The National Electrical Code is revised every three years, and the 2026 NEC is the current edition, but local jurisdictions may still enforce older editions. NFPA's enforcement information shows different states on different NEC editions, so always verify the edition adopted by the project's Authority Having Jurisdiction (AHJ).
The core idea
Think of an electrical circuit like a person carrying weight.
A person can pick up a heavy box for a few seconds. They might even carry it across the room. But carrying that same box for three hours is a completely different problem. Heat, fatigue, endurance, and margin start to matter.
Electrical conductors and overcurrent devices behave the same way. A conductor carrying current produces heat. A breaker inside an enclosure also heats up. A short burst of high current and a long-duration current are not treated the same way.
That is why the NEC separates loads into two broad timing categories:
| Load type | Plain meaning | Typical sizing |
|---|---|---|
| Noncontinuous | Maximum current is not expected to continue for 3 hours or more | Count at 100% |
| Continuous | Maximum current is expected to continue for 3 hours or more | Often 125% when sizing conductors and OCPDs |
The NEC Article 100 definition defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. The important words are maximum current, expected, and 3 hours or more.
The three words that prevent most mistakes
1. "Maximum"
The definition does not say "a load that is energized for 3 hours." It says the maximum current is expected to continue for 3 hours or more.
A thermostat-controlled heater may be energized all day, but if it cycles on and off, its maximum current may not continue for 3 hours straight. A commercial lighting circuit, on the other hand, may sit near its full load for an entire shift. That is a much stronger continuous-load case.
"Will this load sit at or near its full rated current for 3 hours or more during normal use?" When the answer is yes, start thinking continuous load.
2. "Expected"
The NEC is not asking what is physically possible in a weird scenario. It is asking what is expected in normal operation. A toaster could theoretically be held down for three hours, but that is not normal use. A parking lot lighting circuit that turns on at dusk and stays on all night is expected to run for more than three hours.
3. "3 hours or more"
There is nothing mystical about exactly three hours from a physics standpoint. The point is practical: equipment that sits hot for a long time needs more margin than equipment that runs briefly and cools down.
The question is not "Is the equipment on for three hours?" The better question is "Is the maximum current expected to continue for three hours or more?"
Why the 125% rule exists
The 125% rule is really an 80% rule turned around. For a standard breaker used with a continuous load:
Solving for breaker rating:
1 / 0.80 = 1.25
breaker rating ≥ 125% × continuous load
That is why these two statements mean the same thing in ordinary field language:
- "Do not load a standard breaker above 80% for continuous load."
- "Size the breaker at 125% of the continuous load."
UL's circuit breaker marking guide explains that unless a circuit breaker is marked for continuous operation at 100% of its current rating, it is intended for use at no more than 80% of its rating where the load continues for three hours or more.
20 A × 1.25 = 25 A (20 A continuous does NOT fit a standard 20 A breaker)
The basic NEC formula
For many branch-circuit and feeder sizing problems, the working formula is:
Or, in plain language: count the short-duration part normally; add extra margin to the long-duration part.
- Branch-circuit OCPD — NEC 210.20(A): device rating ≥ noncontinuous + 125% × continuous, unless the assembly is listed for 100%.
- Branch-circuit conductors — NEC 210.19(A)(1): same logic for minimum conductor ampacity, with the same 100%-rated exception.
- Feeders — NEC 215.2(A)(1) for conductors and 215.3 for feeder overcurrent protection.
- Services — 230.42(A)(1) requires service conductors to have ampacity of at least 125% of continuous loads plus 100% of noncontinuous loads, before conductor correction or adjustment.
Where the 125% rule usually applies
- Branch-circuit conductors
- Branch-circuit overcurrent protection (breaker or fuse)
- Feeder conductors
- Feeder overcurrent protection
- Service-entrance conductors (service OCPD has its own service-article rules — do not assume identical wording)
Where the 125% rule does NOT automatically apply
1. Not every load calculation
The 2026 NEC relocated load calculations from former Article 220 to new Article 120, and 2026 NEC 120.5(E) states that load calculations are not required to calculate continuous loads at 125%. Continuous loads still affect conductor and OCPD sizing — but might not change the calculated load itself.
Two distinct stages:
- Stage 1 — Calculate the load. Demand factors, nameplate ratings, dwelling or nondwelling methods, EVSE load management.
- Stage 2 — Size conductors and OCPDs. This is where the continuous-load 125% rule usually appears.
