Overcurrent Protection and Devices, Short-Circuit Calculations ...

Electrical Plan Review
Series Ratings
Most electrical distribution systems are fully rated as required by
NEC ® 110.9. A fully rated system is a system where every overcurrent
protective device has an interrupting rating equal to or greater
than the available fault. Fully rated systems are typically preferred
and recommended, but electrical distribution systems are permitted
to incorporate series ratings, provided all the requirements of NEC ®
240.86 and 110.22 are met. However, the actual application of
series ratings is typically limited.
Series rating is a combination of circuit breakers, or fuses and circuit
breakers, that can be applied at available short-circuit levels above
the interrupting rating of the load side circuit breakers, but not above
that of the main or line-side device. Series ratings can consist of
fuses protecting circuit breakers, or circuit breakers protecting circuit
breakers.
Series Rating Circuit Breakers. In the example below, the 20A,
10,000A interrupting rating circuit breaker has been tested, for a
series combination interrupting rating of 65,000A when protected by
the upstream 200A, 65,000A interrupting rating circuit breaker. The
circuit breaker types for this series combination rating would have to
be verified by the evidence of the panelboard or switchboard marking
as required by NEC ® 240.86(A).
While there is only one advantage to utilizing a series combination
rating…lower installed cost, several special requirements or limitations
exist and are discussed below.
Special Requirements For Applying a Series Combination Rating
Special requirements and limitations must be considered for the
application of a series combination rating, which include:
- Motor contribution limitation
- Manufacturer labeling requirements
- Field labeling requirements
- Lack of coordination limitation
- Proper selection of series combination ratings
Motor Contribution Limitation
The first critical requirement limits the application of a series combination
rating where motors are connected between the line-side (protecting)
device and the load-side (protected) circuit breaker. NEC ®
240.86(B) requires that series ratings shall not be used where the
sum of motor full load currents exceeds 1% of the interrupting rating
of the load-side (protected) circuit breaker.
The example to the right shows a violation of 240.86(B) due to motor
contributions. Since the motor load exceeds 1% of the load-side circuit
breaker (10,000 X 0.01 = 100A), this series rated combination
cannot be applied.
Series Rating Fuse and Circuit Breakers. In the example below, a 20A,
10,000A interrupting rating circuit breaker has been tested, for a
series combination interrupting rating of 200,000A when protected by
the upstream Class J fuse. The fuse and circuit breaker types for
this series combination rating would have to be verified by the evidence
of the panelboard or switchboard marking as required by
NEC ® 240.86(A).
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