needs to be made by someone competent to assess it.

On the paperwork: installing a new circuit is notifiable under Part P of the Building Regulations in England and Wales. That means either using a registered electrician who can self-certify, or notifying building control and paying for inspection. The work must be tested — continuity, insulation resistance, earth fault loop impedance, RCD operation — and an Electrical Installation Certificate issued.

That certificate matters beyond compliance. It is what a buyer's solicitor asks for, what an insurer may want after a fire, and what proves the installation was verified rather than merely assembled.

This calculator sizes a cable for planning purposes. It does not verify disconnection times, coordinate the protective device with the cable, assess the earthing arrangement, or account for the specific installation method in the current regulations. All of those are part of a real design.

Assumptions and limitations

• Tabulated values are representative rather than the current BS 7671 tables
• Covers two-core and four-core SWA up to 120 mm²
• Assumes one of two installation methods — real installations have more
• Does not verify earth fault loop impedance or disconnection times
• Does not assess whether the armour is adequate as a protective conductor
• Does not address the earthing arrangement for a detached outbuilding
• A planning aid only — the design must be done and certified by a competent person

Questions people ask about this in United Kingdom