# Minimum circuit ampacity



## CWL (Jul 7, 2020)

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.


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## HertzHound (Jan 22, 2019)

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.


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## emtnut (Mar 1, 2015)

Apprentice5787 said:


> I have a air handler that I need to run wires to. On the nameplate it says MCA of 29.25 amps and MOP 50 amps. My understanding from what I read online I size my wire based on the MCA. So I can have #10 wire ran to it and landed on a 50 amp breaker is that correct? My boss said that’s not correct and I need to always size my wire based on the max Overcurrent rating. There’s nothing wrong with that except for the added costs so I’m just doing what he says since he’s the boss. However I would really like to have any other opinions on if I was doing it the correct way or if his way was correct. Thank you!



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.


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## HertzHound (Jan 22, 2019)

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?


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## emtnut (Mar 1, 2015)

HertzHound said:


> 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:


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## MotoGP1199 (Aug 11, 2014)

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.


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## Kevin (Feb 14, 2017)

emtnut said:


> 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.


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## Apprentice5787 (Apr 10, 2019)

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.


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## paulengr (Oct 8, 2017)

Apprentice5787 said:


> 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.


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## gpop (May 14, 2018)

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.


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## kb1jb1 (Nov 11, 2017)

Apprentice5787 said:


> 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.


Be careful with reading things online. Anybody can post on You Tube or whatever but that does not make things right. Even on this site there are many things that are quoted wrong. I have made mistakes quoting here only to have to eat a plate of crow. My helper sometimes refers to what he learned on You Tube only to find out it is completely wrong. Always good to ask many questions and make sure the answers are backed up.


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## farmantenna (Nov 22, 2012)

Apprentice5787 said:


> I have a air handler that I need to run wires to. On the nameplate it says MCA of 29.25 amps and MOP 50 amps. My understanding from what I read online I size my wire based on the MCA. So I can have #10 wire ran to it and landed on a 50 amp breaker is that correct? My boss said that’s not correct and I need to always size my wire based on the max Overcurrent rating. There’s nothing wrong with that except for the added costs so I’m just doing what he says since he’s the boss. However I would really like to have any other opinions on if I was doing it the correct way or if his way was correct. Thank you!


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


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## farmantenna (Nov 22, 2012)

emtnut said:


> 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.
> ...


that MCA rating includes 125% of the loads so 29.?? is actually lower


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## farmantenna (Nov 22, 2012)

Apprentice5787 said:


> 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.


you're going through what I did as APR. I'd learn things in class and my JM was doing things based on rumors. Confusing.


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## kb1jb1 (Nov 11, 2017)

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.


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