Fully Rated vs. Series Rated Breakers

The NEC requires every device intended to interrupt a fault to have an interrupting rating at least equal to the fault current available at its line terminals (NEC 110.9). There are two ways to meet that. In a fully rated system, every breaker and fuse is individually rated for the available fault current. In a series rated system, a downstream breaker with a lower interrupting rating is protected by a specific upstream device, and the two have been tested together as a combination rated for the higher fault current.

How a series rating works

When a large fault occurs downstream, the upstream device (a current-limiting fuse or breaker) opens fast enough to limit the let-through current and energy. The downstream breaker, which couldn't interrupt the full fault on its own, only has to handle what gets through. The combination is only valid because it was tested that way, under UL 489 for breakers, and it's listed by the manufacturer for those exact devices.

Worked example

A panelboard fed from a 2,000 kVA, 480 V transformer with 5.75% impedance. As an infinite-source estimate at the transformer terminals, available fault current is 100,000 × 2,000 ÷ (1.732 × 480 × 5.75) = about 41,800 A. The real value at the panel will be lower once cable impedance is included, but take 42 kA as the design figure.

• Fully rated: every branch breaker in the panel must be rated at 42 kA or more. That usually means a higher-interrupting frame for every branch circuit.
• Series rated: a main breaker rated above 42 kA, combined with branch breakers of a lower rating, where the manufacturer has tested and listed that exact main-and-branch combination for 42 kA or more.

The series-rated panel is usually cheaper, because a panel full of lower-rated branch breakers costs less than a panel full of high-interrupting ones. The fault-current figure uses the same method as the fault current calculator, and the impedance guide explains why the transformer's tolerance can push it higher.

What the NEC requires for series ratings

• Tested combinations only. New installations must use combinations that have been tested and are marked on the equipment (NEC 240.86). Existing installations can use engineered series ratings selected by a licensed engineer, under specific conditions.
• Field marking. The equipment must be labeled "Caution — Series Combination System Rated ___ Amperes. Identified Replacement Components Required." (NEC 110.22(C)). A like-for-like swap with a different breaker voids the rating.
• The motor limit. A series rating can't be used where motors connected between the two devices contribute more than 1% of the lower-rated breaker's interrupting rating. For an 18 kA breaker, that's 180 A of motor full-load current. Motors feed fault current back through the downstream breaker, which the tested combination didn't account for.

Why series ratings and selective coordination don't mix

A series rating works because the upstream device opens on a large downstream fault. That's the opposite of selective coordination, which requires only the device nearest the fault to open. A series-rated combination is non-selective by design for high-level faults.

That matters wherever the NEC requires selective coordination: emergency systems, legally required standby systems, critical operations power systems, elevator circuits and certain health care feeders. In those systems, series ratings generally can't be relied on, and devices have to be fully rated. Data centers, where losing an upstream breaker takes out far more than the faulted circuit, usually choose fully rated, coordinated designs for the same reason, whether or not the code requires it. The short-circuit and coordination guide covers the study that checks this.

Interrupting rating, SCCR and withstand

All three have to be checked against available fault current, and a new transformer or utility upgrade can invalidate any of them.

Common mistakes

• Replacing a series-rated breaker with a "compatible" one that isn't part of the tested combination.
• Using series ratings in systems that require selective coordination.
• Ignoring motor contribution between the two devices.
• Not re-checking after a system change. A larger transformer, lower impedance or a new utility source raises fault current past the combination's rating.
• Assuming a series rating reduces arc-flash energy. It doesn't. Only faster clearing does, for example from current-limiting fuses, as the arc-flash PPE guide covers.

Frequently asked questions