Sylvania 3 phase breaker for Rooftop

[powerplay]

I have been asked to replace an existing 3 pole 50amp QBH Breaker with an 3 pole 40amp Breaker in an Sylvania Electrical Panel. The Rooftop is going to gas heat to reduce energy used and direct it to another purpose. I hadn't found an new Three Phase Breaker, and was wondering if there are any issues with using an old QBH Breaker? ...should I use the 50amp to avoid nuisance tripping on the old breaker, since the load will only draw so much and the and the cable is properly protected by the Breaker anyways? I see maximum overcurrect protection on nameplates, but if Conductor protected are there any safety issues on Air Conditioners with larger Breakers? ...Thanks again!

 

[GoldDigger]

For safety, the AC unit will incorporate motor overload protection, but the nameplate will specify the maximum allowed short circuit and ground fault OCPD upstream. If you go with a larger breaker in panel you will have to provide the required protection downstream, like with a fused disconnect at the unit.

 

[ptonsparky]

Warranties of the new unit will be voided if you proved overcurrent protection in excess of the label requirements.

Not knowing the installation, can you put a fused disc at the unit with 40 amp fuses? Leave the 50 amp CB as is for the feeder protection.

 

[powerplay]

Warranties of the new unit will be voided if you proved overcurrent protection in excess of the label requirements.

Not knowing the installation, can you put a fused disc at the unit with 40 amp fuses? Leave the 50 amp CB as is for the feeder protection.

I am more curious about what safety issue there is. I can get an 40 amp Breaker, but curious if a used one could possibly be more sensitive? I would think that the overload protection is provided at the Unit, but I was under the impresion that an short circuit would trip most breakers due to a fault. If the Breaker is protecting the cable through the structure and has the same AIC as the existing, if the motor is running higher than it"s rated FLA it would eventually short circuit whether 10amps higher?

 

[Smart $]

It's all in the numbers. A 50A breaker protecting equipment with a 40A MOCPD rating has a higher risk of fire on a short circuit or ground fault event.

It doesn't matter if the used 40A is more sensitive than a new one or not. If you knowingly use a 50A, you are liable.

 

[ptonsparky]

I am trying to follow along but am missing the trail.

Do you have the Nameplate information from the unit?

My home unit has a MCA of 29.6 amps. That means the FLA is about 23.7. The Max Circuit Breaker is 45. An additional 10 amps to the FLA will mean nothing to the 45 amp breaker. Will the breaker eventually trip? Maybe, if the compressor windings eventually break down.

 

[GoldDigger]

And with that setup, as MH covers well in a video, you can use wire good for only 30A and it will be considered protected by the combination of the 45A breaker and the motor overloads.

 

[powerplay]

I am trying to follow along but am missing the trail.

Do you have the Nameplate information from the unit?

My home unit has a MCA of 29.6 amps. That means the FLA is about 23.7. The Max Circuit Breaker is 45. An additional 10 amps to the FLA will mean nothing to the 45 amp breaker. Will the breaker eventually trip? Maybe, if the compressor windings eventually break down.

The existing Panel has an 3 pole 50amp breaker in it that is not working. The Mechanical Engineer asked for an 3 pole 40 amp breaker because the Rooftop was changed to an more efficient one that runs heat off gas. I have not seen the Unit yet, but came across 3pole 50amp breakers that were of the same cost as 40. Speculating that an old Breaker may be weaker that might trips easily, I considered using the same breaker size to protect the cable. The AC Unit Motor I was told would run at FLA, but if it were to run higher amps either the overload kicks out or it would become a dead short which the 50amp Breaker would be able to interrupt just as well?