Feeder Question / Advice

[Javi_M]

That is a very good observation Larry. I also wonder if that design will have to be maintained, 2 runs of 4" conduit.

 

[Strathead]

The required ampacity is 451A. 250kcmil = 290A * 2 sets * 80% adjustment factor = 464A.

You would need 700kcmil to use a single set.

Sorry, I was thinking it was 351 for a minute. Mind phase.

 

[kwired]

If they go with two conduits, don't the paralleling rules require the second conduit also be 4"?

IMO both need to be the same, I would run less than 4 inch if planning to parallel in separate conduits though. If 4 inch is already there, I'd likely increase conductor size as needed instead of adding a second 4 inch.

 

[Javi_M]

I'm going to leave it up to them to determine the most cost effective way. But they already had bought all the 4/0 wire and it has being pulled into the single conduit. To me it seems less costly to add another 4 inch conduit and separate the two pairs instead of exchanging the wire for a 250MCM and run a bigger size conduit to fit. What do you guys think?.

 

[kwired]

I'm going to leave it up to them to determine the most cost effective way. But they already had bought all the 4/0 wire and it has being pulled into the single conduit. To me it seems less costly to add another 4 inch conduit and separate the two pairs instead of exchanging the wire for a 250MCM and run a bigger size conduit to fit. What do you guys think?.

Copper cost more than steel. both are back breaking labor intensive when you get to the sizes in question here. If you can return the copper you have and only pay difference for one size larger could be least costly overall.

 

[JFletcher]

That is a very good observation Larry. I also wonder if that design will have to be maintained, 2 runs of 4" conduit.

I am not seeing in the code where they would be required to be the same size... I'm not even seeing where they have to be the same material, although I am most definitely missing something in 310.4.

Bigger problem is pulling out the conductors, a second conduit is probably not going to be, or routed, the same length as the other, which is a technical violation of the above-mentioned section.

 

[david luchini]

I'm not even seeing where they have to be the same material

Maybe 310.10(H)(3)?

 

[winnie]

4" EMT should have room for 18 4/0 conductors. The conduit currently has 7 (and the ground might me smaller). Would pulling in an additional 3 4/0 phase conductors be a reasonable pull? Or pulling out the existing, adding 3 more and then pulling in the entire set?

3 sets of 4/0 copper would give a derated ampacity of 546A, so the question is if this is a practical method of correction in this case.

-Jon

 

[kwired]

4" EMT should have room for 18 4/0 conductors. The conduit currently has 7 (and the ground might me smaller). Would pulling in an additional 3 4/0 phase conductors be a reasonable pull? Or pulling out the existing, adding 3 more and then pulling in the entire set?

3 sets of 4/0 copper would give a derated ampacity of 546A, so the question is if this is a practical method of correction in this case.

-Jon

If the 4/0 can be returned (helps if full reels and no custom cutting) it is probably less cost to do that and get 250 instead.

 

[winnie]

I also wanted to add some 'comfort' information about the current installation.

Most likely the current installation will work just fine with no issues of overload or safety.

For motor circuits the NEC requires an ampacity that is 125% of FLA (from a table), but permits OCPD of 250% the same value. In this case the 'table' conductor ampacity is 416A (2 * 260A * 0.8), less than the required 451A. However the OCPD is 500A, well below the permitted 903A.

NEC ampacity values are known to be very conservative, and give no credit for 'oversized' conduit. Had this run been in two separate 2" EMT pipes, it would calculate out just fine; well the heat dissipating surface of a single 4" conduit is the same as 2 separate 2" conduits (approximately, I am not dealing with the details of nominal versus true diameter).

None of the above changes the fact that the installation does not meet NEC requirements; it should simply comfort the OP that the installation as is only slightly misses the mark. The NEC is intended to provide 'practical safeguarding', not bleeding edge performance. If it were possible to substitute more detailed analysis for the practical and easy to use tables, then I would bet money that the existing installation is actually as safe as a compliant installation, however such analysis is probably much more expensive than changing to a compliant install.

-Jon

 

[winnie]

If the 4/0 can be returned (helps if full reels and no custom cutting) it is probably less cost to do that and get 250 instead.

Good to know. The OP should also know that the existing conduit is oversized for the 4/0, and would take the 250 without having to change the conduit.

-Jon

 

[wwhitney]

If an inner duct were installed in the 4" conduit, to maintain separation between two sets of feeder conductors, would that eliminate the need for the 0.8 derating factor?

Cheers, Wayne

 

[Javi_M]

Hello everyone. Here is the latest from my electrical contractor. The 4/0 wire that they selected has an insulation rated for 90 C temperature. They are suggesting to do something that I have seen Mike Holt discuss in some of his video series. They want to run 250 Kcmil (2 pair per phase) on a single 4" EMT conduit from the Bus plug to a separate junction box containing a power distribution block with terminals rated for 90 C.

I attached a an image of the way I think it needs to be done. If you guys can help me confirm so I can tell the contractor to follow this instructions.

 

[david luchini]

Hello everyone. Here is the latest from my electrical contractor. The 4/0 wire that they selected has an insulation rated for 90 C temperature. They are suggesting to do something that I have seen Mike Holt discuss in some of his video series. They want to run 250 Kcmil (2 pair per phase) on a single 4" EMT conduit from the Bus plug to a separate junction box containing a power distribution block with terminals rated for 90 C.

The 452A minimum ampacity is required throughout the entire circuit.

 

[Javi_M]

Mike holts discusses on this video, https://www.youtube.com/embed/ItJ0YNOZ4wA, that this setup is a permissible exception to the NEC code. Is this correct or you guys have a different understanding of this possible exception.

 

[david luchini]

Mike holts discusses on this video, https://www.youtube.com/embed/ItJ0YNOZ4wA, that this setup is a permissible exception to the NEC code. Is this correct or you guys have a different understanding of this possible exception.

That discussion is for 215.2. Your motor circuit conductors need to be sized per 430.22.

 

[Javi_M]

So this is completely wrong? That's the solution the electrical contractor is suggesting. Are those the NEC articles, David?

 

[david luchini]

So this is completely wrong? That's the solution the electrical contractor is suggesting. Are those the NEC articles, David?

Yes, I don't believe that that is a proper solution for the motor circuit. And yes, 215.2 and 430.22 are NEC sections.

 

[Javi_M]

Thank you so much David for that clarification.

 

[JFletcher]

Thank you so much David for that clarification.

If they are changing to two parallel runs of copper 250 MCM (instead of the initial 4 / 0) between the 90-degree terminations, that gives 580 amps, derated to 80% for 6 CCC in 1 conduit, 464 amps, Which is higher than the required 452.

So yes, if they upsize from 4 / 0 to 250 MCM, use 90-degree terminations on each end, then appropriately-sized jumpers to the 75 degree terminations, I believe it would be code compliant.