The 80% Rule & Continuous Loads

A continuous load is one where the maximum current is expected to continue for three hours or more (NEC Article 100). Data center IT load, lighting in occupied buildings, and most process loads qualify. The NEC requires conductors and overcurrent devices serving continuous load to be sized at 125% of the continuous load, plus 100% of any non-continuous load. That is the "80% rule" in reverse: 1 ÷ 1.25 = 0.80.

The same requirement appears for branch circuits (NEC 210.19 and 210.20), feeders (215.2 and 215.3) and services (230.42). It exists because standard breakers and their enclosures are tested to carry their rated current in open air, not continuously inside an enclosure next to other heat sources. Running them at full rating for hours can overheat terminations and cause nuisance trips.

What it means at the rack

Usable power is √3 × V × 24 A. Designers and colocation contracts often express rack power in exactly these terms, and a breaker that trips at 80% of its rating after hours of load is usually doing what it was built to do. The 415 V guide shows why the higher voltage doubles the usable power per circuit.

Conductors: two checks, not one

For conductors, NEC 210.19(A)(1) and 215.2(A)(1) require two separate checks, and the conductor must pass both:

• Check A, before derating: the conductor's table ampacity, in the column matching the terminations (usually 75°C), must be at least 125% of the continuous load plus the non-continuous load.
• Check B, after derating: the conductor's ampacity after ambient correction and conductor-count adjustment must be at least the actual load, at 100%.

The 125% factor and the derating factors are not multiplied together. They answer different questions: check A protects the terminations, and check B protects the insulation. The ampacity derating guide covers check B in detail.

Worked example

A feeder carrying 200 A of continuous load, four current-carrying conductors in the raceway (the neutral counts, for non-linear load), 40°C ambient, copper conductors, 75°C terminations.

• Overcurrent device: 200 × 1.25 = 250 A, a standard size.
• Check A: 250 A at 75°C needs 250 kcmil (255 A).
• Check B: 250 kcmil at 90°C is 290 A × 0.91 × 0.80 = 211 A, which is at least 200 A. It passes.

The conductor is 250 kcmil, protected at 250 A.

100%-rated breakers

Some breakers, usually in larger frames, are listed for continuous operation at 100% of their rating when installed under the conditions on their label, which typically cover conductor temperature ratings and enclosure size. With a 100%-rated assembly, the NEC allows the device and conductors to be sized at 100% of the continuous load rather than 125%.

In the example, a 100%-rated 200 A breaker would pass check A with 3/0 copper (200 A at 75°C). But check B still applies: 3/0 at 90°C derates to 225 × 0.91 × 0.80 = 164 A, and 4/0 to 189 A, both below 200 A. The conductor still has to be 250 kcmil. Where derating governs, a 100%-rated breaker saves the breaker size, not the cable. Where the conductors run cool and uncrowded, it can save both.

Common mistakes

• Multiplying 125% by the derating factors. They're two separate checks.
• Treating IT load as non-continuous. It runs around the clock.
• Assuming a 100%-rated breaker shrinks the cable. Derating often governs anyway.
• Loading rack circuits to the breaker rating. The usable figure is 80%.
• Mixing up the check A column. It’s the termination’s temperature column, not the insulation’s.

Frequently asked questions