Breaker Size Calculator

A circuit breaker has one job: to open before the conductor it protects overheats. Sizing it is therefore a question of finding the smallest device that will carry the intended load indefinitely without tripping, while still being small enough to protect the wire. The NEC gets there in two steps.

The first step is the 125% continuous-load factor. Section 210.20(A) requires a branch-circuit device to be rated at not less than 125% of the continuous load plus 100% of the non-continuous load, and 215.3 says the same for feeders. The reason is thermal: a breaker is calibrated in open air on a test bench, but installed in a crowded panel it runs warmer and its trip characteristic shifts. The extra 25% is margin against that, not against the load itself. A load is continuous when its maximum current is expected to continue for three hours or more — a definition about duration, not frequency.

The second step is rounding up to a standard rating. Section 240.6(A) lists the ratings that manufacturers actually build: 15, 20, 25, 30, 35, 40, 45, 50, 60, and onwards in widening jumps. A calculation almost never lands on one of them, so the result goes up to the next listed size. Rounding down is not an option, because the device would then be rated below the load it serves.

Read the two steps together and a useful shortcut falls out: since a continuous load is multiplied by 1.25, any device can serve a continuous load of 80% of its rating. A 20 A breaker takes 16 A continuous, a 30 A breaker takes 24 A, and a 100 A breaker takes 80 A. That 80% figure is where the familiar rule of thumb comes from, and it is simply the 125% requirement stated the other way round.

Two circuits with similar-looking loads land on very different devices, which is the clearest way to see what the load-type question is actually doing.

Case 1 — a 24 A continuous lighting load. A retail floor lit by a run of fixtures drawing 24 A, on from opening to close. That is well past three hours, so the load is continuous and the factor applies: 24 × 1.25 =30 A of required capacity. Looking down the 240.6(A) list, 30 A is itself a standard rating, so the answer is a 30 A breaker with no rounding at all. Worth noticing that this leaves zero headroom — the required capacity sits exactly on the device rating — so if a single extra fixture is added later, the circuit no longer complies.

Case 2 — a 38 A non-continuous load. A commercial cooktop drawing 38 A, used in bursts through a service rather than at full output for three hours. No factor applies, so the required capacity is the load itself: 38 A. That is not a standard rating, so it rounds up to the next one on the list, a40 A breaker, leaving 2 A of headroom.

Now compare them. The second load is more than 50% larger than the first, yet the device only goes up one step, from 30 A to 40 A. And if that same 38 A load were continuous, the arithmetic would be 38 × 1.25 = 47.5 A, which rounds up to a50 A breaker — two steps larger, from the same nameplate current. The load type is doing as much work here as the load itself, and it is the input most often got wrong.

The table lists every standard rating this calculator works in, alongside the largest load each device may actually serve. The two load columns are the 125% rule applied in reverse: the non-continuous column is simply the rating, and the continuous column is 80% of it. Read across from a device you already have in the panel to see what it can legitimately carry.

Use the table when you are working backwards — checking whether an existing breaker suits a load you are about to add, or picking a device from stock. Use the calculator when you are working forwards from a known load, particularly in watts mode, where the conversion and the factor compound.

Two limits on the table. It stops at 600 A because beyond that a single branch device stops being the right question and the job becomes a service or feeder study under Article 220. And none of it applies to motors: Article 430 sizes motor protection from its own full-load current tables at up to 250%, and this tool will undersize a motor circuit if you use it there.