ECN Forum Forums Code Related Discussion NEC & other Code issues Transformer Installations

Perhaps I'll post an article I wrote on transformers here after this dies down a bit

Ryan that would be great, I started this thread for education purposes (my own education included) and your input is more than welcome.

I do like your idea of letting this run as is for a bit.

Bob



[This message has been edited by iwire (edited 09-08-2005).]

In my opinion, creating a true Isolated Grounding system is best done by running an IG to the transformer itself.

How do I size it, since it is not an equipment bonding conductor (according to a freind of mine on panel 5), and therefore 250.30(A)(2) doesn't apply?

Ryan you lost me on that one, I do not see how an IG and 250.30(A)(2) would come up in the same conversation.

I have always used Table 250.122 to size IG conductors.

Typically that would mean a 12 AWG all the way back to the transformer XO terminal would be adequate as the OCP that IG will need to open will be a 20 amp. Very few IG circuits are larger than 20 amps.

Generally I have used a 6 AWG back to XO from a IG bar in the panel just because a 12 landing on XO of a 75 KVA transformer looks bizarre.

Thinking on this some more and it seems a strict reading of 250.122(B) would require the IG for a 20 amp circuit run back with the feeder to be the same size as the feeders ungrounded conductors.

Bob

What happens if you have a phase to IG fault in the secondary raceway? Granted, that is a remote possiblity, but you could have an issue trying to open the primary protection device with a 12 AWG conductor.

I don't think the code addresses this. I am trying to come up with a proposal to address it, but I am at a loss as to how to do it.

What happens?

Hmmm, if I run a 12 AWG all the way back I would say the 12 AWG IG opens at the time of fault and the enclosures of all the IG equipment become 'hot' for just a moment or possibly permanently if the small IG opens on the line side of the fault and welds to an ungrounded conductor on the load side of the fault.

Not real good.

Perhaps a simple proposal that says the IG that is allowed to run back to XO must be sized per 250.122 for whatever OCP protects the the conductors in the same raceway.

No person shall operate a motor vehicle on a road or highway at a speed that is greater than is reasonable and prudent to the existing conditions.

I just for the life of me can't figure why in this world that who ever did this job would want them black wires flopping around between them windings like they are. Is this a problem? Could this cause some kind of induction in them wares? Would this cause them wares to get hot?

Now Bob you promised not to whop me ifen my questions hant no good.

smiley face goes here

edited to explain: My Granddaughter is taking drivers ed and dared me to post a lesson in here

[This message has been edited by jw electric (edited 09-08-2005).]

Welcome new member once your in the 'club' we will let you know the secret smile codes.

You will have to ask the factory about the black wires up around the coils.

Usually a transformer like this will not allow field wiring above the H and X terminals.

Go here https://www.electrical-contractor.net/ubb/faq.html#smilies to learn the smile rules.

Bob

Thank you Bob

You said ask questions so i tried to ask a stupid one for my first, did I do good?

When I see you ask a question like that I usually wait for the other shoe to drop.

A guy as knowledgeable as yourself will many times have something up his sleeve.

Bob