What does NEC 210.19 require?

NEC 210.19(A)(1) sets the minimum size for branch-circuit conductors. The conductor must have an ampacity not less than the noncontinuous load plus 125% of the continuous load, determined before applying any temperature or conductor-bundling adjustment. In plain terms: take the continuous load, multiply it by 1.25, add any noncontinuous load, and that total is the minimum ampacity the conductor must provide.

What is the 125% continuous-load rule?

A continuous load is one that operates for 3 hours or more (NEC Article 100). For continuous loads, both the branch-circuit conductor (210.19) and the overcurrent device (210.20) must be sized at a minimum of 125% of the load. The extra 25% accounts for the heat a conductor and breaker build up under sustained current. Forgetting the 125% is the single most common branch-circuit calculation mistake on electrician exams.

How do you size a branch-circuit conductor for a continuous load?

Three steps. First, confirm the load is continuous (operates 3 hours or more). Second, multiply the continuous load by 125%. Third, pick a conductor from Table 310.16 with an ampacity at least that large, using the termination temperature column allowed by 110.14(C). Example: a 24-amp continuous load needs 24 x 1.25 = 30 amps of ampacity, which a 10 AWG copper conductor provides.

Does the breaker also get sized at 125%?

Yes. NEC 210.20(A) requires the overcurrent device to be rated not less than the noncontinuous load plus 125% of the continuous load, the same as the conductor. A 24-amp continuous load therefore needs both a conductor good for 30 amps and a 30-amp breaker. Sizing the conductor but forgetting the breaker, or the reverse, is a frequent exam trap.

What is the difference between a continuous and a noncontinuous load?

A continuous load runs at its maximum current for 3 hours or more, such as store and sign lighting or EV charging. A noncontinuous load runs in shorter bursts, such as a receptacle for hand tools or a residential range. On a circuit with both, you size for the noncontinuous load at 100% plus the continuous load at 125%, then confirm the conductor still has enough ampacity after any temperature or bundling derating.

Is there an exception to the 125% rule?

Yes. The exceptions to 210.19(A)(1) and 210.20(A) allow 100% sizing where the branch-circuit overcurrent device and its assembly are listed for operation at 100% of their rating. These 100%-rated assemblies are uncommon in the field and rare on journeyman exams, which almost always test the standard 125% rule. Know the exception exists, but expect to apply the 125% version.

Article 210 · NEC 210.19 deep-dive

NEC 210.19: Branch-Circuit Conductor Sizing and the 125% Continuous-Load Rule

Branch-circuit calculations are the most-tested calculation type on the journeyman exam, and the one candidates fail most. Almost every miss traces back to one rule: continuous loads get sized at 125%. Here is the rule, why it exists, and worked examples for the exact spots people lose points.

Last reviewed June 2026

What counts as a continuous load

NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. That is the whole test. It is not about how big the load is. It is about how long it runs. Store lighting, outdoor sign circuits, and EV charging are continuous. A receptacle for hand tools or a residential cooktop is not.

The first thing the exam wants to see is whether you can classify the load before you touch the calculator. If you cannot tell continuous from noncontinuous in the first ten seconds, you do not start the math.

Why 125%

A conductor carrying current for hours runs hotter than one that sees short bursts, and the overcurrent device sitting in the panel heats up the same way. The 25% adder is a thermal margin: it keeps the conductor insulation and the breaker below their rated operating temperature under sustained load. That is why both the wire (210.19) and the breaker (210.20) get the factor, not just one of them.

Worked example 1: a pure continuous load

A 277-volt lighting circuit feeds a 24-amp continuous load. What is the minimum conductor ampacity, and what size breaker?

Classify it: continuous load, conductor sizing. Not voltage drop, not box fill.
Apply 210.19(A)(1): 24 A x 125% = 30 A minimum conductor ampacity.
Select from Table 310.16: a 10 AWG copper conductor (35 A at 75 C) clears 30 A. A 12 AWG (25 A) does not.
Apply 210.20(A) to the breaker: 24 A x 125% = 30 A, so a 30-amp overcurrent device.

The candidate who fails this sees 24 amps, grabs a 12 AWG because 25 is more than 24, and moves on. The 125% rule just cost the question and the candidate never felt it.

Worked example 2: continuous plus noncontinuous

A branch circuit feeds a 16-amp continuous load and an 8-amp noncontinuous load on the same conductor. Minimum ampacity?

Noncontinuous at 100%: 8 A.
Continuous at 125%: 16 A x 1.25 = 20 A.
Add them: 8 + 20 = 28 A minimum conductor ampacity.
Table 310.16: a 10 AWG copper conductor (35 A) clears 28 A.

Only the continuous portion gets the 1.25 multiplier. A common trap is to multiply the whole circuit load by 125%, which oversizes the conductor and produces a wrong answer choice that looks reasonable.

Do not forget the termination temperature

Section 110.14(C) limits which temperature column of Table 310.16 you may use, based on the lowest-rated termination in the circuit. For most equipment rated 100 amps or less, terminations are listed for 60 C unless marked otherwise. If the exam tells you the terminations are 60 C, size from the 60 C column, not the 75 C column. This single detail flips a surprising number of answers.

Where candidates lose the point

Skipping the 125% entirely and sizing the conductor to the bare load.
Applying 125% to the conductor but forgetting the breaker (210.20), or the reverse.
Multiplying the whole load by 125% when only the continuous portion qualifies.
Using the 75 C column when the terminations are only rated 60 C (110.14(C)).
Confusing the 125% sizing step with the separate derating step for temperature and bundling.
Calling a load continuous because it is large. The test is 3 hours or more, not size.

How 210.19 shows up on the exam

Question type 1: classify, then size

A scenario gives a load and tells you it runs 3 hours or more, then asks for the minimum conductor. The work is load x 1.25, then read Table 310.16. The trap is candidates who never confirm it is continuous and skip the multiplier.

Question type 2: mixed load on one circuit

A circuit carries both a continuous and a noncontinuous load. You size the noncontinuous at 100% and the continuous at 125%, then add. The wrong choices apply the factor to the wrong portion.

Question type 3: conductor and breaker together

The question asks for both the wire size and the overcurrent device. Both come from the same 125% number (210.19 and 210.20). Candidates who calculate one and assume the other often pick a mismatched pair.

The 100% exception, briefly

The exceptions to 210.19(A)(1) and 210.20(A) permit 100% sizing where the overcurrent device and its assembly are listed for continuous operation at 100% of their rating. These assemblies exist but are uncommon, and journeyman exams almost always test the standard 125% path. Recognize the exception if it appears, but default to 125%.

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