A continuous load calculation sizes the wires and breakers on any circuit expected to carry its maximum current for three hours or more, and under the National Electrical Code it works by multiplying the continuous portion of the load by 125 percent, adding the non-continuous portion at face value, and applying that total to both the conductor and the overcurrent device. Both pieces of hardware have to meet the number independently. Getting the split wrong, or skipping the multiplier, is what leaves undersized wiring hidden behind drywall.
What Qualifies as a Continuous Load
NEC Article 100 defines a continuous load as one where the maximum current is expected to flow for three hours or more without interruption.1Cummins. Understanding 100% Rated Breaker Assemblies and Their Application Three hours is the threshold where a conductor stops cooling between cycles and reaches a steady-state temperature the whole circuit has to tolerate. A refrigerator compressor or garbage disposal cycles often enough that the wire sheds heat; a storefront sign or parking lot fixture does not.
Two loads catch people off guard. EV charging circuits are classified as continuous no matter how long any individual session actually lasts, so a 40-amp Level 2 charger is sized as if it runs nonstop.2Leviton. Electric Vehicle Branch Circuit Solar PV source and output circuits are also treated as continuous, with NEC 690.8(A)(1) requiring the maximum circuit current to be calculated at 125 percent of the modules’ rated short-circuit current because peak irradiance easily lasts past three hours on a clear day.
Mixed circuits need both categories tracked separately. An outdoor lighting circuit that also feeds a couple of receptacles gets the 125 percent treatment on the lighting portion while the receptacle portion is counted at face value. The classification work is what feeds the formula, so it has to be right before anything else happens.
The 125 Percent Rule
NEC 210.19(A)(1) requires branch-circuit conductors to have an allowable ampacity no less than the non-continuous load plus 125 percent of the continuous load.3Schneider Electric. NEC 210.19 Conductors – Minimum Ampacity and Size NEC 210.20(A) applies the identical formula to the breaker or fuse: it must be rated for at least the non-continuous load plus 125 percent of the continuous load.4UpCodes. Continuous and Noncontinuous Loads NEC 215.2(A)(1) repeats the same math for feeders. Conductor and breaker are sized separately, using the same number, and both must land at or above it.
A worked example. Say a small commercial circuit carries 16 amps of lighting (continuous) and 10 amps of receptacle loads (non-continuous):
- Continuous portion: 16 × 1.25 = 20 amps
- Non-continuous portion: 10 × 1.00 = 10 amps
- Minimum circuit capacity: 30 amps
Both the conductor and the breaker have to be rated for at least 30 amps. In standard conditions, that means 10 AWG copper on a 30-amp breaker. Pair the same load with a 20-amp breaker on 12 AWG wire and the lighting alone pushes the breaker past 80 percent of its rating during normal operation, which is exactly the condition the 125 percent rule is written to prevent.
When the Multiplier Doesn’t Apply
The NEC carves out an exception for equipment specifically listed and labeled for operation at 100 percent of its rating. When the whole assembly including the overcurrent device carries that listing, you size the conductor and breaker for the straight sum of continuous plus non-continuous loads, no 25 percent bump.3Schneider Electric. NEC 210.19 Conductors – Minimum Ampacity and Size Standard breakers are tested at only 80 percent of their rating for continuous duty, and 125 percent is the inverse of 80 percent, which is where the multiplier comes from.5ABB. MCCB Applications – When to Use a 100% Rated Circuit Breaker A 100-percent-rated breaker has to carry the marking “Suitable for continuous application at 100% of rating” per UL on frames 250 amps and larger.6Schneider Electric. What Are the UL Requirements for Marking a Circuit Breaker 100% Rated? Without that marking, the standard 125 percent calculation applies, and the exception rarely shows up outside large industrial installations.
Reading the Load Off the Nameplate
Every calculation starts at the equipment nameplate, because manufacturers are required to label equipment with the amperage or wattage rating.7ANSI Webstore. UL 9691-2021 – Recommended Practice for Nameplates for Use in Electrical Installations Look for FLA (full load amps) or input watts; divide watts by circuit voltage if the nameplate gives you watts. Catalog descriptions and estimates don’t go on the worksheet.
