Xfrmr Wire Sizes

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Alwayslearningelec

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So I thought the primary and secondary transformer( 15,30 & 75kva) wire sizes were always the same sizes not matter the OCPD feeding or the load being fed. No?
 
As Bob said the wire sizes can vary. On the secondary side the wire has to be greater than or equal to the OCPD size. There is no applicable next size you rule.
 
electrofelon said:
Is not it both? Primary conductors are sized to the serving OCPD, which in turn is sized to 450.3. there is provision to feed a transformer with a feeder tap, but I'd have to look at that again. Secondary is sized to 240.21(C) and that OCPD might also need to be sized to 450.3.
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IMO, you use 240.21(C) to size the secondary conductors and their OCPD, to the load, and then use 450.3 to size the primary OCPD, finally you size the primary conductors.
 
infinity said:
Code reference?
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450.3 Overcurrent Protection. Overcurrent protection of
transformers shall comply with 450.3(A), (B), or (C).

(B) Transformers 1000 Volts, Nominal, or Less. Overcurrent
protection shall be provided in accordance with Table
450.3(B).

1647617867069.png

The next size up rule only applies to the secondary, unless you are dealing with primary only protection and that rarely if ever makes any sense.
 
petersonra said:
The next size up rule only applies to the secondary, unless you are dealing with primary only protection and that rarely if ever makes any sense.
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The question was about transformer wire sizing, not transformer overcurrent protection sizing.

On the primary, the conductor can be protected by the next size up OCPD (800A or less.)

On the secondary, the conductor cannot be protected by the next size up OCPD.
 
david luchini said:
The question was about transformer wire sizing, not transformer overcurrent protection sizing.

On the primary, the conductor can be protected by the next size up OCPD (800A or less.)

On the secondary, the conductor cannot be protected by the next size up OCPD.
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I thought that only applied if it was a tap to feed the transformer.
 
david luchini said:
The question was about transformer wire sizing, not transformer overcurrent protection sizing.

On the primary, the conductor can be protected by the next size up OCPD (800A or less.)

On the secondary, the conductor cannot be protected by the next size up OCPD.
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I agree. The table Bob post would be used to size the conductors but on the secondary the next size up rule cannot be used.
 
Wouldn't Primary only protection be used in instances where the primary OCPD would protect the secondary conductors?
 
xformer said:
Wouldn't Primary only protection be used in instances where the primary OCPD would protect the secondary conductors?
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Yes, but it can also be used on transformers with secondary conductor protection.

For example, if you had a 30kVA, 480-208/120V transformer with the secondary supplying a 125A MCB panelboard. The 125A secondary c/b exceeds the maximum allowable secondary protection for "Primary and Secondary Protection." So the primary c/b size would be limited to the maximum allowable under the "Primary Only Protection."
 
xformer said:
Wouldn't Primary only protection be used in instances where the primary OCPD would protect the secondary conductors?
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Those are rare. You can only do that on a two wire to two wire transformer or a 3 wire delta to delta transformer.
 
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