The number that doesn't add up

A customer points at a 20-amp breaker and a piece of equipment stamped 16 amps and asks the obvious question: there's four amps of headroom, so why is the panel schedule telling you that circuit is full?

It's a fair question, and the honest answer surprises a lot of apprentices the first time they hear it. That 20-amp breaker is not rated to carry 20 amps all day. For a load that runs for hours without stopping, the most you should plan to put on it is 16. The breaker's number on the handle and the breaker's real, all-day working capacity are two different things, and the gap between them is one of the most quietly important rules in the entire Code.

It has a few names on the job — the 80% rule, the 125% rule, the continuous load rule. They're all the same idea seen from different ends. Get it wrong and the job still passes a quick look, still works on the day of the final, and then starts tripping a breaker every afternoon three weeks after you've moved on. Understanding why the rule exists is what keeps you off that callback.

What "continuous" actually means

First, the definition, because the Code is precise about it and people guess. A continuous load is one where the maximum current is expected to continue for three hours or more. That's it. Not "on all the time," not "important" — three hours of sustained draw.

That threshold matters because it sorts your loads into two piles. The lights in a retail store that stay on through an eight-hour shift? Continuous. Sign lighting, parking-lot poles, the heat tape on a roof, an HVAC unit cycling under load on a hot day, a battery charger grinding away — continuous. A receptacle a tech plugs a drill into for ninety seconds, the microwave in a break room, a bathroom exhaust fan? Not continuous. They draw their current in bursts and then the conductor gets a chance to cool back down.

That cooling is the whole story.

It's about heat, not magic

Current through a conductor makes heat — the I²R loss every electrician half-remembers from theory class. A wire sized right for a short burst can shed that heat between uses and never get near its insulation limit. Run the same current through it for three hours and the heat has nowhere to go. The conductor and everything it touches climbs toward a steady, elevated temperature.

The weak point isn't usually the wire in the middle of the run. It's the terminations — the screw lugs at the breaker and the device. A breaker is a thermal device by design. It's calibrated to trip on heat, and it's tested and listed for its rating assuming a certain amount of cooling. Standard molded-case breakers are tested to carry 100% of their rating, but the listing standard assumes they're not loaded continuously. Pack a breaker into a warm panel beside forty other breakers, then feed it a steady load right at its number for hours, and the heat building up at its terminals can drift it toward nuisance tripping — or worse, toward cooking the connection over time.

The 125% factor is the engineered margin that keeps the steady-state temperature at those terminations in the safe zone. It is not padding. It's the difference between a connection that lives at a comfortable temperature and one that lives at the edge of its rating every single afternoon.

The same rule, said two ways

Here's where the two names come from, and why they're identical.

From the load's point of view, the Code (NEC 210.20(A) for branch circuits, 215.3 for feeders) says the overcurrent device must be sized at not less than 125% of the continuous load plus 100% of any noncontinuous load. So a 16-amp continuous load needs a breaker of at least 16 × 1.25 = 20 amps. And NEC 210.19 sends the conductor down the same path: the wire's ampacity, before adjustments, also has to cover that 125%.

From the breaker's point of view, flip it: a 20-amp breaker × 0.80 = 16 amps. That's the 80% rule. The most continuous load you should hang on a standard 20-amp circuit is 16 amps. On a 100-amp feeder, 80 amps of continuous load. On a 200-amp service, 160.

Multiply by 1.25 going up from the load, or by 0.80 coming down from the breaker — you land in the same place. Your customer's 16-amp machine genuinely does fill that 20-amp circuit, and now the panel schedule makes sense.

The exception that trips people up

There's a wrinkle worth knowing, because it's where the careful electricians and the guessers part ways. If the entire assembly — the breaker and its enclosure — is listed for operation at 100% of its rating, you're allowed to skip the 125% bump for the continuous portion. These 100%-rated breakers exist, but they're special-order gear, physically larger, usually only found on big feeders and services where the cost of upsizing everything by 25% is real money.

Don't assume it. The default, the thing you reach for on virtually every branch circuit and ordinary feeder you'll ever wire, is the 80% world. Treat 100%-rated as the rare exception it is, and only when the listing on the actual equipment in front of you says so.

Where it actually bites

The rule hides until a real load sits on a circuit for hours. A few places it shows up:

A continuous lighting load in a commercial space. Forty-eight amps of fixtures pulled across a 60-amp circuit looks fine — until you remember 60 × 0.80 = 48. You're at the absolute ceiling with zero margin, and the first time someone adds a fixture you're over.

The terminal temperature rating stacks on top of this. Sizing conductors per the 75°C column is the common case, but if a breaker's lugs are only rated 60°C, your usable ampacity drops before the 125% even enters the math. Two separate limits, and the smaller one wins.

EV chargers are the modern poster child. EVSE is explicitly a continuous load. A 48-amp charger needs a 60-amp circuit and conductors sized for 60 — not 50 — precisely because that car will pull its max for far more than three hours overnight. Size it to the 48 and you've built a callback with a charging cable attached.

Sizing it without the second-guessing

None of this is hard arithmetic. It's a single multiply. What makes it slippery on the job is that it collides with everything else happening at once — the continuous factor, then ambient temperature derating, then conductor bundling, then the terminal temperature column, each one shaving the number a little further. You can run it on paper. The trouble is doing it on a ladder, in a mechanical room, with the inspector due at two.

That's the gap Voltly is built to close. Tell it the load and whether it's continuous, and it carries the 125% through the ampacity tables, applies the right temperature column and any derating, and hands you the breaker and conductor size that actually satisfies the Code — offline, in the field, before you cut a single length of wire. It's the difference between trusting your memory of the 80% rule at the end of a long day and knowing the circuit is right the first time.

The rule itself is simple enough to keep in your head: a continuous load needs a circuit sized to 125% of it, which is the same as loading a breaker to no more than 80%. Heat is the reason, the terminations are where it matters, and three hours is the line. Get that, and the panel schedule stops lying to you.

Run the numbers with confidence at voltly.lumenlabs.works.