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3 phase transformer sizing 1
- Thread starter xformer
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But keep in mind that a 100A main breaker in a panelboard can only carry 80A on a continuous basis.
A 30 kVA transformer with a 208/120V secondary can provide 83 Amps continuously. So a 100A panel works nicely with a 30 kVA transformer at 208/120V.
A 30 kVA transformer with a 208/120V secondary can provide 83 Amps continuously. So a 100A panel works nicely with a 30 kVA transformer at 208/120V.
dpc brings up a good point, and I'd like to take him one step further:
What's the load on your 100 amp panel? If you only have 12kVA of load, and will only ever have 12kVA of load, then a 15 kVA tx with a 50-amp main in your panel will take care of you.
What's the load on your 100 amp panel? If you only have 12kVA of load, and will only ever have 12kVA of load, then a 15 kVA tx with a 50-amp main in your panel will take care of you.
SteveHuck
Electrical
- Jan 9, 2003
- 27
But keep in mind that a 100A main breaker in a panelboard can only carry 80A on a continuous basis.
The above statement . . . Not true!! The most recent that i can think of was a 200A main panel used for temporary power on a construction site.
Phase X 190A
Phase Y 194A
Phase Z 185A
that 80% stuff only works on paper. Size the transformer for the full 100A or put an 80A main in with the 30Kva
The above statement . . . Not true!! The most recent that i can think of was a 200A main panel used for temporary power on a construction site.
Phase X 190A
Phase Y 194A
Phase Z 185A
that 80% stuff only works on paper. Size the transformer for the full 100A or put an 80A main in with the 30Kva
SteveHuck,
I don't understand your point. There is no dispute that a 100A breaker can carry 100A. But per NEC, it may not be used to carry more than 80A for more than three hours. This isn't a question of capability, but rather it's the law. But the 80% rule is based on problem of the breaker ambient temperature when installed in an enclosure. The breaker nominal rating is based on operation in air.
Actually, a 100A breaker can carry a lot more than 100A for a little while.
If you put in an 80A breaker, it can only be loaded to 64A on a continuous (more than three hour) basis.
I don't understand your point. There is no dispute that a 100A breaker can carry 100A. But per NEC, it may not be used to carry more than 80A for more than three hours. This isn't a question of capability, but rather it's the law. But the 80% rule is based on problem of the breaker ambient temperature when installed in an enclosure. The breaker nominal rating is based on operation in air.
Actually, a 100A breaker can carry a lot more than 100A for a little while.
If you put in an 80A breaker, it can only be loaded to 64A on a continuous (more than three hour) basis.
stevehuck,
Look at is this way: The NEC requires that circuits be sized for 125% of the continuous load. This is the same as sizing for 80% of the breaker rating. 1/1.25 = 0.80. It's just a different way of saying the same thing. Article 215.2(A)(1). The only break you get is being able to use the "next larger standard size" breaker or fuse (up to 800A).
In addition, the 80% continuous rating is part of the UL labeling and listing, so it must be followed even it wasn't specifically mentioned in the NEC. But the two requirements are consistent.
Look at is this way: The NEC requires that circuits be sized for 125% of the continuous load. This is the same as sizing for 80% of the breaker rating. 1/1.25 = 0.80. It's just a different way of saying the same thing. Article 215.2(A)(1). The only break you get is being able to use the "next larger standard size" breaker or fuse (up to 800A).
In addition, the 80% continuous rating is part of the UL labeling and listing, so it must be followed even it wasn't specifically mentioned in the NEC. But the two requirements are consistent.
SteveHuck
Electrical
- Jan 9, 2003
- 27
I guess i am just looking at it from the other end.
I think you said you would use a 30Kva with the 100A main breaker panel.
That is fine, Well within the guidelines but what if Joe maintenance loads the panel to 93 amps long after you are gone and the main holds on, Now there is a tranny designed to put out 83 amp putting out 93 and the secondary breaker wont protect it. Chances are the main won't go out because that is sized to the 125+ rule also. Heck, maybe the 250% rule.
If you meant to use the 30Kva with a 80A main, that's fine too but why not go with a 37.5 and reap the entire 100A capacity of the panel.
Thats All I'm Saying!!!
I'm not questioning your intelligence, I'm just trying to inject some real life experience.
That codebook is the absolute worst allowable installation.
Always room to improve on it.
I think you said you would use a 30Kva with the 100A main breaker panel.
That is fine, Well within the guidelines but what if Joe maintenance loads the panel to 93 amps long after you are gone and the main holds on, Now there is a tranny designed to put out 83 amp putting out 93 and the secondary breaker wont protect it. Chances are the main won't go out because that is sized to the 125+ rule also. Heck, maybe the 250% rule.
If you meant to use the 30Kva with a 80A main, that's fine too but why not go with a 37.5 and reap the entire 100A capacity of the panel.
Thats All I'm Saying!!!
I'm not questioning your intelligence, I'm just trying to inject some real life experience.
That codebook is the absolute worst allowable installation.
Always room to improve on it.
There's nothing wrong with putting in a larger transformer, but I don't think 37.5 kVA is a standard size 3-phase unit. You would likely have to go with a 45 kVA.
I just don't accept your argument that using a 100A breaker on the secondary side of a 30 kVA transfomer is any kind of a problem. The breaker will protect the transformer adequately. If the application is critical, a larger transformer could certainly be used.
Actually, for a 30 kVA transformer, you can meet the NEC by using a 110A breaker since 125% of the transformer rating is 104A. You just have to size the feeders accordingly, and this requires a 225A bus panelboard since the next size down is generally 100A. But it's done all the time.
No argument from me on the problems of designing to Code minimum requirements. You're preaching to the choir. But in this case, I don't really see a problem.
I just don't accept your argument that using a 100A breaker on the secondary side of a 30 kVA transfomer is any kind of a problem. The breaker will protect the transformer adequately. If the application is critical, a larger transformer could certainly be used.
Actually, for a 30 kVA transformer, you can meet the NEC by using a 110A breaker since 125% of the transformer rating is 104A. You just have to size the feeders accordingly, and this requires a 225A bus panelboard since the next size down is generally 100A. But it's done all the time.
No argument from me on the problems of designing to Code minimum requirements. You're preaching to the choir. But in this case, I don't really see a problem.
ElectroKing
Electrical
Please pick your breaker based on the inrush current requirements of the load in aadition to sizing it for 125 to 150 % of the nominal. Check out the trip curves before you specify!
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SteveHuck
Electrical
- Jan 9, 2003
- 27
There is no controversy!!!
Just a difference of opinion. Both methods are acceptable and within the guidelines of the NEC.
I just tend to over build a little more than the code book because i have cleaned up the mess after self proclaimed maintenance men who have never seen a code book work their magic.
Just a difference of opinion. Both methods are acceptable and within the guidelines of the NEC.
I just tend to over build a little more than the code book because i have cleaned up the mess after self proclaimed maintenance men who have never seen a code book work their magic.
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