CB Sizing Examples
The following are examples of sizing rules.
Example 1: 50A continuous load and 125A noncontinuous load.
OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 125A) + (1.25 x 50A) = 187.5A
Therefore, a 200A OCPD is needed. If a 100%-rated CB is chosen, a 175A rating (125A + 50A) is acceptable.
Example 2: 300A noncontinuous load.
A 300A device is acceptable; a 100%-rated device is not needed since the load is noncontinuous.
Example 3: 200A continuous load.
OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 0A) + (1.25 x 200A) = 250A
Therefore, a 250A device is needed. If a 100%-rated CB is selected, a 200A rating is permitted.
Example 4: 16A continuous and 30A noncontinuous.
OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 30A) + (1.25 x 16A) = 50A
Therefore, a 50A device can be selected. Although 100%-rated devices typically are not available in sizes this small, the permitted rating would still be 50A (16A + 30A = 46A; rounded up to 50A).
Jim Pauley is Manager, Industry Standards, and Sandi Young is Circuit Breaker Product Specialist with Square D Co., Lexington, Ky. and Cedar Rapids, Iowa respectively.