Transformer Protection

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mull982

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Hello

I have a contractor that needs to put a temporary transformer out in the field in order to run some equiptment for new construction, and wants to feed the transformer from an MCC in one of our electrical rooms. The transformer is a 30KVA transformer and would be fed with 480V from the MCC. I wanted to verify what the proper size of the overcurrent protective device on the primary side of the transformer should be?

I calculated the current draw for this transformer to be about 36 Amps and according to the NEC it states that for a transformer of 600V nominal or less, and a current draw of 9 amps or more, the primary protective device should be rated at 125% of the transformer current. I calculated this to be 45A so was going to reccomend using a 45A breaker to feed the transformer from the MCC. Is this the correct size breaker to use for this application?

Also, do I need protection on the secondary side of the transformer? What size wire do you suggest using?

Thanks for the help

Mull982
 
Mull, You need a little more info to size secondary side. Voltage of secondary and is this 1 phase or 3 phase.

Yes you need secondary ocp and you can size secondary transformer protection at 125%.
Rick
 
The secondary side is is 3 phase 120V. Ok I so it looks like I should size the secondary breaker at 125% as well. Is the 125% breaker rating on the primary side correct? Any suggestions on wire size?
 
If your primary side OCPD is not greater than 125% of the transformer then secondary protection is not required. However your secondary conductors do need to be protected, either by an OCPD or by a tap rule. A panel on the secondary may also require a main OCPD, but it can mounted remote.

If your primary OCPD is larger than 125% then a secondary OCPD is required. This OCPD may also protect the conductors and any panel.
 
To correctly size the OCPD, the instantaneous setting must be something greater then the inrush of the transformer (so it does not trip on energization, and the long time and short time settings need to be lower then the transformer damage curve.

Simply setting the protection by using the NEC percentages, without knowledge of the inrush, or damage curve, could lead to both nuisance tripping and equipment damage.
 
jim dungar said:
If your primary side OCPD is not greater than 125% of the transformer then secondary protection is not required. However your secondary conductors do need to be protected, either by an OCPD or by a tap rule. A panel on the secondary may also require a main OCPD, but it can mounted remote.

If your primary OCPD is larger than 125% then a secondary OCPD is required. This OCPD may also protect the conductors and any panel.
Click to expand...
Anyone know of a version of the NEC written in plain English like this? Well done, Jim.
 
For my info, can you tell me the correct size of conductors for a 75KVA TXFM. 3ph. Pri. 480vac 90.2amps. Sec. 208.2amps.
125amp OCPD Pri. 200amp OCPD Sec.
 
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