What the 125% continuous-load rule is protecting
A circuit breaker is a thermal device. Its calibration assumes it will spend most of its life below its rating, shedding heat between peaks. Run a molded-case breaker at its full nameplate current for hours and the trip unit, the busbar stab and the lug all sit hotter than the standard test conditions assume — which is why the code takes a load that persists for three hours or more and makes you buy 25% more device and 25% more conductor for it.
The definition matters more than the arithmetic. Article 100 defines a continuous load as one where the maximum current is expected to continue for three hours or more. Office and warehouse lighting is continuous. A parking-lot pole circuit is continuous. A sign circuit is continuous by rule under Article 600. A dwelling receptacle circuit is not. A welder is not. A refrigeration compressor cycles and is not, though the code covers it under Article 440 instead. Whether the load meets that three-hour test is a design judgement, and it is the judgement most likely to be argued at plan review.
Two code sections say the same thing about the same circuit. 210.20(A) sets the branch-circuit overcurrent device at not less than 125% of the continuous load plus 100% of the noncontinuous load; 210.19(A)(1) says the branch-circuit conductor must have an ampacity of at least the same figure. For feeders, 215.3 and 215.2(A)(1) repeat both requirements word for word. You size the wire and the breaker to the same number, then round the breaker up.
From watts on a cut sheet to a breaker on a panel schedule
Step one is current. A single-phase load draws P ÷ (V × PF) amperes; a three-phase load draws P ÷ (√3 × VLL × PF), where VLL is the line-to-line voltage. For resistance heat and most modern LED drivers the power factor is close enough to unity to enter 1.00. For motors, use the nameplate amperes rather than any wattage figure — and for motors, come back through the motor full-load amps calculator, because Article 430 sizes motor branch circuits on table values, not nameplate.
Step two is the 1.25 multiplier, applied only to the continuous part. If 40 A of a 50 A circuit runs all day and 10 A is intermittent, the design current is 1.25 × 40 + 10 = 60 A, not 62.5 A. Mixing the two is the commonest arithmetic error on this calculation.
Step three is rounding to a standard rating. 240.6(A) fixes the standard ampere ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200 and upward. A design current of 54.2 A takes a 60 A device. You may go larger than the calculated minimum, but only up to what the conductor and the equipment allow.
Step four is the conductor, and it has three separate gates. First, the ampacity from Table 310.16 in the insulation's own temperature column, multiplied by the ambient correction factor from Table 310.15(B)(1)(1) and the bundling adjustment factor from 310.15(C)(1), must reach the design current. Second, 110.14(C) caps the usable ampacity at the value in the column matching the lowest-rated termination — normally 60 °C for equipment rated 100 A or less and 75 °C above that, unless the terminations are listed otherwise. Third, 240.4(D) refuses to let you protect 14, 12 and 10 AWG copper above 15, 20 and 30 A respectively no matter what the table says. The ampacity derating calculator works the correction and adjustment factors on their own if that is all you need.
The 90 °C column is a derating tool, not a rating. You may start from the 90 °C ampacity of THHN or XHHW-2 when applying correction and adjustment factors, but the finished number can never exceed the 75 °C (or 60 °C) value that the terminations permit. That is why an 8 AWG THHN copper conductor is a 50 A conductor at a 75 °C lug even though the 90 °C column shows 55 A.
Worked example: 12 kW of 277 V continuous lighting
A warehouse lighting circuit carries 12,000 W of high-bay fixtures at 277 V, single phase, at unity power factor. The lights are on for a full shift, so the load is 100% continuous. Copper conductors, THHN insulation, 75 °C terminations, 30 °C ambient, three current-carrying conductors in the raceway.
- Load current. 12,000 W ÷ (277 V × 1.00) = 43.32 A.
- Design current. All of it is continuous: 1.25 × 43.32 = 54.15 A.
- Device rating. The first standard rating in 240.6(A) at or above 54.15 A is 60 A.
- Conductor, first pass. 8 AWG copper is 55 A in the 90 °C column, and with no correction or adjustment that clears 54.15 A. But 110.14(C) sends you to the 75 °C column, where 8 AWG is only 50 A — below the design current. It fails.
