Proper sizing of a breaker
- Thread starter mfatheree
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bphgravity
Senior Member
- Location
- Florida
Re: Proper sizing of a breaker
The NEC generally provides a maximum permitted overcurrent size based on the transformer rated current. The really is no minimum permitted other than being sufficient to supply the load to be served. See Article 450.
The NEC generally provides a maximum permitted overcurrent size based on the transformer rated current. The really is no minimum permitted other than being sufficient to supply the load to be served. See Article 450.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Proper sizing of a breaker
Your motor will draw just over half of the capacity of the transformer. You do not need to have a primary side breaker that can carry the entire capacity of the transformer. I think that is what you were asking.
But keep in mind that we size primary breakers at a value no more than a certain percentage of their full load current. In addition, we size motor branch circuit breakers at a value no more than a certain percentage of their full load current. This gives you two upper bound criteria, and you can't exceed either one.
I would start by looking at the motor load, as seen from the secondary side (i.e., 15 amps). I would pick a secondary side breaker on the basis of Article 430 (i.e., one that will allow the motor to start without tripping, and that still protects the motor branch circuit). Next, reflect the value of that secondary breaker's current rating onto the primary side. Pick a primary breaker that is no more than that reflected current value. Finally, make sure your selected breaker does not exceed the transformer breaker limits of Article 450.
Your motor will draw just over half of the capacity of the transformer. You do not need to have a primary side breaker that can carry the entire capacity of the transformer. I think that is what you were asking.
But keep in mind that we size primary breakers at a value no more than a certain percentage of their full load current. In addition, we size motor branch circuit breakers at a value no more than a certain percentage of their full load current. This gives you two upper bound criteria, and you can't exceed either one.
I would start by looking at the motor load, as seen from the secondary side (i.e., 15 amps). I would pick a secondary side breaker on the basis of Article 430 (i.e., one that will allow the motor to start without tripping, and that still protects the motor branch circuit). Next, reflect the value of that secondary breaker's current rating onto the primary side. Pick a primary breaker that is no more than that reflected current value. Finally, make sure your selected breaker does not exceed the transformer breaker limits of Article 450.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Proper sizing of a breaker
Is it a Delta-Delta (3-wire primary, 3-wire secondary) transformer? Also, is it a "supervised location"? The reasons for my asking come from 240.4(F) and from Table 450.3(A).
Is it a Delta-Delta (3-wire primary, 3-wire secondary) transformer? Also, is it a "supervised location"? The reasons for my asking come from 240.4(F) and from Table 450.3(A).
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Proper sizing of a breaker
240.4(F) says that you cannot protect the secondary conductors of that kind of transformer with the primary breaker. You need to have something on the secondary side. Also, your description does not answer the question of whether this is a "supervised location." That phrase is defined in Note 3 to Table 450.3(A).
240.4(F) says that you cannot protect the secondary conductors of that kind of transformer with the primary breaker. You need to have something on the secondary side. Also, your description does not answer the question of whether this is a "supervised location." That phrase is defined in Note 3 to Table 450.3(A).
mdshunk
Senior Member
- Location
- Right here.
Re: Proper sizing of a breaker
Okay, curiosity is getting the better of me. What's the application for a 3000 VAC motor?
Okay, curiosity is getting the better of me. What's the application for a 3000 VAC motor?
pierre
Senior Member
- Location
- Westchester County, New York
Re: Proper sizing of a breaker
A bedroom ceiling fan... don't forget to protect the circuit with AFCI
A bedroom ceiling fan... don't forget to protect the circuit with AFCI
- Location
- Windsor, CO NEC: 2023
- Occupation
- Hospital Master Electrician
Re: Proper sizing of a breaker
Man, I hate getting stuck with the 480V houses!
Man, I hate getting stuck with the 480V houses!
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