Why use this calculator
Before a circuit is added, a workshop is fitted out or a generator is bought, someone has to answer two questions: how much power will this draw, and what protective device will carry it. Getting the first wrong means a generator that stalls or a bill that surprises you.
Getting the second wrong means nuisance tripping at best and an overloaded cable at worst. This calculator does the arithmetic that sits under both questions, including the square root of three for three-phase supplies and the 125 per cent continuous load factor the US National Electrical Code applies, which people routinely forget.
It is an estimating aid for planning and for briefing your electrician. The actual circuit, cable and protection must be designed and certified by a licensed electrician under AS/NZS 3000 in Australia and New Zealand, the NEC in the United States or BS 7671 in the United Kingdom.
How to use it
- Choose the wiring rules you work under, since that sets the standard breaker sizes and whether the NEC continuous load rule applies.
- Pick single phase for most homes or three phase for balanced industrial loads.
- Enter the supply voltage, using the line-to-line figure such as 400 V for three phase.
- Enter the load current per circuit from the appliance rating plate or a clamp meter reading.
- Enter the power factor, which is 1.0 for resistive heaters and about 0.8 for motors. Enter how many identical circuits carry this load.
- Finally say whether the load runs for three hours or more.
What each result means
- To stay under 80% breaker loading
- What it takes to keep the breaker at or below 80 per cent of its rating: the next standard breaker size, or the watts to remove per circuit. If you are already under, it shows the room left.
- Total real power
- The power actually converted to heat, light or motion across every circuit. This is what the energy meter charges you for per hour.
- Real power per circuit
- Real power drawn by one circuit in watts, useful for comparing against an appliance rating or a generator's running watts.
- Apparent power per circuit
- Voltage multiplied by current. Cables, breakers, transformers and generators are sized on kVA because the current flows whatever the power factor.
- Total apparent power
- Apparent power across all the circuits you entered. Compare it with a generator's kVA rating or the capacity of the supply.
- Design current for the breaker
- The current the protective device must be rated for. It includes the NEC 125 per cent factor for continuous loads where that applies.
- Next standard breaker rating
- The smallest standard rating at or above the design current. The cable must be rated to carry at least this after derating.
- Breaker loading at this current
- How hard the chosen breaker is working. Above about 80 per cent on a long running load, thermal trips become likely.
The formula
Single phase apparent power S in
kVA = V x I / 1000
- V
- supply voltage in volts
- I
- load current in amps
Three phase
S = 1.732 x V(line) x I / 1000
Real power P in
kW = S x PF
Design current = I x 1.25 for a continuous load under NEC 210.20(A), otherwise I
The breaker is the smallest standard rating at or above the design current, and loading = I divided by that rating
How this number is used in practice
Breaker size is never chosen on its own. The rule under AS/NZS 3000, BS 7671 and the NEC is the same in spirit: the design current must not exceed the device rating, and the device rating must not exceed what the cable can carry in its installed conditions.
That cable capacity depends on the installation method, whether the cable is in conduit, buried, clipped to a surface or surrounded by insulation, how many circuits are grouped together, the ambient temperature and the voltage drop over the run. A 2.5 mm² cable that carries 25 A clipped in open air may be rated for well under 20 A buried in ceiling insulation.
Those derating tables live in AS/NZS 3008.1, BS 7671 Appendix 4 and NEC Article 310, and applying them is the electrician's job.
Power factor catches people out with generators and motors. A 5 kW portable generator is usually rated at around 6.25 kVA with an assumed 0.8 power factor, so a motor load of 5 kW real power at 0.8 already fills it before start-up current is considered.
Motors draw three to seven times their running current for a moment at start, so a generator sized on running watts alone will stall. For heaters, kettles and ovens the power factor is effectively 1, so kW and kVA are the same and the arithmetic is simple.
In the United States, residential breakers are commonly 15 and 20 A for general circuits and 30 to 50 A for ranges, dryers and water heaters, with the 80 per cent continuous rule applied throughout. In Australia and the UK, 10 A and 16 A circuits for lighting and power are common, with 20 A and 32 A for ovens, air conditioners and electric vehicle chargers.