- Conductor, second pass. 6 AWG copper is 75 A at 90 °C and 65 A at 75 °C. Both clear 54.15 A, and 240.4(D) does not restrict 6 AWG. The answer is 6 AWG copper.
- Check the device against the conductor. The 60 A breaker is below the conductor's 65 A usable ampacity, so no appeal to 240.4(B) is needed.
- Read the loading. 43.32 ÷ 60 = 72.2% of the device rating, and the 60 A device may carry up to 0.80 × 60 = 48 A continuously. That leaves 48 − 43.32 = 4.7 A of continuous headroom, which at 277 V is about 1.3 kW of additional fixtures.
Note what happened at step 4. The conductor was not chosen by the breaker size and it was not chosen by the 90 °C ampacity. It was chosen by the termination temperature, which is the gate that catches most people. Before you commit to the raceway, take the 6 AWG through the conduit fill calculator and the run length through the voltage drop calculator — on a 250 ft high-bay run, voltage drop often forces 4 AWG anyway.
Standard device ratings, continuous capacity and minimum copper conductor
| Device rating (A) | Max continuous load (A) | Max noncontinuous load (A) | Copper conductor, 75 °C |
|---|---|---|---|
| 15 | 12 | 15 | 14 AWG |
| 20 | 16 | 20 | 12 AWG |
| 25 | 20 | 25 | 10 AWG |
| 30 | 24 | 30 | 10 AWG |
| 35 | 28 | 35 | 8 AWG |
| 40 | 32 | 40 | 8 AWG |
| 45 | 36 | 45 | 8 AWG |
| 50 | 40 | 50 | 8 AWG |
| 60 | 48 | 60 | 6 AWG |
| 70 | 56 | 70 | 4 AWG |
| 80 | 64 | 80 | 4 AWG |
| 90 | 72 | 90 | 3 AWG |
| 100 | 80 | 100 | 3 AWG |
| 110 | 88 | 110 | 2 AWG |
| 125 | 100 | 125 | 1 AWG |
| 150 | 120 | 150 | 1/0 AWG |
| 175 | 140 | 175 | 2/0 AWG |
| 200 | 160 | 200 | 3/0 AWG |
| 225 | 180 | 225 | 4/0 AWG |
| 250 | 200 | 250 | 250 kcmil |
| 300 | 240 | 300 | 350 kcmil |
240.4(B) separately permits the next standard device size above a conductor's ampacity for ratings of 800 A or less where the circuit does not supply receptacle outlets, so a smaller conductor is sometimes legal than this column shows.
How to read the result
The device rating is a minimum, not a target. Nothing stops you installing a larger breaker as long as the conductor supports it, and nothing forces you to fill it. What you cannot do is put a continuous load above 80% of the device rating on it later, which is why the largest continuous load this device may carry figure is worth writing on the panel schedule.
Loading above 80% is not automatically a violation. It only means part of the load is noncontinuous. A 100 A device carrying 100 A of purely intermittent load complies; the same device carrying 85 A continuously does not. Read the percent-loaded figure together with the continuous fraction you entered, never on its own.
If the conductor size surprises you, check the termination temperature first. Nine times out of ten a size that looks a step large is the 110.14(C) limit doing its job. The second suspect is 240.4(D): a 25 A device cannot protect 12 AWG copper even though the 75 °C column shows 25 A for it.
A design current that lands just above a standard rating is worth a second look. At 51 A of design current you buy a 60 A device and 6 AWG conductor; at 49 A you buy 50 A and 8 AWG. Shaving one fixture off a lighting run, or splitting a load across two circuits, is often cheaper than the larger wire — particularly on long runs where voltage drop would push the size up again.
This calculator sizes one circuit. The feeder or service that supplies several of them is an Article 220 exercise: see the dwelling load calculation calculator for residential services, and remember that 220.40 requires a feeder to be no smaller than the sum of the branch-circuit loads it serves after any demand factors.
Where this calculation goes wrong
- Applying 125% to the whole load when only part of it is continuous. The multiplier attaches to the continuous portion alone; the rest is counted once.
