240V single phase circuit with unequal length leads - Electrician Talk - Professional Electrical Contractors Forum
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Old 02-18-2015, 06:35 AM   #1
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Default 240V single phase circuit with unequal length leads

Hello all

I am wiring an invertor-type mini-split AC system in a residence requiring a 240v 15a breaker.

Due to the topography of the house, running an additional circuit from the breaker would pose some difficulty and substantially increase the cost of the install.

I have 2 existing 120v 20a single phase circuits supplying outlets in the area (not in the same conduit) and am interested in removing the breakers from these 2 circuits and replacing them with a double pole 240v 15a circuit breaker (with common trip) and tapping these for use with the minisplit. The neutrals for each circuit are tied in to the same block at the service entrance breaker box and there are no sub-panels.

1) My understanding is that a multipole breaker with common trip will allow full rated amperage through any pole as long as combined amperage from all poles does not exceed rated amperage. i.e. in this case if 1 pole was pulling 0 amps the other pole can pull the full 15 amps without the breaker cutting the circuit.
Is this correct?

2) I know that magnetic interactions between oppositely phased leads within the same conduit act to decrease heat buildup. This would not be the case here. However, I would be de-rating my circuit.
Will this be a problem?

3) I estimate that the wire runs for each of the 2 circuits are approximately 50-75 feet long but are unequal length.
Will unequal length leads cause phase shifting problems?

4) Last but not least, is this expressly prohibited by the NEC?

Thanks in advance for any and all input!
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Old 02-18-2015, 06:52 AM   #2
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Old 02-18-2015, 06:53 AM   #3
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If this is in conduit there is no way to fish 2 extra 14 guage wires?
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Old 02-18-2015, 07:13 AM   #4

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