XFMR secondary OCPD

[jim dungar]

Re: XFMR secondary OCPD

I don't know why it uses the phrase "required".
My guess is - if the primary protection happens to be greater than 125% then of course secondary protection of 125% is required.

Again my guess, the code assumes that a primary protective device rating of 125% max. will always protect the secondary windings of 600V or less transformers regardless of the configuration. The secondary conductors themselves are protected by 240.4(F) and not by anything in 450.3(B)

My interpretation is: Adding a secondary protective device is an option that allows you to use the second row of table 450.3(B), which allows the primary to be sized larger. My point is, there is nothing in the NEC 450 that says you must exceed a 125% primary OCPD.

Maybe my argument is nothing more than what comes first, the chicken or the egg? So my reasoning is; start with the panel 408.36, then protect the conductors 240.4(F), and finally choose the applicable row of table 450.3(B).

forgot the word "not"

[ December 06, 2005, 06:21 PM: Message edited by: jim dungar ]

 

[George Stolz]

Re: XFMR secondary OCPD

Originally posted by charlie b:
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• <font size="2" face="Verdana, Helvetica, sans-serif">VA = 1.732 x V x I</font>

<font size="2" face="Verdana, Helvetica, sans-serif">Now, what do you know, and what are you trying to find? In this case, you know the rating of the transformer, in units of VA. You also know voltage. You are looking for current.

Ooooooooooohhhhhhhhhhh. I get it. Thank you for spelling it out like that, that's clear.

 

[George Stolz]

Re: XFMR secondary OCPD

I will have to ponder the rest later in the evening.

 

[sceepe]

Re: XFMR secondary OCPD

Bob,
When you said:

you are not allowed to go the the next size breaker on the secondary.

I thought that note 1 of table 450.3(B) lets you do this. For example, with the 75 KVA dry type in question, 1.25*208=260A. So can you use a 300 amp secondary OCPD?

 

[bob]

Re: XFMR secondary OCPD

sceepe

I thought that note 1 of table 450.3(B) lets you do this. For example, with the 75 KVA dry type in question, 1.25*208=260A. So can you use a 300 amp secondary OCPD?

You can increase the breaker to 300 amps but you must use conductor with 300 amp capacity.
In your case #350 is required.
We are used to seeing a 400 amp panel with 500kcm feeding a 400 amp breaker. With transformers that is not allowed.
See 240.21.C.2b