NEC 210.23: The 80% Rule, the 50% Rule, and What One Circuit May Feed
Everybody on a job site quotes the 80% rule. Half of them are quoting it about the wrong thing, and the exam is built to catch exactly that. 210.23 holds two limits: 80% for a single portable appliance, 50% for fastened-in-place equipment sharing a circuit with lighting. Our data says each catches about half of candidates.
Last reviewed September 2026
The 80% limit, precisely
On a multi-outlet 15- or 20-amp branch circuit, the rating of any ONE cord-and-plug-connected appliance may not exceed 80% of the circuit rating. Twenty amps times 0.80 is 16 amps. The rule exists because a shared circuit is presumed to have other things plugged into it. One appliance eating the entire rating leaves no room for the coffee maker beside it, so the code caps the single biggest portable load and leaves headroom.
The 50% limit nobody memorizes
The quieter rule is 210.23(A)(2). Where the circuit also supplies lighting or portable equipment, anything fastened in place, other than light fixtures, is capped at 50% of the circuit rating. A 20-amp office circuit that carries the lights cannot also carry a 12-amp fastened heater: the heater's ceiling on that circuit is 10 amps. In our diagnostic data this rule scores worst of the two, because most candidates have never heard the 50% number at all.
Worked examples
- Break room, 20-amp circuit with several receptacles. Largest legal cord-and-plug appliance: 20 x 0.80 = 16 A.
- Office, 20-amp circuit feeding lighting plus a fastened-in-place unit. Largest legal fastened load: 20 x 0.50 = 10 A.
- Same fastened unit on its OWN individual circuit: no 50% cap. The circuit simply has to be sized for the load.
- 15-amp shared circuit, one portable appliance: 15 x 0.80 = 12 A maximum.
Where candidates lose the point
- Quoting 80% as a cap on total circuit loading. It caps one portable appliance on a shared circuit.
- Never having met the 50% fastened-in-place rule. It is the harder question and the more common one.
- Missing that an individual branch circuit escapes both splits. One load, one circuit, size it and move on.
- Confusing this section with the 125% continuous-load rule in 210.19/210.20. Different rule, different trigger, different math.
- Accepting a 25-amp multi-outlet circuit. Multi-outlet circuits come in 15, 20, 30, 40, and 50 only.
How it shows up on the exam
The question names a circuit rating, tells you what else the circuit serves, and asks for the maximum load a specific appliance may draw. The entire skill is picking the right limit: portable appliance on a shared circuit takes 80%, fastened equipment beside lighting takes 50%, and a dedicated circuit takes neither. Read what else lives on the circuit before reaching for a percentage, because that sentence is the whole question.
Would these two rules have cost you points today?
The free 15-minute diagnostic pulls from the same branch-circuit rules the exam tests hardest and shows you exactly which ones you would miss.