112.5 KVA / 480volt to 120-208 volt transformer wire sizing ?

Davebones

Senior Member
We had to relocate a 112.5 KVA transformer on the floor that feeds a milling machine . Machine is rated max 270 amps at 208 volts . Realized this transformer has been fed on the primary side with #1 thhn wire fused at 125 amps . The secondary wire is two #1/0 per phase into the machine to the main 300 amp breaker . It looks like the wire was sized one size smaller looking at the machine amperage . This has been like this for probably 20 years with no issues . Is this safe to continue to use this transformer in this configuration ?
 

JoeStillman

Senior Member
Location
West Chester, PA
The wires are protected at their ampacity by the 300A CB. The transformer is protected from overload by both the primary and secondary breakers. Remember that the breaker sizes in table. 450.3(B) are maximum sizes. A transformer only draws the current required by the load on it. It is not a load by itself. The only thing I would worry about would be inrush current on the primary, but that seems to already be coordinated since nobody has ever complained.

IMHO, it is safe to use that way.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
We had to relocate a 112.5 KVA transformer on the floor that feeds a milling machine . Machine is rated max 270 amps at 208 volts . Realized this transformer has been fed on the primary side with #1 thhn wire fused at 125 amps . The secondary wire is two #1/0 per phase into the machine to the main 300 amp breaker . It looks like the wire was sized one size smaller looking at the machine amperage . This has been like this for probably 20 years with no issues . Is this safe to continue to use this transformer in this configuration ?
270 Amps at 208 V is less than 100 kva. I would not be worried about the transformer.
 

Davebones

Senior Member
So this transformer wiring was sized to what the machine was rated . If you are installing a 112.k kva 480v to 120-208v transformer for panel boards I believe that primary wire size would need to be 1/0 per code . This transformer now has #1 primary and two #1/0 secondary per phase . I was asked by another electrician would using smaller wire be considered code compliant when feeding just one machine ? This is in a manufacturing plant and the transformer only feeds the one machine . I'm not sure how to answer his question .
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The conductors have to be sized so they are protected by their respective over current protection devices. Two 1/0 conductors are properly protected by a 300 amp breaker.

The number one conductors on the primary side are properly protected by the 125 amp fuse. I don't see any problems here.

If you made the wire smaller, you would need to adjust the overcurrent protection device rating accordingly.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I was asked by another electrician would using smaller wire be considered code compliant when feeding just one machine ? This is in a manufacturing plant and the transformer only feeds the one machine .
It isn't a question of 1 machine vs a panel.

It is a question of: is the transformer protected and are the wires protected.

IMHO this transformer was installed with wire and OCPD as if it had a 100kVA rating. The wires are protected at their ampacity and the transformer protected by OCPD less than the maximum permitted. So kosher on that point.

If the same wire and OCPD were used to feed a panel it would also be fine, however with unused transformer capacity.

About the only possible issue is if the machine is a 270A continuous load, in which case the OCPD and the wires would both need to be larger.

Jon
 

Davebones

Senior Member
Well after relocating the machine we have a distance issue .Is 25ft not the max length for secondary with out over current protection ? The machine disconnect is 25ft across the floor from the transformer but the secondary wire goes up to the ceiling and down to the machine making it about 35 ft long . Am I correct we can just add a set of fuses at the transformer to protect the secondary wires . This is and industrial manufacturing plant . Probably never have an issue with the way it is ..
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Well after relocating the machine we have a distance issue .Is 25ft not the max length for secondary with out over current protection ? The machine disconnect is 25ft across the floor from the transformer but the secondary wire goes up to the ceiling and down to the machine making it about 35 ft long . Am I correct we can just add a set of fuses at the transformer to protect the secondary wires . This is and industrial manufacturing plant . Probably never have an issue with the way it is ..
that would seem to be the simplest solution.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Well after relocating the machine we have a distance issue .Is 25ft not the max length for secondary with out over current protection ? The machine disconnect is 25ft across the floor from the transformer but the secondary wire goes up to the ceiling and down to the machine making it about 35 ft long . Am I correct we can just add a set of fuses at the transformer to protect the secondary wires . This is and industrial manufacturing plant . Probably never have an issue with the way it is .
The secondary length may be able to exceed 25' if the installation is a Supervised Industrial Installation per the definition in Article 240.
 
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