Flexible Cord in Conduit

[deedubya]

Hello. New subscriber. First time posting.

My interpretations of NEC 400.10(A)(10), 400.12(6), and 501.140(A)(3) provide for use of flexible cords in conduit from a submersible pump (465 full load amps) to its power source (VFD). However, 400.12(2) says the cord can't run through "holes in walls...or floors". My submersible pump vendor supplies a German-manufactured flexible cord, (3) 120mm2 area (237MCM), stamped "SOW" on the jacket, 2 cords per pump.

My questions are:
1. Can I ignore 400.12(2)? This pump is in a Class I, Division 1 wet pit. It is unreasonable to expect that the VFD and the pump will be in the same space. There is no way to get a cord from the pump to its power source without going through "holes in walls or floors". I'm thinking 501.140(A)(3) waives this rule although it doesn't specifically say so.

2. Table 404.5(A)(1) only shows ampacities of flexible cords up to 2AWG for SOW cord. The vendor claims each cord is good for 344A per phase in conduit. 2 questions:

a. Is there any difference between ampacity in free air vs. in conduit? I was unable to find any difference.
b. Given the table only goes up to 2AWG, do I have to go through Neher-McGrath calcs in Annex B for larger conductors? My application has fairly large underground duct banks for part of the route. I have glance at the tables in Annex B and they only mention building wire insulations. And duct bank table ampacities I see in Table B.310.15(B)(2)(6) would indicate that ampacity of 237MCM is nowhere close to 344A.

It would be very clean and less expensive installation if I could extend these cords unbroken from pump to VFD. But my gut instinct is they are not sufficient per NEC. I don't trust myself to correctly calculate the ampacity of these cables and I don't trust that my vendor can either, or did. It will be a few hundred thousand $ in cord. I can't afford to be wrong. Thanks for your help.
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[gar]

190423-0753 EDT

deedubya:

You are getting no responses, and I can't answer your question.

The NEC is a rule based system based on theory and empirical data. If you can not find a rule that fits, then you need to go back to basic theory and experiment.

I don't know how flexible fits in to the question. Possibly this has something to do with insulation, or wire positioning, or thermal conductivity.

Ultimately the problem or question relates to temperature, and/or mechanical forces.

A bare wire in free air will have a lower thermal resistance, and therefore temperature rise than the same wire insulated and in a confined space. Also in free air it is not coupling to other conductors and magnetic materials.

One long straight wire with a fixed current and air as the magnetic material has a magnetic field strength that varies as the inverse of radial distance.

Two straight parallel wires with the same current, but flowing in opposite directions have a combined magnetic field that drops off based on K*(1/r1-1/r2). So as you move away from the wires the field drops off much more rapidly than for a single wire.

AC unbalanced current flow inside a ferromagnetic material causes heating. All heat generated in a conductor and what is around it determines temperature rise. Thus, insulation and resistance problems.

Both conduction and convection heat flow are of concern.

How can you use these comments I have no idea. Possibly you need to do some experiments.

.

 

[kwired]

Nothing in NEC prohibits running some other wiring method through walls or other partitions then installing a junction to convert to the flexible cord. I know the size cord you are talking about it would be a big pain compared to smaller cords.

And yes 501.140(A)(3) allows you to extend through wet pit enclosure in a raceway. You would need to hit your drive on other side of that wall or transition to another wiring method though, can't continue with flexible cord through other walls should the drive be in yet another room.

 

[deedubya]

gar: I guess I could have asked a simpler question: "What is ampacity of 120mm2 flexible cord in underground duct bank?" I find no path through the NEC that leads me to an answer. Flexible cord ampacity table doesn't include conductors that big and do not distinguish as to how they are installed (in free air, in conduit in free air, in underground duct bank, etc). I have an answer from a vendor but I don't believe it but not sure how to prove them wrong.

kwired: I have to go through a wall and a 2 floors. Seems if I could go through one, I could go through 100 if I wanted. Regardless, I was trying to avoid a junction i.e. VFD to pump with the pump cord. Regardless, I do not think the 120mm2 cord has enough ampacity when installed in a duct bank. So I'll be forced to route larger conductors through the underground conduit portion and transition to the pump cord in the pump structure.

Thanks to both for weighing in.

 

[Chamuit]

+1 KWired


It looks like 400.5.C addresses the ampacity question.

 

[oldsparky52]

My submersible pump vendor supplies a German-manufactured flexible cord, (3) 120mm2 area (237MCM), stamped "SOW" on the jacket, 2 cords per pump.

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From the AWC website

Please note that SO and SOW have been superseded by SOOW. SOOW has both an oil resistant insulation and an oil resistant outer jacket while SO and SOW have only an oil resistant outer jacket. Any SOW or SO Cord parts entered for RFQs will be quoted as SOOW.

My 1st question would be if this cable has a listing that is acceptable in the USA.

IMO once you install a cord in conduit (if allowed), you can no longer use the ampacity tables in 400, you would need to use 310.15(B)(16).

Then there is 110.14(C)(1)

 

[kwired]

From the AWC website

My 1st question would be if this cable has a listing that is acceptable in the USA That was sort of on mind also.

IMO once you install a cord in conduit (if allowed), you can no longer use the ampacity tables in 400, you would need to use 310.15(B)(16).What is the conductor type to use with the table? I guess temp rating is sort of biggest key here, but still the table lists conductor types.

Then there is 110.14(C)(1)

..