A series-rated system cannot be used where motors are connected on the load side of the higher-rated overcurrent device or on the line side of the lower-rated device, and where the sum of the motor full-load currents exceeds what percent of the interrupting rating of the lower-rated circuit breaker?

A series-rated system cannot be used where motors are connected on the load side of the higher-rated overcurrent device or on the line side of the lower-rated device, and where the sum of the motor full-load currents exceeds what percent of the interrupting rating of the lower-rated circuit breaker?

• A. 0.5%
• B. 1%
• C. 5%
• D. 10%

Answer: (B)

In series-rated motor protection, protection is arranged so the line-side device carries the higher interrupting capacity, while the motors on the load side must not draw more than a tiny portion of the lower-rated device’s interrupting ability. The rule specifies that the sum of the motors’ full-load currents on the load side should be kept to about one percent of the interrupting rating of the lower-rated circuit breaker. This small fraction ensures the lower-rated device can still interrupt fault current and that protective coordination remains valid for the series-rated setup. For example, with a lower-rated breaker that can interrupt a large fault current, the total motor load current must be limited to about one percent of that rating. Choosing a higher percentage would allow more current through the load-side devices, risking improper interruption or protection. The one-percent limit is the standard used, so that is the best answer.

In series-rated motor protection, protection is arranged so the line-side device carries the higher interrupting capacity, while the motors on the load side must not draw more than a tiny portion of the lower-rated device’s interrupting ability. The rule specifies that the sum of the motors’ full-load currents on the load side should be kept to about one percent of the interrupting rating of the lower-rated circuit breaker. This small fraction ensures the lower-rated device can still interrupt fault current and that protective coordination remains valid for the series-rated setup. For example, with a lower-rated breaker that can interrupt a large fault current, the total motor load current must be limited to about one percent of that rating. Choosing a higher percentage would allow more current through the load-side devices, risking improper interruption or protection. The one-percent limit is the standard used, so that is the best answer.