Circuit Breaker Sizing Helper
Recommend a circuit breaker's ampere rating from a load's current (or watts + voltage). Applies the standard continuous-load rule — breaker ≥ 1.25× continuous current + 1.0× non-continuous current — and rounds up to the nearest standard breaker size. Runs entirely in your browser.
Input
A load running 3 hours or more at a stretch — lighting, HVAC, most fixed equipment.
Short-duration load on the same circuit, if any — leave at 0 if none.
100%-rated breakers are specifically UL-listed for it on a matching panel — comparatively rare.
Output
| Item | Value |
|---|---|
| No data yet | |
Guides
Turn a load's current — or its watts and voltage — into a recommended circuit breaker rating, applying the standard continuous-load rule instead of just rounding up to the nearest breaker size.
Most quick calculators skip the part that actually matters: a load running 3 hours or more (lighting, HVAC, most fixed equipment) can't use the breaker's full rating. Standard breakers are limited to 80% of their rating on a continuous load, which is the same thing as saying the breaker has to be sized at 1.25× the continuous current, plus the full non-continuous current on top.
How to use it
- Choose how you know the load — directly in amps, or in watts with a voltage (the tool converts watts ÷ voltage to amps, assuming unity power factor).
- Split continuous vs. non-continuous if the circuit carries both. If you're not sure, treat it as continuous — it's the conservative choice.
- Pick the breaker type. Almost every residential and most commercial breakers are "standard" — the 80%/1.25× rule applies. Only select 100%-rated if you specifically have a breaker AND panel combination UL-listed for 100% continuous duty (uncommon, mostly larger commercial/industrial gear).
- Read the required minimum capacity and the recommended breaker, rounded up to the nearest standard size (15, 20, 25, 30… 600A).
The formula
Required capacity = 1.25 × continuous current + 1.0 × non-continuous current
For a 100%-rated breaker, the multiplier on the continuous term drops to 1.0 — the whole point of that (rarer, more expensive) listing is that it's allowed to run at its full rating continuously.
Why 1.25× and not just "round up to cover the load"?
Because a breaker sitting at 100% of its rating for hours trips on thermal buildup even without a fault — that's it doing its job, not a defect. The 80% continuous-load rule (NEC 210.19(A)(1) / 210.20(A) in the US, and the same underlying idea in most other electrical codes) exists specifically so a breaker carrying a steady load all day isn't sitting at the edge of nuisance-tripping.
Does this size the wire too?
No — deliberately not. Conductor sizing depends on the wire's insulation temperature rating, how many other conductors it's bundled with, ambient temperature and the terminal ratings at each end (NEC Table 310.16 and 240.4(D)), which needs its own reference table to get right rather than a bolted-on guess here. Once you have a breaker size from this tool, size the conductor separately against that table — the wire generally needs to be rated for at least the breaker's ampacity.
Is this a substitute for a licensed electrician?
No. This estimates the breaker's ampere rating from a load current using the standard textbook rule. It doesn't know your panel's fill, your local code amendments, or whether a load classifies as continuous in your specific installation. Get a licensed electrician to review before wiring or modifying a real circuit.
Related tools
Sizing the generator that would feed this panel? The Generator Sizing Calculator turns an appliance list into a recommended generator size and its own amp-draw figures. For the underlying volts/amps/watts relationship, see the Ohm's Law Calculator.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/circuit-breaker-sizing-helper \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"inputMode": "amps",
"continuousAmps": "16",
"nonContinuousAmps": "4",
"breakerType": "standard"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `circuit-breaker-sizing-helper` tool (Circuit Breaker Sizing Helper) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.