2. Not noncontinuous loads
A 30 A load expected to run at full current for 45 minutes is not a continuous load. Other rules may apply, but the general continuous-load 125% rule is not triggered just because a load is large.
3. Not the same way when equipment is listed for 100% operation
For an assembly listed for operation at 100% of rating, sizing may be permitted at:
But this is not the same as simply buying a breaker with "100%" in the catalog. The breaker AND the installation conditions (enclosure, cubicle, terminations) must be evaluated and listed for that use.
4. Not blindly double-derated with conductor adjustment/correction
NEC 210.19(A)(1) and 215.2(A)(1) use "larger of" logic — compare required ampacity for continuous/noncontinuous loading with required ampacity after adjustment and correction, then use the larger. Do not chain them in one multiplication.
5. Not a replacement for special equipment rules
Motors, HVAC, welders, EV chargers, signs, fire pumps, and other equipment have specific code rules. Motor conductor sizing per Article 430 uses 125% of FLC — but that is a motor-specific 125%, not the general continuous-load 125% stacked on top.
Continuous-load and noncontinuous-load examples
Continuous-load examples that make intuitive sense
- Commercial lighting — stores, warehouses, corridors, offices, parking lots, exterior signs.
- Signs and outline lighting — Article 600 treats sign branch circuits as continuous.
- Electric vehicle charging — NEC 625.42 treats EV charging loads as continuous.
- Fixed storage-type water heaters (≤ 120 gal) — treated as continuous for branch-circuit sizing.
Noncontinuous-load examples that make intuitive sense
- Kitchen appliances with short heating cycles (toaster, microwave, coffee maker).
- Shop machines used for short production cycles (saw, press, pump, welder).
- General-purpose receptacles where expected use is not continuous max current.
The "maximum current" trap
A load can operate for more than three hours without being a continuous load at maximum current. A rooftop HVAC unit may be energized all day — fan running, compressor cycling, crankcase heater cycling — but that does not mean the maximum current of every component is continuous for three hours. Meanwhile, a row of LED parking-lot fixtures sits at roughly the same input current from dusk to dawn. That is much closer to the continuous-load definition.
Worked calculations
Example 1 — A simple continuous load
A fixed load draws 16 A at full output and is expected to run more than 3 hours.
A standard 20 A branch circuit is the minimum. Equivalently: 20 A × 0.80 = 16 A continuous.
Example 2 — A 20 A continuous load
A load draws 20 A continuous.
A standard 20 A breaker is not enough. The circuit needs at least 25 A of rating, but 12 AWG copper is generally limited to 20 A under the small-conductor rule, so the answer is rarely "just upsize the breaker" — the conductor must be evaluated too.
Example 3 — Mixed continuous and noncontinuous load
A branch circuit supplies 24 A continuous + 18 A noncontinuous.
Example 4 — EV charger sizing
EV charging is continuous per NEC 625.42.
Example 5 — Water heater
Fixed storage water heater rated 4,500 W at 240 V.
18.75 A × 1.25 = 23.44 A
Conductors and OCPD must be based on at least 23.44 A, before terminal/ampacity checks.
Example 6 — Parking lot lighting feeder
208Y/120 V, 3-phase feeder supplies 12 kVA of continuous lighting.
33.3 A × 1.25 ≈ 41.6 A
Example 7 — The derating trap
Feeder supplies a 40 A continuous load; adjustment/correction factor = 0.67.
Check 2: 40 / 0.67 ≈ 59.7 A
Use the larger: 59.7 A
Do not chain (40 × 1.25) / 0.67 in one step. Read the structure of the rule.
Example 8 — Motor feeder with other loads
Motor A: 40 A · Motor B: 20 A · Continuous non-motor: 15 A · Noncontinuous non-motor: 10 A.
Other portion: 15 × 1.25 + 10 = 28.75 A
Total feeder load: 70 + 28.75 = 98.75 A
100%-rated equipment: the exception that gets misused
Where the assembly — including the overcurrent device — is listed for operation at 100% of its rating, the minimum rating may be allowed to equal continuous + noncontinuous (no 1.25 multiplier on the continuous portion). But the key word is assembly.
Load calculation vs equipment sizing
A load calculation answers: "How much load is this system expected to serve?" A conductor sizing calculation answers: "What ampacity is required to carry that load safely?" An OCPD calculation answers: "What rating is required and allowed for this load and conductor?" Related, but not the same question.
