Backup wiring: size the changeover feed, not the biggest appliance

Why a circuit runs hotter on backup than on the grid

Householders notice this and assume the inverter is faulty. It is arithmetic, and three things stack up.

One cable does the work of four. A board splits the house into ways: lights, fans, sockets, kitchen, each on its own conductor. Put all of them behind one changeover and its output carries the sum, not the largest. Nothing in the board reports that, because the individual MCBs still see their own small currents.

Constant-power loads draw more current at lower voltage. An inverter air conditioner, a switch-mode charger, a fridge compressor and a motor on a fixed mechanical load all hold their output as the voltage falls, which means the current climbs to compensate. On a battery bank near the end of its charge, the same appliances that drew 18 A at 220 V draw 21 A at 190 V.

The ambient is not 30 °C. Current-capacity tables assume 30 °C air. A generator in a closed store or an inverter on a roof in June sits well above that, and the first metres of the feed sit in it too. Derate for the position — take the correction factors for your own method and ambient from BS 7671 rather than a rule of thumb. If a run already gets hot on backup, work through cable overheating before adding anything else to it.

Changeover feed size by backup load

A changeover feed is sized for the sum of every circuit behind it, not for the largest appliance on it, and the current is worked out at the lowest voltage the inverter or generator actually delivers rather than at 220 V. A 4,000 W backup load is 18.2 A at 220 V and 21.1 A at 190 V, so the cable has to clear 21.1 A before any derating — on BS 7671 reference method C at 30 °C that is 2.5 mm² at 27 A on paper, and 4 mm² at 36 A once the hot enclosure the feed runs through is allowed for.

Add the running watts of everything you intend to put behind the changeover, from the rating plates, then read the row. The load figure is watts, not the inverter's kVA badge — a 3 kVA machine feeding 1,500 W of house needs a 1,500 W feed, and a 3 kVA machine that you intend to load fully needs a 3,000 W one.

The breaker on that feed follows the cable, never the load: its rating must sit under the cable's capacity after derating. Fitting a larger device because the backup feed keeps tripping is the mistake set out on wire size too small.

Worked: a house on a 5 kVA inverter

Take a double-storey house that backs up lights, fans, sockets, the fridge and one inverter AC. Read every figure off its own rating plate; these are typical values to show the method.

Ten ceiling fans at a typical 75 W each is 750 W. Twenty-five LED points at a typical 12 W each is 300 W. A fridge running at a typical 180 W, a television at 100 W, an inverter air conditioner holding a typical 1,200 W in steady state, and 1,500 W of sockets and kitchen appliances actually in use. Total: 750 + 300 + 180 + 100 + 1,200 + 1,500 = 4,030 W.

At 220 V that is 4030 ÷ 220 = 18.3 A. At a sagging 190 V it is 4030 ÷ 190 = 21.2 A. The smallest copper that carries 21.2 A on method C is 2.5 mm² at 27 A — which leaves under 6 A of margin before a single derating factor is applied, in a cupboard that is warmer than 30 °C. The feed we would fit is 4 mm², rated 36 A, three-core so the earth runs at full size: 4mm 3 core standard cable. Add a second AC to the backup later and the same sum lands on 6mm 3 core standard cable.

Length rarely decides this run, because the inverter usually stands within a few metres of the board. Where it does not — a generator in an outhouse, an inverter on the roof of a double-storey — check the drop as well as the heat, using the run-length limits on voltage drop.

The changeover itself, and the one thing never to do

• The changeover transfers live and neutral together, so the grid and the backup source can never be connected at the same instant. That is the whole purpose of the switch, and it is why a manual changeover with two positions and no overlap is safer than any improvised arrangement.
• Never feed a generator into a socket with a double-ended lead. It backfeeds the street through the meter, which endangers anyone working on the line during an outage, it leaves the generator's own output live on exposed pins, and it puts the whole backup load through one final circuit's cable.
• The earth is not switched. The backed-up circuits keep the installation's existing earthing arrangement through the changeover. How the machine's own frame is bonded depends on the machine, and belongs to the electrician installing it.
• Decide what is behind the changeover before you size anything. Most disputes about backup cable are really disputes about which circuits were meant to be on it. Write the list, total the plates, then buy.

Three other runs, and none of them is this cable

A backup installation is four separate cable decisions and people routinely price only one. The battery leads carry very high current at 12, 24 or 48 V over a short distance, so they are sized by a different rule entirely — that is the inverter-to-battery cable. A solar array feeds the same inverter on DC cable rated to a higher temperature, typically 90 °C for XLPE or XLPO against 70 °C for ordinary PVC, covered on solar panel to inverter cable. The AC feed into a UPS or inverter input is on cable for a UPS or inverter. This page is the last of the four: the AC output of the changeover into the circuits it backs up.

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