For multimotor circuits, wire sizes are to be 125% of the largest load plus all other loads. Where multiple loads are present, how is each fused disconnect sized? For example, if two roof top units each need 50A fuses, code would allow them to be placed on a single circuit with the breaker and wire sized properly. I understand how to size the wire and breaker, but I'm not sure how to size the disconnects. Would 60A disconnects be adequate if the circuit is fed from a 90A breaker? That is what I get when reading 2008 NEC 440.12.B and 430.110. It says the disconnecting means must be sized for all simultaneous loads. Since each disconnect will only carry the load of a single RTU, wouldn't a 60A disconnect be adequate? I'm assuming that the disconnect would be fed from a junction box (i.e., a tap) so that only the current from a single RTU passes through the disconnect.
Fused disconnect size for multimotor circuits
- Thread starter Steve-R
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If I understand the question. The disconnect for the individual units would be sized for that unit
(in your case a 60 amp disconnect with 50 amp fuses).
The fact that you have 90 amp OCP on the feeder does not effect the size of the individual disconnects connected to that feeder.
Does that answer your question ?
(in your case a 60 amp disconnect with 50 amp fuses).
The fact that you have 90 amp OCP on the feeder does not effect the size of the individual disconnects connected to that feeder.
Does that answer your question ?
Yes, that answers my question and agrees with my interpretation. I probably should've worded the question better. I.e., a fused disconnect in a multimotor circuit shall be at least 115% of its load (as the load is defined by the NEC), and the size of the OCP will have no bearing on that size.
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