Transformer secondary conductors

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Dustin Foelber

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I have a project where my client is replacing an X ray. Currently the have an incoming service voltage to the gear of 208/120 3 Phase. They came out of this gear with a 200A breaker to feed a 75 KVA 208 to 480 DELTA step up transformer. The #4awg 480V secondary conductors come right out of the transformer and run about 80' and land directly on a 60A Fusible disconnect. I am not sure if this is NEC compliant? I would assume it should have some type of local OCPD on the secondarys but maybe this is OK?

Any help would be greatly appreciated. Thanks
 
I did not check any of the math, but the immediate problem I see is the length. Except for outside installs 240.21(C) limits the length of the secondary conductors to 25' max between tranny and OCPD.
 
If the secondary is 480 volt 3 wire delta - is possible this is considered protected by the primary overcurrent device? I didn't look at the numbers to see if what is there is compliant if it can be protected by primary.
 
This is where my confusion comes in. Is the 200A OCPD the only required protection for primary, secondary, and transformer? The secondary 480V does only terminate on the disconnect but it is in another room about 70-80 away. 60A fused disconnect
 
Dustin Foelber said:
This is where my confusion comes in. Is the 200A OCPD the only required protection for primary, secondary, and transformer? The secondary 480V does only terminate on the disconnect but it is in another room about 70-80 away. 60A fused disconnect
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IF the transformer is 208V delta to 480V delta, you would be able to protect the secondary conductors with the primary OCPD per 240.21(C)(1). That being said, the 200A primary OCPD does NOT properly protect the #4 secondary conductors per that section.

IF the transformer is 208V wye to 480V delta, you would not be able to protect the secondary conductors with the primary OCPD.

Do you know if the transformer is delta-delta or wye-delta?

The 200A primary OCPD protects the transformer as "primary only" protection per T450.3(B).
 
david luchini said:
IF the transformer is 208V delta to 480V delta, you would be able to protect the secondary conductors with the primary OCPD per 240.21(C)(1). That being said, the 200A primary OCPD does NOT properly protect the #4 secondary conductors per that section.

IF the transformer is 208V wye to 480V delta, you would not be able to protect the secondary conductors with the primary OCPD.

Do you know if the transformer is delta-delta or wye-delta?

The 200A primary OCPD protects the transformer as "primary only" protection per T450.3(B).
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OP said:

Currently the have an incoming service voltage to the gear of 208/120 3 Phase. They came out of this gear with a 200A breaker to feed a 75 KVA 208 to 480 DELTA step up transformer.
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That means wye-delta, no?
 
Dustin Foelber said:
This is where my confusion comes in. Is the 200A OCPD the only required protection for primary, secondary, and transformer? The secondary 480V does only terminate on the disconnect but it is in another room about 70-80 away. 60A fused disconnect
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After looking this time - I think what you have is almost compliant.

Transformer is protected by the primary device, in fact would still be considered protected by a 225 amp device

You didn't mention primary conductor size but it would need to be 200 amp ampacity.

75 kVA would be 208 amps primary 90 amps secondary.

If you figure the primary to secondary ratio and consider you have a 200 amp protective device on the primary - the secondary conductor needs to be 86.6 amps - 4 AWG is just slightly too little as it's ampacity is 85 amps.

If the secondary conductor were 3 AWG I'd say it is compliant.
 
Smart $ said:
I know 240.21(C)(1) refers to delta-delta only, but do you know why wye-delta is not permitted?
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With a delta-delta, and overload on one phase on the secondary results in a corresponding overload on one primary phase, and the primary OCP should trip.

Add a neutral on the secondary, and I believe an overload on the secondary gets split between two primary phases. So the primary OCP may not trip.

Would adding a neutral connection on the primary would result in the same situation?

I'm not sure, and not willing to attempt the math to find out. Not sure if I could even do it. :)
 
steve66 said:
With a delta-delta, and overload on one phase on the secondary results in a corresponding overload on one primary phase, and the primary OCP should trip.

Add a neutral on the secondary, and I believe an overload on the secondary gets split between two primary phases. So the primary OCP may not trip.

Would adding a neutral connection on the primary would result in the same situation?

I'm not sure, and not willing to attempt the math to find out. Not sure if I could even do it. :)
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What about a wye primary, but with a floating neutral? Unbalance current has to involve more then one phase coil in that circumstance.
 
Transformer is a wye 208 but no neutral is connected to HO. Secondary is a 480V corner grounded Delta. By NEC is this application permitted without secondary protection for 80'. It lands directly into a 3 phase disconnect.
 
Dustin Foelber said:
Transformer is a wye 208 but no neutral is connected to HO. Secondary is a 480V corner grounded Delta. By NEC is this application permitted without secondary protection for 80'. It lands directly into a 3 phase disconnect.
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Can you cite the Code reference?
 
dkidd said:
Can you cite the Code reference?
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He's asking our opinion if this installation is allowed by code.


In short I say no.
In addition to that, I doubt the 480v side is actually a corner grounded delta.

JAP>
 
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