Sort every device sharing the circuit into one of two columns, continuous or non-continuous, and document both. A break-room coffeemaker is non-continuous, an always-on vending machine is continuous, and if they share a receptacle circuit the worksheet has to show both. Specification sheets help when the duty cycle isn’t obvious. Getting the split right is what separates a calculation that passes inspection from one that gets red-tagged.
Picking the Wire and the Breaker
Once the minimum ampacity is calculated, three references work together to pick actual hardware.
NEC Table 310.16
This table, numbered 310.15(B)(16) in NEC editions before 2020, lists allowable ampacity by wire gauge and insulation temperature rating, assuming no more than three current-carrying conductors in a raceway at 30°C (86°F) ambient.8Schneider Electric. Conductor Ampacity Based on the 2017 National Electrical Code For common copper, the 60°C column gives 15 amps at 14 AWG, 20 amps at 12 AWG, and 30 amps at 10 AWG. The 75°C column reads higher, but you can only use it if the equipment terminations are rated for 75°C.
The Small-Conductor Rule
NEC 240.4(D) caps overcurrent protection on 14, 12, and 10 AWG wire at 15, 20, and 30 amps respectively, regardless of what the ampacity table says in a higher-temperature column.9HELUKABEL. Allowable Ampacity Tables NFPA 70 NEC – 2023 A 12 AWG wire with 90°C-rated insulation may show 30 amps in the table and still be limited to a 20-amp breaker.
Terminal Temperature: NEC 110.14(C)
The weakest thermal link in the circuit controls the usable ampacity. For equipment rated 100 amps or less, NEC 110.14(C)(1)(a) generally requires the 60°C column unless the equipment is specifically listed for higher-temperature conductors.10Schneider Electric. Wire Temp Ratings and Terminations Above 100 amps, the 75°C column applies. The equipment labeling controls, not the lug rating; manufacturers often install 90°C-rated lugs on equipment rated for only 60°C or 75°C, and reading the lug instead of the equipment label is a common error.
Derating for Heat and Bundling
Table 310.16 assumes 30°C ambient and no more than three current-carrying conductors in a raceway. When the installation differs, the conductor’s allowable ampacity has to be derated before it is compared to the calculated load.
NEC Table 310.15(B)(2)(a) gives ambient-temperature correction factors. For 75°C conductors, the factor drops to 0.88 at 36–40°C, 0.75 at 46–50°C, and continues down from there.11Schneider Electric. Correction and Adjustment Factors A southern-US attic can reach 50°C in summer, which strips a quarter of the rated ampacity off any wire run through it.
Bundling adjustments stack on top. More than three current-carrying conductors in the same raceway or cable reduces each conductor’s ability to shed heat. Four to six drops ampacity to 80 percent of the table value; seven to nine drops it to 70 percent. Apply both the ambient correction and the bundling adjustment, and the final number still has to meet or exceed the calculated load.
This is where higher-temperature insulation earns its place. You can start derating from a 90°C conductor’s ampacity even when the terminal temperature limits you to the 60°C or 75°C column for the final answer. The derated number just has to land at or below the terminal-limited ampacity.10Schneider Electric. Wire Temp Ratings and Terminations
Voltage Drop
The NEC does not require a maximum voltage drop. Informational notes in 210.19(A)(1) and 215.2(A)(1) recommend 3 percent on a branch circuit and 5 percent for the combined feeder-and-branch run, and some local jurisdictions have adopted the recommendation as mandatory. Continuous loads are especially sensitive because the loss is constant rather than intermittent. On a long run where the 125 percent calculation already puts you at the minimum wire size, bumping up a gauge is often the practical answer.
Running the Sequence
The order of operations matters. Classify every load as continuous or non-continuous. Multiply the continuous total by 1.25 and add the non-continuous total at face value. Use that number to pick the breaker. Verify the conductor against Table 310.16, but confirm the terminal temperature rating first so you know which column applies. Apply ambient-temperature correction and bundling adjustments if the install calls for them. Check voltage drop on longer runs.
Document the math on a load calculation worksheet. Inspectors expect to see it in the permit application, and the same sequence applies at every level of distribution, so a building with multiple panels needs the calculation repeated at each feeder and at the service.3Schneider Electric. NEC 210.19 Conductors – Minimum Ampacity and Size Skip a step and the whole chain breaks, which is how a conductor ends up overloaded while the breaker reads fine.