OCPD Load side of Isolation Transformer

[Mddocstanley]

First my question is must I have overcurrent protection on the secondary side immediately at the transformer?
I have a 7.5kva isolation transformer/panel that is required for equipment.
Supply voltage is 208V 1Ph and secondary will be 208V 1Ph
Conductor size will be #8cu xhhw
The paramaters being dealt with dictate that the isolation panel and the transformer cannot be built as one unit, the transformer will have to be remote from the panel.
According to Art.450-3(b) if I keep the OCPD on the Line side 125% or less as a percentage of the transformer rated current (@36amps so 45amps or less) then
I will not need secondary protection on the conductors extending to the panel.
Art. 240.4(f) requires OCPD on the secondary with 2 exceptions and I believe
that this qualifies as one of the exceptions.
That exception being that it is single phase and 2-wire.
Thanks for any input.
MD

 

[raider1]

According to Art.450-3(b) if I keep the OCPD on the Line side 125% or less as a percentage of the transformer rated current (@36amps so 45amps or less) then
I will not need secondary protection on the conductors extending to the panel.

Actually you are reading that section wrong. 450.3(B) only deals with secondary protection of the secondary windings of the transformer not the secondary conductors to the panelboard.

For secondary conductor protection check out 240.21(C).

Also most likely the panelboard will have to have it's overcurrent protection device located on the secondary side of the transformer.

Take a look at 408.36(B).

Chris

 

[don_resqcapt19]

In this case, with a two wire to two wire transformer, 240.21(C)(1) applies and the primary OCPD can be used to protect the secondary conductors. While the code is not clear on the use of the primary device to protect the panel, I see no technical or safety reason why it can't in the case of a two wire to two wire transformer.

 

[Mddocstanley]

Just a note before I have to run.
I do know that when this unit is built as a single piece that there is an OCPD
on the secondary side ahead of the busbars. I am trying to get info on how this particular setup will be built.
I believe from what I have already read that with the current being 36 amps and the ratio being 1, that I can install a 35 amp OCPD ahead of the transformer and meet all requirements for the transformer and the conductors.
This also would be ok for the single circuit that would be taken from this panel as the unit has a 30 amp load requirement.

Thanks all for the great input.
MD

 

[Mddocstanley]

Misread again

Misread again

On my last post I made an error I believe.
In 240.21(C)1 I misread "multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio."
I misread it to mean the secondary ampacity of the transformer.
So since the conductor ampacity is 50amps at 75deg.C and the ratio is still 1,
then the Primary overcurrent protection can be 50 amp.
I really appreciate this site as each time I come here I am enticed to study and think.
Something that as I get older I seem less inclined to do.
MD