Overcurrent protection for a Rooftop AC unit

Status
Not open for further replies.
Sparky2791

Sparky2791

Senior Member
Location
Northeast, PA
Occupation
Electrical Design
A rooftop AC unit lists the MOCP on the nameplate as 50A. This unit is replacing a Rooftop unit which currently has a 90A circuit breaker protecting it (the area the new unit is serving got smaller so the tonnage went down hence the smaller load)
My Question - If I use a fused disconnect switch at the unit with 50A fuses at the unit is it permissible to leave the 90A breaker in the panel as the circuit breaker feeding it. Obviously the branch circuit conductors are large enough.

Thanks for replying!
 
They are but I am curious as to why. Wouldn't the 50A fuses limit the ampacity to 50A on the BCC's if mounted at the RTU?

Plus Article 440 specifically 440.35 would cover smaller wire, no?
 
LibertyEngineering said:
They are but I am curious as to why. Wouldn't the 50A fuses limit the ampacity to 50A on the BCC's if mounted at the RTU?

Plus Article 440 specifically 440.35 would cover smaller wire, no?
Click to expand...

The 50A fused switch becomes the branch circuit overcurrent device, making the 90A c/b a feeder overcurrent device. The conductors from the 90A c/b to the 50A fused switch are feeder conductors and must be protected as such.
 
Ahhhhhhhh.... That makes sense. Thank's for your input. I poured over the NEC for over an hour and it only took me 10 minutes to post the thread. I knew I should have started here but I am stubborn sometimes and want to figure it out alone. Have a great day!
 
What size are (what will become) the feeder conductors, and how long are they?

You might be able to use tap rules to keep the breaker and wires as they are.
 
Generally if they originally had an unfused switch at the old unit then the original breaker and wire was sized to the old unit nameplate and it was a branch circuit. Putting fuses at the new unit is ok but the old branch circuit becomes a feeder (as @david luchini ) mentioned and the breaker and wire are sized differently
 
The original circuit was sized for 90A (CU #4) so we will add the fused disconnect keep the original circuit wiring to the unit, which is only about 12 years old, use it as a feeder to feed the fused disconnect which has 50A fuses than feed the unit from the load side of the disconnect with CU #8. Thank you all for your responses.
 
Status
Not open for further replies.
Top