Electrical load calculation per circuit, a quick estimate
How to estimate the load on each circuit of a house from what is on it, at 230 V or at 120 V, why the figure is only indicative, and a calculator with the typical loads to try on your own circuits.
A quick load estimate for a circuit adds up a typical load for every device on it, in volt-amperes, and divides by the supply voltage to get the current in amps. Compare that with the breaker's rating and the circuit that needs attention stands out. It is a rough figure, with no diversity, and it is not the demand calculation your regulations ask for; it is the check that tells you which circuit to calculate properly first.
Try it on a circuit
Pick the supply, choose the breaker, and add what is on the circuit. The loads are the ones ePlan assumes in its Loads tab, so the figure here is the figure the app would show.
What the figures mean
Volt-amperes, not watts. The table works in VA, the apparent power, which is what the breaker sees. For a heater, a kettle or a lamp the two are the same; for motors and electronics watts are a little lower. For a rough estimate the difference does not matter.
Amps are what the breaker trips on. Divide the VA by the voltage: 3,000 VA is 13 A at 230 V and 25 A at 120 V. That is why the same kitchen is comfortable on a 16 A circuit on one side of the Atlantic and needs two 20 A circuits on the other.
Each device at its own voltage. On a North American plan the cooker, the dryer and the water heater sit on 240 V while the sockets sit on 120 V. The estimate divides each device by its own voltage before adding the currents, so a circuit that mixes the two is not divided by one figure.
Typical loads
The allowances the estimate uses. A socket is given a typical connected load, not the 3 kW a kettle could draw, because the point is the comparison between circuits, not the worst case on one.
| Device | 230 V plans | 120 / 240 V plans |
|---|---|---|
| Double socket | 500 VA | 180 VA at 120 V |
| Single socket | 300 VA | 180 VA at 120 V |
| Ceiling light | 100 VA | 100 VA at 120 V |
| Recessed light | 50 VA | 50 VA at 120 V |
| Extractor fan | 50 VA | 50 VA at 120 V |
| Fixed appliance on a fused spur | 1,500 VA | 3,000 VA at 240 V |
| Cooker | 7,000 VA | 8,000 VA at 240 V |
| Electric heater | 2,000 VA | 1,500 VA at 240 V |
| Immersion heater | 3,000 VA | 4,500 VA at 240 V |
| EV charger | 7,400 VA | 7,700 VA at 240 V |
The 180 VA per receptacle follows the customary North American allowance; the 230 V figures are ballpark allowances for a comparison between circuits.
Where it goes wrong
The circuit that fails this check in most houses is the kitchen. Six double sockets above the worktop come to 3,000 VA on their own, 13 A at 230 V, before the dishwasher, the washing machine and the fridge on the sockets under it. On one 16 A circuit that is over before anything is switched on. The fix is on the plan, not in the wall: the appliances on their own circuit, or the kitchen sockets split in two.
The second is the circuit that collected the extras over a renovation: the outdoor sockets, the garden lights, the shed, the pond pump, all on what was the garage circuit. Each addition was small. The sum is not.
The proper calculation
A design calculation applies diversity, the fact that not everything draws its full load at once, with factors from your regulations: the On-Site Guide's tables that accompany BS 7671 in the UK and Ireland, Article 220 of the NEC in the US, the equivalent elsewhere. It also sizes the cable for the current, the length of the run and how the cable is installed. None of that is in the estimate above, and none of it should be. The estimate's job is to point at the circuit that needs the calculation before the walls are closed, and the checking article puts it beside the two other checks a plan needs.
Questions
Is a load calculation required?
A demand calculation for the installation, and sizing for each circuit, are required by the regulations wherever they apply. The quick estimate on this page is not that calculation. It is the screening that tells you which circuit to look at first.
Why is a socket 500 VA and not 3,000?
Because a kettle is not plugged into every socket at once. The allowance stands for the typical load a socket contributes to its circuit when the house is in use, which is what makes one circuit comparable with another. The worst case on one socket is a different question, answered by the socket's own rating and the circuit's protection.
What is diversity?
The reduction a designer applies because the devices on a circuit are never all on together. Ten lights on a circuit are often all on; ten sockets never are. The regulations give factors for each kind of circuit, and a design calculation uses them. The estimate here does not, which is why its figure is high rather than low.
Does the app do the design calculation?
No. ePlan's Loads tab shows the estimate above per circuit, against the breaker you choose, with the voltage it assumed written underneath, and says on the screen that the figure is indicative.