Circuit load and breaker sizing
Load current, breaker size with the 125% continuous rule and a matching copper conductor.
Get this tool with its role plan
This is a premium tool. It comes with its role's plan: sign in, then add the role in Billing.
Comes with the electrical engineer plan: every tool for the role, from $10.20 a month at the launch price.
- Your own numbers, worked out as often as you like
- The full circuit sheet you can print or save to My work
- 3 cited sources, linked, with the checks to make
- Every other tool for the same role, in the same plan
- A plain-words read of your result (a fuller AI read with the Bifrost AI add-on)
Included in these role plans
Example result · fictional numbers
What you get: circuit sheet
Worked out by this tool from its default inputs. Its role plan lets you run your own numbers, print the sheet and save it.
40 A 2-pole breaker, 8 AWG copper, 10 AWG ground, 1/2 in EMT, 1.5% drop
- Load current
- 30.0 A
- Required breaker rating
- 37.5 A
- Standard breaker
- 40 A
- Copper conductor (75 °C)
- 8 AWG THHN
- Breaker loading
- 75% of rating
Branch circuit
- Breaker
- 40 A 2-pole (next standard size at or above 37.5 A, 125% of the continuous load)
- Conductors
- 8 AWG copper THHN/THWN-2
- Ampacity after adjustment
- 55 A at 90 °C × 100% (2 current-carrying conductors) × 1.00 (ambient) = 55.0 A; carries 30.0 A and is protected by the 40 A breaker
- Terminal check
- 50 A at 75 °C, at least the 40 A breaker (terminals are rated 75 °C)
- Equipment grounding conductor
- 10 AWG copper for a 40 A breaker (Table 250.122); size it up in proportion if you upsize the hots for voltage drop
- Conductors in the run
- 2 hots, 1 ground (3 wires)
Also on the sheet
- Conduit and voltage drop · 5 lines, starting with “EMT for this circuit”
- Panel schedule line · 4 lines, starting with “Circuit”
The plain-words read of this example
A 7,200 W load at 240 V draws 30.0 A, so it takes a 40 A 2-pole breaker after the 125% continuous-load rule. 8 AWG copper is the smallest wire that fits: 55.0 A after adjusting for 2 conductors and the ambient. It needs a 10 AWG copper ground and fits 1/2 in EMT. Over 75 ft the drop is 1.5%, inside the 3% the NEC recommends. Check the nameplate first: motors, HVAC and welders follow their own articles, and the inspector has the last word.
3 cited sources
- NFPA 70, National Electrical Code (NEC) (National Fire Protection Association)
- NEC Chapter 9 (Tables 1, 4, 5 and 8) and Informative Annex C (NFPA free access (read-only, free account))
- OSHA 29 CFR 1926.404, Wiring design and protection (construction sites) (U.S. Occupational Safety and Health Administration)
Plus 6 step-by-step checks, a print layout and a link that reopens it with your numbers.