Grounding secondary of a 30KVA single phase transformer 480 to 208

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
With no tap on the secondary, your only options are to ground either line, or leave it floating.

I wonder if there's a way to derive a "center tap" for you to ground?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I think this transformer is a poor choice due to not having a center tap for grounding. Yes, you can use this transformer and ground one end of the secondary winding but it is going to complicate things to do it this way.
 

Mezani

Member
Location
Washington
The only way to derive a center tap is to send it back to the manufacturer. Texie, what would complicate this. It seems as though 240.21(C)(1) would allow going directly from the transformer to the equipment without overcurrent protection on the secondary. (#4 Cu secondary conductors) I did consider 250.20(B) (1) not allowing it as it is 208 to ground.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
The only way to derive a center tap is to send it back to the manufacturer. Texie, what would complicate this. It seems as though 240.21(C)(1) would allow going directly from the transformer to the equipment without overcurrent protection on the secondary. (#4 Cu secondary conductors) I did consider 250.20(B) (1) not allowing it as it is 208 to ground.
The issue is that the secondary has to be grounded in compliance with the NEC but in your case you will now have a hot and a grounded conductor going the the appliance instead of 2 hots. This will be a problem in some equipment that may have fuses in the appliance as you can't fuse a grounded conductor. It is also non standard and will be confusing to anyone working on this.
You also will need a GES. The correct way to do this is with a center tap transformer terminating in a fusible disconnect where you can bond the center tap and connect the GES. Then take an EGC to the appliance.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
Hopefully Texie or another member will correct me if need be.
As I read 250.21, you could operate this as an un-grounded system and install a grounding-electrode system per 250.32(B), a SSBJ per 250.32(A) and a ground detector per 250.21(B)
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Hopefully Texie or another member will correct me if need be.
As I read 250.21, you could operate this as an un-grounded system and install a grounding-electrode system per 250.32(B), a SSBJ per 250.32(A) and a ground detector per 250.21(B)
That is correct, as the system would be greater than 150V to ground.
This is really no different than the case of transformer with a delta output of 240V 3-Phase 3-wire.

Probably this transformer is dedicated to a specific machine. Hopefully the machine is only a 2-wire input.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Hopefully Texie or another member will correct me if need be.
As I read 250.21, you could operate this as an un-grounded system and install a grounding-electrode system per 250.32(B), a SSBJ per 250.32(A) and a ground detector per 250.21(B)
Well, yeah, you could do this but seems far more complicated and expensive than just getting the correct transformer.
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
It almost looks like this transformer is meant to be a single phase STEP UP transformer instead of being a STEP Down transformer. But it does have the TAPS to adjust for the Primary voltage. Other wise you have to comply with a whole bunch of stuff and have only qualified people in the area of the transformer. Either get it changed by the manufacturer and have a center tap added to the LV side or get a new transformers.

Does the machine its hooking up to have a voltage range "IE" 208-230" or 240v. That makes finding the correct transformer a lot easier.

Here is an ACME 480 v to 240v Single phase, 25 KVA, with tap for ground

ACME 480v to 240v Single PHase, 37.5 KVA, w/ tap for ground
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
It almost looks like this transformer is meant to be a single phase STEP UP transformer instead of being a STEP Down transformer. But it does have the TAPS to adjust for the Primary voltage.
No; it's labeled 480-208 and calls the HV the primary in the tap table.
 

Electricmo

Senior Member
Location
Missouri
Occupation
Lineman
OP stated this feeds a two wire cooking appliance. He must be in a building that has existing 480. Looks like he needs to run two wires to appliance. A neutral and hot. This transformer will work.
 

Jamesco

Senior Member
Location
Iowa
Occupation
Master Electrician
OP stated this feeds a two wire cooking appliance. He must be in a building that has existing 480. Looks like he needs to run two wires to appliance. A neutral and hot. This transformer will work.
This is feeding a cooking appliance that only requires 2 wires.

It could also could be fed by 2 ungrounded 208V conductors.

It would be nice if the OP could provide a wiring diagram for the piece of "cooking appliance".
I assume the piece of equipment will be hardwired.

Who furnished, ordered, the transformer? Electrical contractor? Other?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
How is this any different than a glorified Buck/Boost Transformer?
A buck-boost is slang for an autotransformer arrangement of windings. In an autotransformer there is a conductor which is common to both the HV and the LV sides of the transformer. The output is not a separately derived system

The OP has an isolation transformer which does not have a conductor (other than ground) common to both sides of the transformer. The out is a separately derived system.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't see how this would be in violation of the code. I don't see how 250.21 (B) would require this system to be grounded.

250.21 (B) Alternating-Current Systems of 50 Volts to 1000 Volts.
Alternating-current systems of 50 volts to 1000 volts that supply
premises wiring and premises wiring systems shall be grounded
under any of the following conditions:
(1) Where the system can be grounded so that the maximum
voltage to ground on the ungrounded conductors does
not exceed 150 volts
(2) Where the system is 3-phase, 4-wire, wye connected in
which the neutral conductor is used as a circuit conductor
(3) Where the system is 3-phase, 4-wire, delta connected in
which the midpoint of one phase winding is used as a
circuit conductor
Clause (1) would be the operative clause but since the transformer does not have a center tap on the secondary it can't be grounded there so (1) does not apply.

A better argument against this idea would be the listing instructions of the appliance. It might well require a grounded supply. However, it might also require that grounded supply be two lines of a 3 phase system, rather than a single phase system.
 

jap

Senior Member
I see problems with this transformer in more ways than 1.

Most pieces of equipment that are 208v Single Phase are dual voltage 208/240v.

If so, I'd be looking hard at a 480/ 120/240v single phase transformer that I could ground, keep the voltage stable, and, reduce my worries about it.


JAP>
 
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