Wire size for rooftop units

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samcon

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I have a situation where the old RTUs were replaced with new. The old RTUs were fed with #6 THHN copper on a 70A breaker. The new RTUs are fused at 50A at the unit and are fed with the same #6 wire. They are still on the 70A breakers at the panel. Is it OK to leave the 70A breakers in place to protect the #6 wire up to the disconnect switch at the unit? Obviously the #6 wire is fine from the disconnect to the unit since it is protected by 50A fuses. It would seem to me that the 70A breakers would be OK since they are protecting only against short circuit and not overload. The fuses would take care of any overload condition. Any help on this is much appreciated. Thanks.
 

david luchini

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Staff member
Location
Connecticut
Occupation
Engineer
Assuming your equipment has 75deg rated terminations, #6 has an ampacity of 65, and would be properly protected with a 70A c/b per 240.4(B). It would be acceptable on this basis.
 

cowboyjwc

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Staff member
Location
Simi Valley, CA
I have a situation where the old RTUs were replaced with new. The old RTUs were fed with #6 THHN copper on a 70A breaker. The new RTUs are fused at 50A at the unit and are fed with the same #6 wire. They are still on the 70A breakers at the panel. Is it OK to leave the 70A breakers in place to protect the #6 wire up to the disconnect switch at the unit? Obviously the #6 wire is fine from the disconnect to the unit since it is protected by 50A fuses. It would seem to me that the 70A breakers would be OK since they are protecting only against short circuit and not overload. The fuses would take care of any overload condition. Any help on this is much appreciated. Thanks.

You are correct.
 
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