Generator Cord 10/4 SOOW Ampacity Confusion

smaggardJune 15, 2011


I am installing a manual transfer switch for a 7000 running watts generator. Many of the cords from Reliance Controls, Gentran etc state they are 30A cords made with 10/4 wire and 14L30 connectors.

I am looking at having a longer cord and want to construct a 75' cord from SOOW 10/4 90C cable and attach the 14L30 ends myself. It doesn't seem voltage drop will be siginificant based on the calculators I have found.

However, ALL of the 10/4 SOOW cable states it is rated at 25A, while 10/3 is 30A. I'm confused since all of the major generator manufacturers are selling these cables advertised as 30A cables good for 7200 watts but all of the 10/4 SOOW bulk vable I can find is 25A.

Am I making this more difficult and worrying too much? One thing I was wondering is if the 25A rating is assuming current would be flowing through all 4 wires, but with the 14L30 there will be 2 hot, 1 neutral, and 1 ground so current should only flow through 3 conductors, therefore is it ok to assume that a 30A load will be ok?

Any advice on this would be appreciated! Otherwise I suppose I could just buy the 40' Reliance Controls cord as the 75' Gentran cord (which is 10/4) is too much $$ whereas I can make my own 75' for about the same $$ as thr Reliance 40'.

thanks in advance!


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Ron Natalie

According to table 400.5(A)1 10 gauge SOOW is good for 25A (Column A). You can't use Column B because there are more than two current carrying conductors.

SOOW is cord by the way, not cable.

    Bookmark   June 15, 2011 at 5:40PM
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Thanks for the quick response...very helpful! Based on the table it seems the 10/4 cords sold by generator companies are in fact 25A cords rather than 30A (even though they have 30A connectors), is that a safe assumption? If so I would rather put together the longer cord, if all else will be equal, minus a small increase in voltage drop.

Thanks again for the help!

    Bookmark   June 15, 2011 at 10:08PM
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Ron Natalie

The NEC delegates the responsibility for pre-made extension cords to UL.
I'm not really up on what UL's rules are for conductor sizes and connectors.

    Bookmark   June 16, 2011 at 8:49AM
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They may alaso be playing games with the "7000 W."

Just like fantasy horsepower.

At 100% efficiency ~746 W is 1 HP.

That means you would need at least a ~9.4 HP engine.

When include inefficiency (alternators are pretty far from 100%) the horsepower can increase significantly.

I would be very surprised if you can run 7,000 W of resistive load, let alone inductive.

    Bookmark   June 16, 2011 at 9:46AM
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Thanks again for the input! Yes Brick I sort of took that 7000W number with a grain of salt. While the engine is a 13/14 HP Honda, I'm sure they stretch those ratings for the absolute best scenario etc. That being said I'm starting to think the generator isn't going to be cranking out 30A continuous anyways and the 25A cord is likely sufficient.

Thanks again.


    Bookmark   June 16, 2011 at 10:07AM
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It probably puts out 7,000 W just as the engine grinds to a halt.

I have a 3 HP Baldor motor on my table saw, that by its locked rotor KVA code is 'Sears' rated over 60 HP.

The motor shaft would fail at that kind of load (it is only 3/4 inch).

    Bookmark   June 16, 2011 at 4:21PM
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