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 DIY Chatroom Home Improvement Forum Cable Size for Sub Panel vs chart
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11-08-2008, 07:28 PM   #1
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## Cable Size for Sub Panel vs chart

I have read people suggest #4 thhn for an in building sub panel at 100 amps, and 2/0 for a inside panel at 200 amps. (I see other have suggested a #3 for a 100A sub, which appears to me to be correct based on the NEC number of 102 for a #3)

The chart in the NEC book says that a #4 90 degree THHN wire is good for 89A and the 2/0 is good for 186A.

Are these able to be rounded up due to them being odd numbers (or is the #4for 100A and 2/0 for 200A incorrect)? I would just like to understand how to use this chart, so I can make the proper wire choices for my projects.

Thanks for the clarification.

Jamie

Last edited by jamiedolan; 11-08-2008 at 07:53 PM. Reason: new info / type

11-08-2008, 10:52 PM   #2
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Quote:
 Originally Posted by jamiedolan I have read people suggest #4 thhn for an in building sub panel at 100 amps, and 2/0 for a inside panel at 200 amps. (I see other have suggested a #3 for a 100A sub, which appears to me to be correct based on the NEC number of 102 for a #3)
Table 310.15(B)(6) allows the service entrance conductors or feeders in a dwelling unit to be sized as you have stated. This sizing does not apply to branch circuits or subfeeds.

Quote:
 The chart in the NEC book says that a #4 90 degree THHN wire is good for 89A and the 2/0 is good for 186A.
Where are these numbers coming from? Table 310.16 lists the ampacities of #4, 3 and 2/0 as 95, 110, and 195 A, respectively, in the 90 degree column. There is virtually no equipment that has terminations rated for 90 deg. For wires pulled into conduit, such as THHN, we almost always use the 75 degree column. The temp rating of a device or terminal is printed somewhere on it. And the ampacity must be selected based on the lowest temperature rating encountered in the circuit.

For instance, if the breaker is rated at 75 deg and the receptacle is rated at 60 deg, then you must size the conductor based on the 60 deg column.

Quote:
 Are these able to be rounded up due to them being odd numbers (or is the #4for 100A and 2/0 for 200A incorrect)? I would just like to understand how to use this chart, so I can make the proper wire choices for my projects. Thanks for the clarification. Jamie
240.4(B) allows you to use the next highest size of circuit breaker, as long as the three conditions are met. And I do believe I neglected to tell you about condition 1 in the post about derating conductors in ENT. I had forgotten it myself.

 11-09-2008, 12:30 AM #3 DIY'er     Join Date: Sep 2008 Location: Neenah, Wisconsin, USA Posts: 2,044 Rewards Points: 1,024 Blog Entries: 2 Where are these numbers coming from? Table 310.16 lists the ampacities B.310.1 - 90 degree column, it is the only column that lists THHN, -- Maybe I was just looking at the wrong chart, I found 310.16. In table 310.16 - under the coper side of the table, the 90 degree column is the only column that lists THHN wire. But you use the 75 column even if your using THHN? (The 75 degree column lists RHW, THHW, THW, THWN, XHHW, USE, ZW). of #4, 3 and 2/0 as 95, 110, and 195 A, respectively, in the 90 degree column. There is virtually no equipment that has terminations rated for 90 deg. For wires pulled into conduit, such as THHN, we almost always use the 75 degree column. The temp rating of a device or terminal is printed somewhere on it. And the ampacity must be selected based on the lowest temperature rating encountered in the circuit. I looked all over my pushmatic panel - the main label is printed clearly, but lists no temperature on the panel. The main breaker does not have a temp list on it, other than it says 40 C - without the degree mark. I found a temp rating on one of the other breakers it says 40degrees C, use 60 degreeC wire. I wonder if these old Pushmatic Panels can only operate at 40degrees C. Jamie

11-09-2008, 09:01 AM   #4
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Quote:
 Originally Posted by jamiedolan In table 310.16 - under the coper side of the table, the 90 degree column is the only column that lists THHN wire. But you use the 75 column even if your using THHN? (The 75 degree column lists RHW, THHW, THW, THWN, XHHW, USE, ZW).

B.310.1 is for engineering calculations only. The THHN is listed in as a 90 deg wire, true, but there are virtually no terminals or lugs that are rated for it. Also, see 110.14. That pretty much sums it up. The reason electricians love THHN is because the Code allows us to use the higher temp rating as our starting point for derating. So, while we may only be able to use a 90 deg wire in a 60 or 75 deg ampacity, we can use the 90 deg ampacity when we adjust for various factors such as ambient temp or number of conductors in a conduit.

Quote:
 I looked all over my pushmatic panel - the main label is printed clearly, but lists no temperature on the panel. The main breaker does not have a temp list on it, other than it says 40 C - without the degree mark. I found a temp rating on one of the other breakers it says 40degrees C, use 60 degreeC wire.
Quote:
 I wonder if these old Pushmatic Panels can only operate at 40degrees C. Jamie

40 degrees is the temp the breaker is rated for use at. In other words, the breaker will perform as specified up to 40 degrees ambient temperature. The wire terminations are rated for 60 degrees. This means that any wire you place on the breaker, you must determine the ampacity based on the 60 degree column for that size wire.

Last edited by InPhase277; 11-09-2008 at 09:03 AM.

 11-09-2008, 12:12 PM #5 Idiot Emeritus   Join Date: Mar 2008 Location: Fernley, Nevada (near Reno) Posts: 1,849 Rewards Points: 1,492 In a nutshell, article 110.14 C 1 a states that all terminations are assumed to be rated at 60C unless otherwise marked for a higher temperature. Most terminations these days are marked for 75C. Conductors must be sized for the lowest temperature rating at any point in the system. Pushmatic panels were built before manufacturers began marking their equipment with temperature ratings, so it is assumed to be 60C. Therefore, the ampacity of any conductor connected to this panel must be based on the 60C column. If this were a newer panel, and the breakers were marked 75C (nearly all of them are these days), and the device at the other end of the circuit is also marked 75C, (again, usually the case) then you would size the wire based on the 75C column. One notable exception here is than NM cable (romex) is always sized based on the 60C column. This is found in 334.80 The 90C column is used with wire rated at 90C when de-rating factors must be applied because of more than 3 current-carrying conductors are installed in a raceway (conduit). Any wire can be used at less than its temperature rating, just like it can be used at less than its current rating. Rob
11-09-2008, 01:30 PM   #6
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Thank you both for your help in clarifing this.
Jamie

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