Circuit Load & Breaker Size
List the loads on a circuit or distribution board with their quantities and demand factors, and get the connected and demand load, the design current, the current on each line with the phase imbalance, the next standard breaker or fuse rating, and a starting cable size. Single- and three-phase, fully offline.
Open Circuit Load & Breaker Size →What is the circuit load & breaker size calculator?
A free load-schedule calculator that runs entirely in your browser. Build a list of the loads on a circuit or distribution board — a name, a quantity, a wattage and a demand factor for each — and it adds up the connected load, applies each demand factor to give the diversified demand load, and converts that to a design current using your supply voltage and power factor. On a three-phase board you can tie each load to L1, L2 or L3, or leave it spread across all three, and the calculator shows the current on every line and the phase imbalance so you can even the board out. It then names the next standard breaker or fuse rating above your design current plus a margin, and suggests a starting cable size. Presets for common loads get you going quickly. Everything stays on your device.
How to use Circuit Load & Breaker Size
- Set the supply — Choose single- or three-phase, then enter the supply voltage — line-to-line for three-phase — the power factor and the device margin.
- Add your loads — Use Add load for a blank row, or pick a preset such as lighting, sockets or an EV charger to start from a typical figure. Edit the name, quantity, watts and demand factor freely.
- Balance the phases — On a three-phase board, set each row's line to L1, L2 or L3 for single-phase loads, or leave it on All lines for a balanced three-phase load. Watch the imbalance figure as you move loads about.
- Read the sizing — The design current, next standard device rating, per-line currents and a starting cable size update as you type.
Frequently asked questions
What is a demand factor?
It is the fraction of a load you expect to be running at the same time as everything else. Socket circuits are rarely all loaded at once, so a diversity allowance well below 100% is normal; a water heater or an EV charger that runs at full power is usually taken at 100%.
Why is the device margin 25% by default?
A protective device must be rated for at least 125% of a continuous load — the NEC rule in 210.20(A) and 215.3. If your load is not continuous, set the margin to 0 and the calculator sizes straight from the design current.
What does the phase imbalance number mean?
It is the difference between the most and least loaded lines as a percentage of the most loaded one. Zero is perfectly balanced. A large imbalance overloads one line, wastes capacity and can cause neutral current problems, so shuffle single-phase loads between lines to bring it down.
Is the suggested cable size final?
No. It is the smallest cable that carries the current in the reference condition only — 30 °C ambient and no more than three current-carrying conductors. Use the Wire Size & Ampacity tool to apply the real ambient and grouping derating for your installation.
Tips
- Enter each load in watts and let the power factor field convert the whole board to current — that is more accurate than mixing kW and kVA rows.
- Aim to keep three-phase imbalance in single figures; moving one medium single-phase load between lines usually does it.
- Keep the schedule as a record — it is saved on your device, so you can reopen the board later and add the next circuit.