Transformer

Status
Not open for further replies.
meternerd said:
A long time ago, I posted about testing a 480 to 120v step down control transformer for ratio as a nuclear plant test technician. We had many that failed because we applied 120 to the secondary and measured for 480 on the primary. We got MANY wrong ratios. Someone here explained why. Reverse it to apply 480 and measure for 120 and they were correct. You might search for the thread. Just because they're rated for step-down, does not mean they'll work as step up and vice versa. Eye opener for me.
Click to expand...

Brian Blackadar said:
they are 25kva, I will run an EGC - I don't see a need for a neutral conductor?
Click to expand...

Brian -
Your plan will work fine. You are correct, there is no need for a neutral. The 480V link is two wire.

Other things you didn't ask about:
Consider grounding one side of the 480V at the step-up transformer. This will alleviate the need for any ground detectors (art 250.20).

The issue about the ratio not being exactly 2:1 (480/240) is of no concern. You are using the same model transformer to step-up as is used for step down. So it doesn't matter if the ratios are exact 2:1 - they are the same, that is all that matters.

I don't know if you are planning on the terminal end being 120/240. I ask because the application sounds like extending a residential service 1200' up an access road. If so, a couple of notes:
The first transformer is two wire to two wire. The primary OCP protects the first transformer, the 480V two-wire feeder, and the second transformer. The second transformer is two wire to three wire. You will need OCP on the terminal transformer secondary conductors/panel. A 150A main CB in the panel would work fine.

And you will need to bond the 120/240 neutral at the panel.​

I have helped put in a few residential extensions using the method you outline. They all worked fine.

ice
 
Do you need a disconnect for the first transformer's conductors near the first transformer and a primary disconnect for the second transformer?
 
david luchini said:
Big mistake. I'm finding that copious amounts of bacon keep my mind sharp (and my body round.)
Click to expand...

LOL.

My family and I spent a week camping, I am ashamed to admit how many pounds of bacon we used that week. (Normally we don't even have it in the house, too tempting)
 
ASG said:
Do you need a disconnect for the first transformer's conductors near the first transformer and a primary disconnect for the second transformer?
Click to expand...

I don't know the code answer for that. However, I would not expect to put in a feeder transformer secondary disconnect, the primary disconnect should be right close. However, for the terminal transformer, considering the feeder disconnect is 1200' away, I'd put a primary disconnect right next to the terminal transformer.

just thinking - not mandating

ice
 
Status
Not open for further replies.
Top