Temp Scenario

[Alwayslearningelec]

I know this is exhausting cause it sure is for me.
So they changed the location of my temp 45 kva transformer.
Instead of 100' away from breaker feeding it they want it right next to the panel/breaker feeding it.
I do have some concerns with others plan as indicated below.
So.......
Loads are
(3) temp trailer panels 120/208v single phase.
Trailer A - load current draw is ~80A
Trailer B - load current draw is ~35A
Trailer C - load curent draw is ~35A

1. 277/480V 3P-70A breaker feeds primary of 45KVA step down xfrm 5' away.
2. 120/208v side of 45kva xfrm #3/0 goes to splice box 100' away. My concern here is don't you need a disconnect on the secondary side here and I believe a splice would not be allowed. We are splicing the 3/0 to (3) SER cables, a 3/c #2/0, and (2) 3/C #4
3. We are running those cables to (3) exterior disconnect switches.

 

[augie47]

You have to abide by one of the 240.21(C) tap rules. In your situation, likely the 10' or 25' rule.
You can run from your secondary disconnect (required above) to one location and then use the 240.21(B) tap rules to your individual trailer fusible disconnects.

Short primary and long secondary is the LEAST cost effective but that may not be your call.
(The transformer outside by your disconnects would be the most ideal)

 

[Alwayslearningelec]

You have to abide by one of the 240.21(C) tap rules. In your situation, likely the 10' or 25' rule.
You can run from your secondary disconnect (required above) to one location and then use the 240.21(B) tap rules to your individual trailer fusible disconnects.

Short primary and long secondary is the LEAST cost effective but that may not be your call.
(The transformer outside by your disconnects would be the most ideal)

I had it so we were distributing for 480 V further, which would’ve been most ideal as you said, but it was changed as the owner now wants the transformer right by the panel, so we have a very short primary

 

[augie47]

His money 10 ft secondary to fused disconnect to j box with taps to your disconnects

 

[infinity]

His money 10 ft secondary to fused disconnect to j box with taps to your disconnects

I didn't see this in his installation even though it would be required.

 

[Alwayslearningelec]

Like I said we are now running one set of secondary conductors to a splice box and splicing down to (3) sets of cable that will feed (3) disconnects that ill then feed 120/208v single phase trailer panels.
The disconnects are 3 pole(only thing we could get in time...NEMA 3R) but we are only using 2 poles.

The foreman is saying he needs to run 3 phase conductors from the xfrmr to balance out the phases. Is that correct?

I'm only asking because the disconnect are only requiring 2 phase conductors so I'm confused. Maybe when he splices from the one cable to the three something can be done with the extra phase conductor.

 

[infinity]

You have 3.phases A/B, B/C and A/C. What you've proposed is not code compliant.

 

[Alwayslearningelec]

You have 3.phases A/B, B/C and A/C. What you've proposed is not code compliant.

What do you mean what I proposed? Which scheme or part?

Your 3 phases are A,B.C.

 

[wwhitney]

What do you mean what I proposed? Which scheme or part?

Have you read 240.21(C) as Augie indicated in the first response? Here's a direct link for you:

https://up.codes/viewer/new_jersey/nfpa-70-2020/chapter/2/wiring-and-protection#240.21_(C)

Cheers, Wayne

 

[Alwayslearningelec]

Have you read 240.21(C) as Augie indicated in the first response? Here's a direct link for you:

https://up.codes/viewer/new_jersey/nfpa-70-2020/chapter/2/wiring-and-protection#240.21_(C)

Cheers, Wayne

Thank you. I read through but honestly it's so hard to understand and comprehend some of the code;
What I'm vaguely getting is that the splice box/splices for the secondary conductors can't be more than 25'?

 

[augie47]

In your scenario you must have secondary ovwercurrent protection within 25 of your transformer. You need a breaker or 3 pole fused disconnect. From there you run 3 phases, neutral and equip ground to you j box and tap to your individual disconnects per 240.21 tap rules.

 

[infinity]

What do you mean what I proposed? Which scheme or part?

Your 3 phases are A,B.C.

The 25' rule part.

Your three loads are single phase 208/120 so they should be connected 1) A-B, 2) B-C, and 3) A-C.

 

[don_resqcapt19]

In your scenario you must have secondary ovwercurrent protection within 25 of your transformer. You need a breaker or 3 pole fused disconnect. From there you run 3 phases, neutral and equip ground to you j box and tap to your individual disconnects per 240.21 tap rules.

My guess would be that this is an outside installation, and if so, there is no length restriction on the secondary conductors.

 

[infinity]

My guess would be that this is an outside installation, and if so, there is no length restriction on the secondary conductors.

Regardless of length, can you run one set of conductors from the transformer secondary and then splice on three other sets before any OCPD?

 

[don_resqcapt19]

Regardless of length, can you run one set of conductors from the transformer secondary and then splice on three other sets before any OCPD?

No, I am not suggesting that. If you run one set of conductors, you must hit an OCPD before splitting off to the three loads. The OCPD will trigger another outside unlimited length for the tap conductors on the load side of the OCPD.

One of my instructors over the years, like to call this the "million feet" tap rule.

 

[wwhitney]

My guess would be that this is an outside installation

Pretty sure from earlier posts on this project that the primary side panel is indoors. Which means that with the transformer 5' away, it would be indoors as well.

Seems like it would make a lot more sense to set the transformer outside by the 3 new secondary disconnects.

Cheers, Wayne

 

[Alwayslearningelec]

Everything indoors to the splice box the. After that outdoors

 

[Alwayslearningelec]

Regardless of length, can you run one set of conductors from the transformer secondary and then splice on three other sets before any OCPD?

No, I am not suggesting that. If you run one set of conductors, you must hit an OCPD before splitting off to the three loads. The OCPD will trigger another outside unlimited length for the tap conductors on the load side of the OCPD.

One of my instructors over the years, like to call this the "million feet" tap rule.

Yes from the secondary we are going to a fused disconnect within 10’

 

[Alwayslearningelec]

The 25' rule part.

Your three loads are single phase 208/120 so they should be connected 1) A-B, 2) B-C, and 3) A-C.

My confusion was I didn’t think it was possible or “electrically” correct to splice/tap a 3 phase conductor cable to (3) cables each with 2 phases.

 

[Tulsa Electrician]

My confusion was I didn’t think it was possible or “electrically” correct to splice/tap a 3 phase conductor cable to (3) cables each with 2 phases.

Sure you can. Your tapping single phase from a three phase four wire feeder.

Just follow the tap rules on sizing.