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# 3 phase transformer sizing

## 3 phase transformer sizing

(OP)
How do you properly size a 4 wire, 3 phase delta-wye transformer with 480Vac primary to supply a 100A distribution panel for 120Vac loads (do I use 208Vac or 120Vac in my calculations)?
Replies continue below

### RE: 3 phase transformer sizing

Simply, 100A x 120V x 3 = 36,000VA  Some make ∆-Y drytypes rated 37.5kVA, else 45kVA.   {Remember 120V x 3 = 208 x sqrt3}  150°C-rise transformers are blast furnaces compared to 80° or 115° units.

### RE: 3 phase transformer sizing

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.

### RE: 3 phase transformer sizing

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.

### RE: 3 phase transformer sizing

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

### RE: 3 phase transformer sizing

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.

### RE: 3 phase transformer sizing

But per NEC, it may not be used to carry more than 80A for more than three hours.

You have a code section on that? I believe the 80% rule is for banch circuits 30A or less.

### RE: 3 phase transformer sizing

Hi

Do we still do loss evaluation when sizing transformers.

### RE: 3 phase transformer sizing

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.

### RE: 3 phase transformer sizing

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.

### RE: 3 phase transformer sizing

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.

### RE: 3 phase transformer sizing

I don't think 37.5 kVA is a standard size 3-phase unit.

Siemans makes a 37.5

I agree with you that the installation is within the guidelines of the code book.

Hey, smoke'em if ya got'em

### RE: 3 phase transformer sizing

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!

### RE: 3 phase transformer sizing

ElectroKing,

Sorry, we've been talking about the secondary side, which doesn't see any inrush.  You're right, sizing of the primary breaker is driven more by inrush than by full load amps.

### RE: 3 phase transformer sizing

(OP)
I can't believe the controversy that my seemingly simple question has stirred up.  Nice to know that there's a place where the finer details of design are hammered out.  Thanks again for the help guys.

### RE: 3 phase transformer sizing

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.

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