Hello all,

I recently went out to do an IR survey at a pump station because the Alumunim conduits from the MCC to the motor were said to be hot and there were electrical connection failures. I would like you opinions, please.

COLLECTED DATA:

Each motor has (6x) 4/0 AWG THHN wires inside a 3" aluminum conduit running in parallel and have an aprox 50 feet run (From MCC: 3 feet up, 20 feet inside a false ceiling-horizontal then dropping down to a pit about 30 feet - add a few more feet of run to the motor junction box) I have pics.

The alum. conduit was measured at about 115 F thru-out the conduit.

The motor was measured at 173 F (skin) and 225 F internal.

Customer had two failures, at the junction box - Burndy Connectors.

Ambient Temp rose 15-20F from morning to early pm from 72 F - Final = 87, almost 93 F.

HP rating is 350 (2x Motors)FLA=412

Measured amps:
As PAIRED Conductors - 1=370, 2=370, 3=364
as SINGLE Conductors- 1a=186, 1b=186, 2a=210, 2b=215, 3a=182, 3b=183

Here are my questions:

What size wire (if in parallel) do I need to carry heat away from the motor junction box, incidently this is where the failure occurred.

AND...

Do I need to address the ambient temperature of the pit? I calculated a BTU rating of 1.8 MBTU. (1watt=1.34 HP, and 1HP=3.928 BTUit)

My calculations says the wire should be 2x derated therefore a 400 MCM wire would be better. My collegue says they are fine (THHN).

IR Pic 2

IR PIC 3

quote:

As PAIRED Conductors - 1=370, 2=370, 3=364
as SINGLE Conductors- 1a=186, 1b=186, 2a=210, 2b=215, 3a=182, 3b=183

Two strange things:
item 1 - The paired measurements are balanced, but the individuals aren't. (2a and 2b are higher).
item 2 - The single measurements add up to the paired measurements except for 2a and 2b which add up to much more than 2.

It's a strange pattern and readings 2a and 2b cause the stangeness in both item 1 and item 2. Is it possible the 2a and 2b were measured during a brief voltage droop? Can you double-check the measurements?

I will take a guess at the rating question, but cable sizing is not part of my job so I'm not that familiar with it and my answers should be viewed with caution.

NEC Table 310-6 gives "Allowable ampacities of insulated conductors rated 0 thru 200VAC....not more than three current carrying conductors in raceway, cable or earth, based on ambient temperature of 30C."

For 90C insulation (THHN) they give an ampacity of 260 for 4/0 and 290 for 250MCM.

There are two deratings to apply as you mentioned: ambient, and more than 3 cables.

The ambient corrections are given in the same table:
31C-35C (88-95F) - multiply by 0.96.
36-40C (96-104F) - multiply by 0.91

The >3 conductors is given in table 310-15(b)(2)(a) - "Adjustment factors for more than 3 current carrying conductors in raceway or cable":
For 4-6 conductors they give an adjustment of 0.8. For 7-9 the adjustment would be 0.7

So if we apply 0.91 temperature correction and 0.8 adjustment for 6 conductors, we get 0.73 adjustment. This brings the ampacities down to 190A for 4/0 and 211 for 250MCM.

What should it be? I would normally expect the overload protection to be somewhere around 115-125% of motor FLA, and I think the cable needs to have a derated ampacity at least as high as the overload setting. So I would expect the derated cable ampacity should be at least 1.15*206=237A (the 190A calculated above for 4/0 doesn't meet this).

For a double-check of the derating question, you might also try the power engineering forum at www.eng-tips.com

This message has been edited. Last edited by: electricpete,

E-Pete,

I knew you would be one to reply and I thank you for your input. Your thought process was near to what I was hoping. But in my calcs. I suspected the best cable is 400 MCM. In either case, we both agree that the 4/0 is not the best selection for this pump station.

ANY MORE TAKERS? Your opinions are appreciated.

DAN