What Is a Series-Rated Electrical System per NEC 2023 240.86?

A series-rated system allows a downstream circuit breaker with a lower interrupting rating (AIC) to be used where the available fault current is higher, provided it is protected by a specific upstream breaker or fuse combination that has been tested and listed to work together.

Simple Example

If the available fault current is 42 kA, a 10 kAIC downstream breaker may be used with a 65 kAIC upstream breaker only when that specific combination is manufacturer-tested and listed for a series rating of at least 42 kA.

65 kAIC Upstream Breaker → 10 kAIC Downstream Breaker

A higher-rated upstream breaker does not automatically make the downstream breaker series rated. The specific combination must be approved for series operation.

What Should Be Shown on the Electrical Drawings?

When a series-rated system is used, the electrical drawings should clearly indicate:

• Available fault current at the applicable equipment.
• Required series rating in kA.
• Upstream and downstream protective devices that form the listed combination.
• Series-rating requirements on the single-line diagram and/or panel schedules.
• A requirement that the contractor verify the exact breaker/fuse combination using the manufacturer’s published series-rating tables.
• Required permanent series-rating field labeling.

Series-rated systems must be properly selected, coordinated, installed, and labeled in accordance with NEC 240.86 and 110.22(C).

Important: Replacing or substituting a breaker can invalidate the series rating. Always verify replacement devices against the manufacturer’s listed series-rated combinations.

Example Using the Square D Series-Rating Chart

The Square D chart shown above provides a good example.

Assume the calculated available fault current at a 240V system is:

Available Fault Current = 50 kA

From the Square D chart, we can identify a tested combination with a:

Series Combination Rating = 65 kA at 240V

For the highlighted combination, the chart identifies:

Upstream Square D Breaker:
LH, MG, MJ, MH, PG, PA (1600), or RG (2000), as applicable to the manufacturer’s table

Downstream Square D Breaker:
QD, maximum 225A

Listed Series Rating:
65 kA at 240V

Therefore:

Upstream Listed Breaker
↓
QD Breaker — Up to 225A
↓
Series Combination Rating = 65 kA

If the available fault current is 50 kA:

50 kA Available Fault Current < 65 kA Series Rating

Therefore, this combination can be acceptable from a series-rating standpoint, provided the exact equipment configuration and all manufacturer and NEC requirements are satisfied.