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Thursday, January 12, 2017
Question:
For example, you have a transformer with a secondary with 360A full load. 360 x 125% = 450A, so you use a 450A breaker and conductors with 450A ampacity. The breaker is feeding an automatic transfer switch. What amp rating of the ATS is required? The breaker is 80% rated and ATS is 100% rated, so can the ATS be 400A which is greater than the transformer FLA or does the ATS have to have an amp rating equal to or greater than the breaker (600A)?
For panelboards 408.36 requires the breaker to not be greater than the panelboard rating. So for the same example above, a 400A panelboard though bigger than the transformer FLA can’t be connected to a 450A breaker, a 600A panelboard is required.
NEC 408 does not have the same requirement for switchboard and switchgear. Are 100% rate breakers normally use on this equipment?
Thanks, Kevin
A
Answer:
Hey Kevin thanks for your question. The automatic transfer switch needs to be rated for the load as stated in 700.4, 701.4 or 702.4 depending on which type of system it is. The 125% factor only applies to continuous loads, as defined in Article 100, and only for the feeder conductors as stated in 215.2(A)(1) and for the feeder overcurrent device as stated in 215.3. These minimum sizes are based on the load, not the transformer secondary current rating. If none of you load is continuous the overcurrent devices and conductors can be sized at 100%. The 80% factor (80% of 125 = 100) is in the product standard which limits a continuous load unless the device and its assembly is listed for operation at 100% of its rating as stated in 215.3 Exception 1. You are correct panelboards must be protected with overcurrent devices that do not exceed the panelboard rating as stated in 408.36. The same rule does not exist for switchboards, switchgear or transfer switches.