Feed thru lugs feeding step up transformer

[Rhazel]

Hi everyone. I couldn't find my answer via the search and hope someone can help.

Is it permissible to use the feed thru lugs to feed a transformer?

600A MLO panel fed with (2) 350mcm > feed thru with (2) 350mcm to 150 KVA step up transformer > (2) 100A fused disconnects on secondary feeding chargers.

All lengths are 10' or less.

 

[infinity]

The feed through lugs are protected by the 600 amp OCPD?

 

[Rhazel]

Yes, correct

 

[jim dungar]

Your protective device, supplying the feeds through panel and lugs, needs to comply with the transformer primary side sizing requirements of 450.3.

 

[jap]

Doesn't it though?

I'm assuming 208v primary at 519 amps and next size up.

Granted, wouldn't leave much power for whatever the 600 amp panel was feeding in the first place, but,


JAP>

 

[jim dungar]

I'm assuming 208v primary at 519 amps and next size up.

I don't know that rule.

My experience has been step up transformers have high amounts of in rush current and likely need protection sized more than 125% of FLA.

But the basic idea outlined in the OP could be NEC compliant.

 

[Rhazel]

My reasoning; and feel free to make corrections:
1. True step up transformers windings are compensated unlike reverse wired step down. Link at bottom.
2. Need to size @250% to comply with the 600A ocpd.
3. 150KVA is good for around 180A @ 480V. I require around 140A. This should put me roughly around 415A on the primary 208V side. This should leave plenty for the 600A panel?
4. I will have (2) fused 100A disco w/ 80A fuses on secondary. I believe this satisfies the secondary protection.
5. I see feed thru's for panels all the time but not for equipment or transformers. Is there anywhere that specifically prohibits this?

 

[jim dungar]

5. I see feed thru's for panels all the time but not for equipment or transformers. Is there anywhere that specifically prohibits this?

No.
I have often suggested this type of system as one way to comply with specifications requiring full selective coordination.

My previous point about the 'next size up rule' is it should be not applied to transforms if it exceeds the 125% or 250% limits in 450.3.

 

[Rhazel]

Jim, i appreciate your reply and went through 450.3(B) as you suggested.
My previous point about the 'next size up rule' is it should be not applied to transforms if it exceeds the 125% or 250% limits in 450.3.
Is there anywhere that says the next size up rule shall not apply?
Would 240.4(B) not apply?
240.4(B) Overcurrent Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.

 

[jim dungar]

240.4(B) addresses protection for conductors.
450.3 addresses protection for transformers and includes the adjective maximum.