How do you determine the number of spare breakers required in a panel to meet the spare requirement of FBC 423.17.3, Spare Capacity?
First it must be understood that the number of spares is set by the number of breakers the panel is capable of holding, not the number of breakers the design requires. To give an example, the design calls for 32 circuits each of which require one 20 amp breaker. The panel of choice is capable of holding 42 single phase breakers. The number of spare breakers to meet the 20% criteria would be 8 not 6. If the total number of breakers exceeded 42, then a sub-panel or feed-thru would be required. It should be noted that the total number of spare breakers would increase by the size of the sub-panel. As a second example; the design calls for 54 circuits of which each require one 20 amp breaker. To see if a 42 breaker main panel and a 30 breaker sub-panel or feed-thru will be sufficient, we take the total number of breaker slots available, 72, and take 20% of that number. That number is 14 and is added to the total number of required circuits, 54. This gives a total of 68. Therefore, a 42 brea
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