transformer size and cb size

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MissLiz

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have customer that is moving to new building and has air compressor that is 240/460 but service is 208y. he wants to install transformer to boost to 240 or 480 the air compressor is 40 amp @ 480. I figured that a 50kva would be needed but the service there is only 200 amp on either panel @ 208v can I use a 20kva and just boost to 240v and be able to use 150amp cb to feed transformer or am I way off
 

iwire

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Location
Massachusetts
I am not a motor expert by any means but here is what I see.

the air compressor is 40 amp @ 480.

Leaving the power factor out of it that is about 33,000 watts.

Forgetting transformer looses at 208 that will be about 91 amps.

The starting current will be six to eight times that 91 amps.

Seems unlikely to me that the 200 amp service has enough capacity to handle an additional 91 amps never mind the starting current.

At the very least the lights are going to dim when this tries to start, I say tries because it might not even start due to voltage drop on the conductors supplying the 200 amp panel.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
have customer that is moving to new building and has air compressor that is 240/460 but service is 208y. he wants to install transformer to boost to 240 or 480 the air compressor is 40 amp @ 480. I figured that a 50kva would be needed but the service there is only 200 amp on either panel @ 208v can I use a 20kva and just boost to 240v and be able to use 150amp cb to feed transformer or am I way off

Standard KVA ratings for 3 phase transformers are 30, 45, 75, 112.5 etc. Now, taking into consideration iwire's math, your looking at at a 45KVA just to operate the motor under steadystate condition.

Starting through a transformer is tricky, as pointed out, VD could be a problem and I'm not talking about the STD type although that may be worse, but I digress.

Say you use a 45KVA, assume standard impedance of 5.75% for transformer, in that case the inrush of the motor (assuming 6x FLA) is 40 x 6 = 240A. The rated current of the 45KVA transformer is 54A @ 480V. So the inrush is 444% of the rated. The VD is going to be..........wait for it......... around 26%. Ouch!

Working backwards; say you want to limit the starting VD to 15% (85% bus voltage) then the %FLA needs to be 260% or less. The rated current needs to be around iwire's number of 91A, and thus you need probably a 112.5KVA (75KVA just not quite big enough).

Rule-of Thumb is that a standard motor starting through a transformer needs about 2.5X the KVA rating under steady state. Utilizing standard sizes, this would hold true such that 2.5X 33KVA = 82.5KVA, rounding up to a 112.5KVA standard size.
 

Jraef

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Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Before buying anything, I would check to see if your motor is dual voltage, ie 230/460V, most likely it is, in which case you could just hook it up and try it. You still may not have enough available headroom in your 200A service to start it without a voltage drop, but at least you haven't spent a bunch of money first before finding that out. If it is a single voltage 460V motor, it also may be less expensive to go buy a dual voltage motor rated 208-230/460 as well, then sell that 460V motor you have. The net cost difference will likely be less than a transformer and all the other stuff needed to go with it.

If your motor is dual voltage, and your system can handle starting it, then you can deal with the 230 vs 208V issue later. it's going to depend on how closely that motor is sized to the HP requirements of the compressor. If it's too close and running it on 208V makes it exceed the FLA rating, then you may need a buck-boost transformer, but that will cost a lot less than a 112.5kVA distribution transformer.
 
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topgone

Senior Member
Here's my take on this problem:

Supply the air compressor motor with a boost transformer bank composed of 2 x 10 kVA in open-delta arrangement. It is not feasible to boost up to 480 because the boost ratio goes beyond 2 (480/208).
figure_h.jpg
The 2 X 480/120V single-phase transformers will however give you:

(208 x (120+480)/480 = 260V, 3-phase

The common winding current of the banked transformers will be 92-80 = 12A; 92A being the current your 208V system will have to supply as discussed by other posters and 80A is the current your motor will draw @240V. The 10kVA primary winding can carry 10,000/480 =20.8A thru it, it will be safe, 20.8> 12A.

Your only problem now is whether your 208V/200A system can allow starting your big motor load.
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
Here's my take on this problem:

Supply the air compressor motor with a boost transformer bank composed of 2 x 10 kVA in open-delta arrangement. It is not feasible to boost up to 480 because the boost ratio goes beyond 2 (480/208).
View attachment 11279
The 2 X 480/120V single-phase transformers will however give you:

(208 x (120+480)/480 = 260V, 3-phase

The common winding current of the banked transformers will be 92-80 = 12A; 92A being the current your 208V system will have to supply as discussed by other posters and 80A is the current your motor will draw @240V. The 10kVA primary winding can carry 10,000/480 =20.8A thru it, it will be safe, 20.8> 12A.

Your only problem now is whether your 208V/200A system can allow starting your big motor load.

Very good example. thanks
 
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