circuit protection for new branch

[QGeek]

I need to modify a piece of equipment to include a circuit for an additional piece. The equipment is powered at 460VAC, 3-phase. The original equipment contains one step-down xfmr to 120VAC, and is protected by two primary and one secondary fuses. The add-on is 120VAC, nearly 10A max inrush. It is 120VAC input with a controller that powers a couple of 24VDC motors. Can someone tell me, or point me to the standard for circuit protection?

 

[QGeek]

I found a great article on sizing circuit protection for transformers and their loads:

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1492-in001_-en-p.pdf

In it, a helpful standard is listed:
Circuit breaker protection for the primary side of the transformer can be
sized per NEC (National Electrical Code) Article 450-3(b)(2) which states:
”All transformers of 600V, nominal, or less, shall be protected by an
individual overcurrent device on the primary side . . .”

 

[DRWeig]

Be careful, there...

It would be most helpful if you posted a schematic of your proposed system. Then we can give you advice if needed.

Best to you,

Goober Dave

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[iceworm]

If you (and the equipment) are in NEC land, then yes, NEC 450 is good. However, for small control transformers, the max fuse allowed is pretty high.

Unless you are inside of a panel - then you need to follow the panel listing, maybe look at UL508A

Unless you can't (follow the panel listing) - then probably NEC 409, NFPA 79.

Personally I like tight primary and secondary protection of control transformers. Smoked (overloaded or internally failed) control transformers have a nasty habit of burning up the insides of control panels - while not manageing to open the fuses. Nearly iratating.

Some likely would think it is way overkill, but I don't mind getting out a fuse TCC, and verifying the energization is within the fuse rating - especially if I had not done one before.

This way you could consider it engineering - not design by regulation, which usually is not a good idea.

ice

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