- Sizing the conductor from the 90 °C column and stopping there. The 90 °C ampacity is a starting point for correction and adjustment factors. 110.14(C) still holds you to the termination column.
- Forgetting 240.4(D). 14, 12 and 10 AWG copper are capped at 15, 20 and 30 A of overcurrent protection regardless of what the ampacity table shows, with a short list of exceptions in 240.4(E) and (G).
- Using this method on a motor circuit. Article 430 sizes motor conductors at 125% of the Table 430.248 or 430.250 full-load current and the short-circuit device at a much higher multiple. Use the motor overload and breaker sizing calculator instead.
- Ignoring the neutral in a bundling count. A neutral carrying only the unbalanced current of a balanced three-wire circuit is not counted, but the neutral of a circuit supplying nonlinear loads is a current-carrying conductor under 310.15(E)(3).
- Assuming 100% rated equipment. Some assemblies are listed for continuous operation at 100% of rating, which removes the 1.25 multiplier under the exceptions to 210.20(A) and 215.3. The listing applies to the assembly, not the breaker alone, and it must be marked on the equipment — you cannot infer it.
- Sizing the equipment grounding conductor from the load. It comes from Table 250.122 based on the device rating — see the equipment grounding conductor calculator.
Code edition and scope
The rules and tables here follow NFPA 70, the National Electrical Code, 2023 edition: 210.19(A)(1) and 210.20(A) for branch circuits, 215.2(A)(1) and 215.3 for feeders, 240.4 and 240.6(A) for overcurrent protection, 110.14(C) for terminations, and Table 310.16 with 310.15(B)(1)(1) and 310.15(C)(1) for ampacity. The 125% figure and the ampacity tables are unchanged from the 2017 and 2020 editions; some section numbers are not. This calculator covers conductors in raceway or cable, not in free air, not direct-buried, and not in parallel — and it does not cover motors, air-conditioning equipment, welders, or circuits over 1,000 V, each of which has its own article.
Where the 80% rule came from and where it does not apply
"80% rule" and "125% rule" are the same rule seen from opposite ends. Multiplying a load by 1.25 is identical to limiting the load to 1 ÷ 1.25 = 0.80 of the device rating. Electricians in the field usually say 80%; the code text says 125%, because the code is written from the load toward the equipment.
Standard breakers are tested at 100% in open air, not in a panel. UL 489 calibration is performed on a specified conductor length in specified conditions. Inside an enclosure, packed with other poles, the same device runs hotter. The 125% margin is what reconciles the laboratory calibration with the installed condition, and it is also why a breaker listed for 100% continuous duty carries a specific enclosure and conductor requirement in its listing.
Several loads carry their own multipliers instead. Electric vehicle supply equipment is continuous by definition in 625.41 and always takes the 125% treatment. Fixed electric space-heating equipment is treated as a continuous load under Article 424. Motors get 125% of the table full-load current under 430.22 and a short-circuit device sized by percentage from Table 430.52. Air conditioning follows Article 440 and the nameplate minimum circuit ampacity, which is the manufacturer's own version of this calculation — if the nameplate gives an MCA and a maximum fuse or breaker size, use those and do not recalculate.
For three-phase work, confirm the current first. Feeding a wattage figure and the wrong phase selection into this calculator changes the answer by a factor of 1.73. The three-phase power calculator is the quickest way to sanity-check the current before you size anything around it.
Key terms
- Continuous load
- A load whose maximum current is expected to continue for three hours or more, as defined in Article 100.
- Ampacity
- The current a conductor can carry continuously under the conditions of use without exceeding its temperature rating. It is a property of the installation, not just the wire.
- Design current
- 125% of the continuous load plus 100% of the noncontinuous load — the single number both the device and the conductor must meet.
- Correction factor
- The multiplier from Table 310.15(B)(1)(1) that reduces ampacity when the ambient temperature is above 30 °C, or raises it below.
- Adjustment factor
- The multiplier from 310.15(C)(1) that reduces ampacity when more than three current-carrying conductors share a raceway or cable.