Transformer secondary conductors

[wwhitney]

Could I have connected three sets of lower ampacity conductors instead of two to the end of the feeder in the 480V gutter without "tapping a tap"?

Yes, an arbitrary number. When "end of the feeder" means the end of the 400A conductors, which is the end of the portion of the feeder that is not a feeder tap.

Cheers, Wayne

 

[don_resqcapt19]

Got it. Thanks for your patience; I'm asking these questions because the what is easier to remember if I understand the why.

Could I have connected three sets of lower ampacity conductors instead of two to the end of the feeder in the 480V gutter without "tapping a tap"?

That is based in the 400 amp conductors being protected in accordance with 240.4. If that is the case, the 400 amp conductors are feeders and can supply any number of sets of tap conductors.

 

[ggunn]

That is based in the 400 amp conductors being protected in accordance with 240.4. If that is the case, the 400 amp conductors are feeders and can supply any number of sets of tap conductors.

OK, so the "cannot tap a tap" argument against three 200A conductors on the transformer secondary was incorrect? Apparently it was because of the OCPD location(s), not the number of tap conductors; if I had had a 600A OCPD on the transformer secondary I could have split the 600A conductors on its load side three ways, correct?

 

[wwhitney]

OK, so the "cannot tap a tap" argument against three 200A conductors on the transformer secondary was incorrect?

Literally yes, as transformer secondaries aren't tap conductors, per the definition. But correct in spirit.

A tap conductor has OCPD conductively upstream of it, but at too high a value to comply with 240.4. Transformer secondary conductors have no OCPD conductively upstream of them.

Thus neither one complies with the first sentence of 240.21. The only way to supply other conductors from them is through an OCPD that complies with 240.4.

So once you add a transformer into the mix, the "can't tap a tap" rule becomes "can't tap a secondary conductor". Not quite as catchy as "can't tap a tap" but same idea.

Cheers, Wayne

P.S. In case it helps, let me restate the big picture: for any conductor segment, you can look at it and say "does it have OCPD that complies with 240.4, and is that OCPD located at the supply end of the segment"? If yes to both, that complies with the first sentence of 240.21 and thus all of 240.21. If not, then you have to find the applicable subsection of 240.21 and comply with the restrictions therein.

 

[ggunn]

So my drawing in post #38 is good to go, right?

Thanks again for y'all's help; as long as I have been designing PV systems I have run into the occasional experienced electrician who has gotten all balled up in code questions because the current is flowing the other way. This time the shoe is firmly on the other foot.

 

[don_resqcapt19]

So my drawing in post #38 is good to go, right?

Thanks again for y'all's help; as long as I have been designing PV systems I have run into the occasional experienced electrician who has gotten all balled up in code questions because the current is flowing the other way. This time the shoe is firmly on the other foot.

Almost, but the supply side bonding jumpers that you show as grounding conductors for the transformer secondary conductors are too small. They need to be sized per Table 250.102(C)(1) and that will require that they be 4 AWG. If they were equipment grounding conductors, the 6 AWG that you show would be ok, but they are on the line side of the 200 amp OCPDs, so they are supply side bonding jumpers and not EGCs.

 

[augie47]

Has nothing to do with your transformer but you show a #8 Ground from your gutter to the line side of your 100 amp disconnect.
The #3 tap conductors are protected by the 400 amp breaker so the equipment ground should be #3
(If the phase conductors were smaller the equipment ground could be smaller)
On the load side of yiour 100 amp breaker #8 is fine,.

 

[wwhitney]

What about the question of whether the 300 kVA transformer with nominal primary current capacity of 361A is too large given the other OCPD and equipment sizes? I.e. the primary fuses at 300A; the service OCPD at 400A; and the 200 kW generator with a nominal current of capacity of 240A (unless its kVA rating is higher). Is there a likelihood that transformer startup current could trip any of those?

Cheers, Wayne

 

[Tulsa Electrician]

Looking at your drawing. I see you have 3-350+1#3 in a 2".
For a nipple I would say ok.
For the primary side of the transformer it will require a larger raceway if over 24" in lenght. Maybe 2.5"

 

[ggunn]

Looking at your drawing. I see you have 3-350+1#3 in a 2".
For a nipple I would say ok.
For the primary side of the transformer it will require a larger raceway if over 24" in lenght. Maybe 2.5"

Thanks; that should have been 2-1/2". Good catch.

 

[ggunn]

Has nothing to do with your transformer but you show a #8 Ground from your gutter to the line side of your 100 amp disconnect.
The #3 tap conductors are protected by the 400 amp breaker so the equipment ground should be #3
(If the phase conductors were smaller the equipment ground could be smaller)
On the load side of yiour 100 amp breaker #8 is fine,.

Done.

 

[ggunn]

Almost, but the supply side bonding jumpers that you show as grounding conductors for the transformer secondary conductors are too small. They need to be sized per Table 250.102(C)(1) and that will require that they be 4 AWG. If they were equipment grounding conductors, the 6 AWG that you show would be ok, but they are on the line side of the 200 amp OCPDs, so they are supply side bonding jumpers and not EGCs.

Done.

 

[ggunn]

Thanks for the education, you guys. I owe you all a beer.

 

[Engser18]

one note, why would you used 300KVA Xformer, the 225KVA xformer would do the jobs. Cheer.

Please ensure the spec on 100A Fuse disconnect to be rated and mark 75 degree C. Otherwise, it is not compliance 110.14.