Worked examples
An Australian 15 A continuous load at 230 V, single phase, power factor 0.9
3.11 kW real power, 3.45 kVA apparent, next standard breaker 16 A loaded to 94%
A US 240 V water heater drawing 18.75 A, resistive, continuous
4.50 kW, design current 23.4 A under the 125% rule, so a 25 A breaker
Two three-phase 400 V motors each drawing 22 A at power factor 0.85
25.91 kW total real power, 30.48 kVA total apparent, next standard breaker 25 A per motor circuit
When people use it
- Estimating the load of a new workshop or shed before briefing an electrician
- Checking whether a generator's kVA rating covers the tools you want to run
- Converting an appliance's amp rating into kW to estimate running cost
- Comparing single-phase and three-phase supply options for a large load
- Understanding why an existing circuit trips when a heater and kettle run together
Tips
- Read the rating plate rather than guessing. It gives current or wattage, and sometimes both along with the power factor.
- Size generators on kVA and allow for motor start current, which can be several times the running figure.
- Under the NEC, a continuous load must not exceed 80 per cent of the breaker, which is the same as the 125 per cent rule written the other way.
- Never fit a larger breaker to stop nuisance tripping. The breaker protects the cable, and an oversized one lets the cable overheat.
- Long cable runs lose voltage. Voltage drop limits of around 5 per cent often force a larger cable than current alone would suggest.
- Keep a record of each circuit's load when planning so the electrician can check the main switch and supply capacity too.
What it assumes, and where it stops being accurate
- Three-phase results assume a balanced load with equal current on every phase. Unbalanced loads need each phase calculated separately.
- Voltage is the nominal supply. Real supply voltage varies by several per cent, which changes current for constant power loads.
- No cable derating, voltage drop, fault level or discrimination is checked. Those decide the real design and are outside this tool.
- Motor starting current and harmonic distortion from electronic loads are ignored.
- Standard ratings are the common NEC 240.6(A) and IEC 60898 series. Your supplier's range may differ.
- The result is an estimating aid. Electrical work must be designed and certified by a licensed electrician under the local wiring rules.
Terms used on this page
- Power factor
- The ratio of real power to apparent power, from 0 to 1. Motors and electronics have a power factor below 1.
- Apparent power (kVA)
- Voltage multiplied by current, the load that cables, breakers and generators must actually carry.
- Continuous load
- Under the NEC, a load where the maximum current is expected to continue for three hours or more.
- Design current
- The current a circuit is designed to carry in normal service, written Ib in AS/NZS 3000 and BS 7671.
- Derating
- Reducing a cable's current rating for grouping, insulation, ambient temperature or installation method.
Questions people ask
- Can I choose my own breaker size with this?
- No. The rating shown is the next standard size above the design current, which is a useful starting point. The real choice also depends on the cable, its installation method, grouping, ambient temperature and voltage drop. Only a licensed electrician working to AS/NZS 3000, the NEC or BS 7671 can design and certify the circuit.
- What is the difference between kW and kVA?
- kVA is apparent power, simply volts times amps. kW is real power, the part that does useful work, and equals kVA times the power factor. For heaters they are identical. For motors and electronics the kVA is higher, and cables and generators must carry the full kVA current.
- Why does the NEC add 25 per cent for continuous loads?
- Standard breakers are built to carry their full rating only for shorter periods. For loads running three hours or more, NEC 210.20(A) requires the device to be rated at 125 per cent of the load, which keeps the breaker at or below 80 per cent and prevents heat-related nuisance tripping.
- How do I work out three-phase power?
- Multiply the line-to-line voltage by the line current and by the square root of three, about 1.732, to get volt-amps, then multiply by the power factor for watts. A 400 V three-phase motor drawing 22 A at 0.85 power factor uses about 12.96 kW.
- What power factor should I use?
- Use 1.0 for resistive loads such as heaters, kettles, ovens and incandescent lamps. Use 0.8 to 0.85 for induction motors running near full load, lower for lightly loaded motors. Mixed household loads with electronics are typically around 0.9 to 0.95.
- Does Australia use the 80 per cent rule?
- Not as a blanket rule. AS/NZS 3000 requires the design current to be no more than the device rating, and the device no more than the cable's current-carrying capacity from AS/NZS 3008.1 after derating. Designers still leave headroom on long running loads, so the loading figure here remains useful.
Use this with
Electrical Load Calculator answers one part of the question. These pick up where it stops: Ohm's Law Calculator, Power Bill Calculator, Solar Panel Savings Calculator and HVAC BTU Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- NFPA 70: National Electrical Code. US continuous load rule and standard breaker ratings
- Standards Australia: AS/NZS 3000 Wiring Rules. Australian and New Zealand wiring rules and AS/NZS 3008.1 cable selection
- IET: BS 7671 Requirements for Electrical Installations. UK wiring regulations
Electrical Load Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.