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Author Topic: What does NEC table 400-6(b) explain?  (Read 4410 times)

Jeffery Foster

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What does NEC table 400-6(b) explain?
« on: January 08, 2015, 09:10:54 pm »

I'm trying to determine the max amp capacity of Southwire 6 gauge SC cable, shown here;

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheetOEM146

It says 105 amps, with a note to reference 400.6(b).  I can only find 400.5(b).  Any suggestions?

Thanks in advance!
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Stephen Swaffer

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Re: What does NEC table 400-6(b) explain?
« Reply #1 on: January 09, 2015, 12:13:04 am »

The correct table to use is NEC 2014 400.5(A)(2)-that table lists the same ampacity under 90 deg C column D1 as Southwires chart.  That column is only good for a single wire not in contact with another wire for more than 24 ".

You may already know this, but  keep in mind that you can only use the ampacity that corresponds to the temperature rating of your connections-lugs/breakers, etc, and it is rare to find one of those rated 90 deg-the ampacity at that temp is almost always used to derate from-very rarely is that actually a usable ampacity. Finding the true ampacity usually involves a bit more than just looking up a number in a table.

For a cable using two current carrying wires with 75 deg rated lugs the ampacity is 88 amps, for a 3 phase cable/3 current carrying wires it is 77 amps.
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Steve Swaffer

Tim McCulloch

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Re: What does NEC table 400-6(b) explain?
« Reply #2 on: January 09, 2015, 12:38:02 am »

The correct table to use is NEC 2014 400.5(A)(2)-that table lists the same ampacity under 90 deg C column D1 as Southwires chart.  That column is only good for a single wire not in contact with another wire for more than 24 ".

You may already know this, but  keep in mind that you can only use the ampacity that corresponds to the temperature rating of your connections-lugs/breakers, etc, and it is rare to find one of those rated 90 deg-the ampacity at that temp is almost always used to derate from-very rarely is that actually a usable ampacity. Finding the true ampacity usually involves a bit more than just looking up a number in a table.

For a cable using two current carrying wires with 75 deg rated lugs the ampacity is 88 amps, for a 3 phase cable/3 current carrying wires it is 77 amps.

And further in the Code (520, Theaters...) IIRC the neutral of a 3 phase Wye system is considered a current-carrying conductor, so additional derating would be needed.
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Stephen Swaffer

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Re: What does NEC table 400-6(b) explain?
« Reply #3 on: January 09, 2015, 08:22:14 am »

Correct. The two likely derates that really should be followed would be number of conductors in the cable (4 if you have 3 phase Y with a neutral) as well as ambient temp-the table is good for 86 deg F only (how often is that true?)

However, to make things interesting, these would be applied to the 90 deg ampacity -99 amps and 87 amps respectively.  Then, the result is compared to the rating that is acceptable with a 75 deg rated connection and the lower is used.

For example:  A 4 wire cable requires a derate to 80%., 80 % of 87 is 69.6.  This is lower than the 77 amp 75 deg rating so if you had a 4 wire cable with a current carrying neutral your ampacity would be 69.6.

The adjustment factor for number of wires is table 400.5(A)(3), for ambient temp it is table 310.15(B)(2)(a). 
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Steve Swaffer

TJ (Tom) Cornish

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Re: What does NEC table 400-6(b) explain?
« Reply #4 on: January 09, 2015, 08:55:35 am »

Correct. The two likely derates that really should be followed would be number of conductors in the cable (4 if you have 3 phase Y with a neutral) as well as ambient temp-the table is good for 86 deg F only (how often is that true?)

However, to make things interesting, these would be applied to the 90 deg ampacity -99 amps and 87 amps respectively.  Then, the result is compared to the rating that is acceptable with a 75 deg rated connection and the lower is used.

For example:  A 4 wire cable requires a derate to 80%., 80 % of 87 is 69.6.  This is lower than the 77 amp 75 deg rating so if you had a 4 wire cable with a current carrying neutral your ampacity would be 69.6.

The adjustment factor for number of wires is table 400.5(A)(3), for ambient temp it is table 310.15(B)(2)(a).
In addition to all this good information, in our industry, practical ampacity is frequently limited by voltage drop.  Even though a single conductor #6 cable has a higher ampacity than a multi-conductor cable like 6/4 SOOW, it has the same resistance, and if you load it up anywhere the theoretical ampacity at lengths we typically use - 50', 100', etc., you're going to be way over your 5% voltage drop recommendation. 

I'm trying to think of a situation where #6 SC would be appropriate.  The only one I can come up with is short tails (less than 10') to a breaker box providing a 14-50 receptacle.  For anything else, #6 is pretty small.  Normally #2 wire is used for 100A tails, 2/0 for 200A, and 4/0 for 400A:  http://www.motionlabs.com/c-100-20-cam-feeder.aspx

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Jeffery Foster

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Re: What does NEC table 400-6(b) explain?
« Reply #5 on: January 09, 2015, 12:27:00 pm »


I'm trying to think of a situation where #6 SC would be appropriate.  The only one I can come up with is short tails (less than 10') to a breaker box providing a 14-50 receptacle.  For anything else, #6 is pretty small. 

Thank you to everyone for the excellent information.  Tom, this was almost exactly what I encountered in this situation.  Just wanted to learn, and I have!
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Re: What does NEC table 400-6(b) explain?
« Reply #5 on: January 09, 2015, 12:27:00 pm »


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