48Ov primary to 480v/277v transformer

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t9jrt7homas61

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I have a 480 volt primary and a 480v/277v secondary. The input amps and output amps are the same (273 amps)Do I have to have primary and secondary overcurrent protection
 
Probably yes.

Because your transformer has a wye connected secondary, it is considered a 'multi-voltage' transformer, so the primary cannot protect the secondary, see 240.20(C)(1).
 
It sounds to me like the transformer was installed to create a neutral for 277 volt loads.

It has straight 480 volt delta supplying it and 480Y/277 leaving it.

The voltage has not really changed.
 
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I can think of no other reason it has to be for 277 loads as Bob says, ie:277v lighting

Do you know specifically what this transformer is dedicated to?

dick
 
If the facility 'service' voltage is ungrounded, corner grounded, or high resistance grounded L-N loads are not allowed, so a 480-480Y/277 transformer is employed to create an isolated 4-wire system (usually for lighting).
 
jim dungar said:
If the facility 'service' voltage is ungrounded, corner grounded, or high resistance grounded L-N loads are not allowed, so a 480-480Y/277 transformer is employed to create an isolated 4-wire system (usually for lighting).
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In our faciltiy we have the same type of transformer for lighting, to keep the harmonics produced by no linear lighting off of our 480V system.
 
We use the 480-480/277 transformers to feed lighting circuits. We see several advantages:

1. Able to use High Resistance Grounding on the main 480 power systems.
2. 3-Phase, 3-Wire saves the neutral on the large feeders to MCC's and switchboards.
3. Transformer reduces short circuit current on lighting circuit, dropping the arc flash level and increasing safety.
4. Lower short circuit allows use of less expensive lighting panel circuit breakers.
5. Process loads and lighting loads are kept on separate panels. An electrician trying to find or kill a lighting circuit doesn?t dump the plant.
 
t9jrt7homas61 said:
I have a 480 volt primary and a 480v/277v secondary. The input amps and output amps are the same (273 amps)Do I have to have primary and secondary overcurrent protection
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You will need protection on each side, but you have two choices. You can protect the "primary only" per T450.3(B) and use a max 350A c/b in the primary and use any size c/b on the secondary, such as if you wanted to feed a 400A mcb panel.

Or you can use "primary and secondary" protection per T450.3(B) and use a max 350A c/b on the secondary side and a max 600A c/b on the primary side.

Though I can't see any reason not to just use 350A OCPD on both primary and secondary.
 
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rcwilson said:
We use the 480-480/277 transformers to feed lighting circuits. We see several advantages:

1. Able to use High Resistance Grounding on the main 480 power systems.
2. 3-Phase, 3-Wire saves the neutral on the large feeders to MCC's and switchboards.
3. Transformer reduces short circuit current on lighting circuit, dropping the arc flash level and increasing safety.
4. Lower short circuit allows use of less expensive lighting panel circuit breakers.
5. Process loads and lighting loads are kept on separate panels. An electrician trying to find or kill a lighting circuit doesn?t dump the plant.
Click to expand...

Can we call it tertiary winding transformer ?
 
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