A clean spreadsheet should have separate columns for:
- Connected load
- Demand factor or calculated load
- Continuous portion
- Noncontinuous portion
- Conductor sizing basis
- OCPD sizing basis
- Correction/adjustment basis
- Final selected equipment
If a spreadsheet has one column called "load × 1.25," it is probably hiding too much.
A clean workflow for honest NEC math
Common mistakes
Mental models
Beginner
A continuous load is a marathon load, not a sprint load.
A circuit can handle a sprint closer to its full rating. A marathon load needs breathing room. That breathing room shows up as 125% extra, or 80% maximum continuous use — the same relationship from two sides.
Advanced
The continuous-load rule is a thermal coordination rule. It coordinates load duration, conductor ampacity, breaker heat rise, enclosure dissipation, terminal temperature limits, equipment listing, ambient temperature, conductor bundling, and insulation rating. Continuous current makes small thermal errors visible — that is why the NEC treats it differently.
Quick reference
| Situation | Use 125%? | Why |
|---|---|---|
| 16 A EV charger load on standard breaker | Yes | EV charging is continuous (625.42) |
| 16 A toaster load | No | Not expected at max current for 3+ hours |
| Parking lot lighting | Yes | Expected to run for hours |
| General dwelling receptacle circuit | Not automatically | Possible use ≠ expected continuous max |
| Fixed 4,500 W water heater ≤120 gal | Yes | Code treats it as continuous for branch sizing |
| Standard breaker, 100 A continuous load | Yes | Needs 125 A min unless 100%-rated assembly |
| Listed 100%-rated assembly, 100 A continuous | Not necessarily | 100% listing may allow 100 A continuous |
| Motor branch circuit | Use Art. 430 | Similar multiplier but motor-specific |
| HVAC with MCA/MOCP nameplate | Use nameplate rules | Do not rework blindly |
| Load calculation under 2026 NEC Article 120 | Not automatically | 120.5(E) clarifies load calcs do not require 125% |
A practical lab you can connect to this note
Why Duration Matters
Goal
Show that current duration matters. The same current that seems harmless for a short time can produce meaningful temperature rise when it continues.
Safe setup concept
Use a supervised training setup with listed equipment only. Do not open energized panels. Do not overload cords, receptacles, or breakers.
- Plug-in power meter (Kill A Watt or equivalent)
- Clamp meter with a line splitter
- Listed resistive load — e.g., a small space heater within its listing
- Thermal camera or infrared thermometer
- Timer
- Worksheet comparing 80% and 125% math
What students observe
Run the load for a short period and record current and temperature. Then run it longer within safe limits and record how the temperature stabilizes over time. The lesson is not to overload anything — the lesson is to see that heat is time-dependent.
Discussion questions
- Did the current change much over time?
- Did the temperature change over time?
- Why does the NEC care about 3 hours?
- Why does a 16 A continuous load fit a 20 A breaker, but a 20 A continuous load does not?
- What would change if the breaker assembly were listed for 100% continuous operation?
Final field rule
The 125% rule is not a magic tax on electricity. It is a heat-management rule for equipment expected to carry maximum current for a long time.
References & code notes
- NEC Article 100 — definition of Continuous Load (maximum current expected for 3 hours or more).
- NEC 210.19(A)(1) — branch-circuit conductor sizing for continuous and noncontinuous loads.
- NEC 210.20(A) — branch-circuit OCPD sizing.
- NEC 215.2(A)(1) — feeder conductor sizing.
- NEC 215.3 — feeder overcurrent protection.
- NEC 230.42(A)(1) — service-entrance conductor sizing.
- NEC 240.4(D) — small conductor rule (e.g., 12 AWG Cu generally limited to 20 A).
- NEC 240.6(A) — standard OCPD ratings.
- NEC 430.24 — motor feeder conductor sizing (125% of largest motor + sum of others).
- NEC 600.5(B) — sign branch circuits treated as continuous.
- NEC 625.42 — EV charging loads treated as continuous.
- 2026 NEC Article 120 / 120.5(E) — load calculations are not required to compute continuous loads at 125%.
- UL Circuit Breaker Marking Guide — 80% standard vs 100%-rated breaker listing conditions.
- NFPA NEC adoption map — verify the edition adopted by the local AHJ.
Always verify the edition adopted by the local Authority Having Jurisdiction and confirm any specific section numbers against the adopted edition before applying them to a project.
The 125% rule is a heat-management rule — not a tax.
Run the related lab to feel the difference duration makes, or browse more Field Notes on NEC sizing, protection, and grounding.