The breaker is in place to protect the wiring in the structure more than to protect the equipment. Undersizing the conductors for the breaker can be a good way to get a fire started. How far the unit is from the electrical source is also something to take into consideration.

 

You are correct. But since he’s the boss, do it the way he wants.

The breaker will protect the short circuit current. Usually there are overloads that protect for running protection.

 

You're probably under NEC so I won't give you CEC rules (consider adding your location in your profile)


You're on the right track, but, you can't size your conductor at 100%. #10 would be too small.



Mfr says MOP 50A. So the motor has overload protection... the reason you can oversize the breaker.

 

You're probably under NEC so I won't give you CEC rules (consider adding your location in your profile

You're right....
@Apprentice5787 please take a few minutes to fill out your profile as per the user signup agreement. This includes your general location so we know what code books to use.

 

Even at 60c 10 wire is good for 30A. Since MCA is already at 125% the #10 is already “derated”. Unless I’m missing something, #10 would be fine under the NEC. Not sure where the OP is from?

 

You're right ... my brain fart :biggrin:

 

HertzHound is correct.

#10 is fine as the MCA is already at 125% and the MOP is 50 so you size it at 50. This falls under rules for Chapter 4 (430 motors, or 440 AC). Your boss is referencing Chapter 3 rules. Chapter 4 rules modify Chapter 3 rules. So your wire is not sized based upon OCP. The wire is protected by Overload protection either in the motor or in the air handler equipment. The 50 amp breaker is for short circuit and ground fault protection only. The larger breaker is sized this way to allow the motor to start with LRC (locked rotor current) and not trip the breaker. The wire is sized for the amperage draw under running conditions.

 

Ya I’m from the USA so we would fall under NEC rules. I’m still learning and he’s been a journeyman for a while so that’s where I’m getting confused. Everything I read online says I can size wire based on the MCA and at 75 degrees #10 can do 35 amps. Otherwise what would even be the point of having the MCA on the name plate? However he’s the boss so I just did what he said I just want to know for the future when it’s me making these decisions.

 

The point of having MCA on the name plate is that if it was something simple say just a single motor, you can use the NEC rules to figure out the proper wire and short circuit protection rules. But with air handlers there are a lot of moving parts so it's not often easy to determine the correct sizes. So the manufacturer provides those for you.

 

NEC is the minimum requirement.

Sometime during your career you will be digging up a line cussing that fact that the NEC said it would work all while trying to remember the 20 bucks you saved.

 

looks like your boss doesn't understand what he's doing

 

Rumors are a good description for many ideas out there. I always like checking the books and when I am wrong, I will never forget it. A good learning experience.