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Old 12-05-2009, 07:28 AM   #1
94yj
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Default 125 amp service what wire size to use

i would like to get some opinions on what size wire to use from my elec pole to my building and it is a 35 foot run and would be underground for about 18 feet a friend said i could use 4 ga for this and would be plenty is this correct or should i go with another size opinions and help please
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Old 12-05-2009, 08:27 AM   #2
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Default Re: 125 amp service what wire size to use

minimum #2. #4 is 95 amps.#2 is 130 amps.

Last edited by oleguy; 12-05-2009 at 08:31 AM.
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Old 12-05-2009, 09:53 AM   #3
Gary S
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Default Re: 125 amp service what wire size to use

Agreed. I wouldn't use anything smaller than #2 copper.
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Old 12-06-2009, 10:05 AM   #4
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Default Re: 125 amp service what wire size to use

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Originally Posted by oleguy View Post
minimum #2. #4 is 95 amps.#2 is 130 amps.
This is true if the wire is rated for 90 degrees.
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Old 12-06-2009, 10:23 AM   #5
94yj
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Default Re: 125 amp service what wire size to use

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This is true if the wire is rated for 90 degrees.
i did not know that wire is temp sensitive how can i tell what temp is rated at and how does the rating work does it carry less amps if it is colder
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Old 12-06-2009, 10:55 AM   #6
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Default Re: 125 amp service what wire size to use

Wire is rated by the temp that the insulation is capable of withstanding without degradation or damage. This is seperate from the ambient temp that the wire is in. The supplier should be able to specify the temp rating of the wire you purchase. For instance, #2 AWG THHW rated for 75 degrees C has an allowable ampacity of 115 amps while the same wire rate for 90 degrees C has an allowable ampacity of 130 amps. There is also temperature derating based on the application of the conductor based on the equipment(service, feeder, branch circuit) that the conductor is connected to. So in some applications 90 degree C wire ampacity would be derated to 75 or even 60 degrees C allowable ampacities. This can become confusing but the allowable ampacity of a conductor is actually based on the specific use of that conductor.
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Old 12-06-2009, 10:58 AM   #7
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Default Re: 125 amp service what wire size to use

To make it dirt simple:

You'll need #1 copper minimum without knowing any details.

But, if you can provide the inspector with a calculated load sheet proving your load is less than 115 amps you can use #2 copper.
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Old 12-06-2009, 11:08 AM   #8
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Default Re: 125 amp service what wire size to use

no!!!the temp rating is ambeient temp rating.ratings are based on 30*c 86*f.the correction factors are for a temp value other than 30*c.
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Old 12-06-2009, 11:17 AM   #9
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Default Re: 125 amp service what wire size to use

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no!!!the temp rating is ambeient temp rating.ratings are based on 30*c 86*f.the correction factors are for a temp value other than 30*c.
Different issue than what I was stating. I believe that you are refering the Ambient temperature correction factors (Table 310.16) and I was refering to the temp limitations of "Termination provisions" as stated in 110.14(C)(1)(b). Two different issues.

Last edited by hidollartoys; 12-06-2009 at 11:19 AM.
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Old 12-06-2009, 11:30 AM   #10
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Default Re: 125 amp service what wire size to use

and in a 2" conduit...
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Old 12-06-2009, 11:32 AM   #11
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Default Re: 125 amp service what wire size to use

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Originally Posted by hidollartoys View Post
Different issue than what I was stating. I believe that you are refering the Ambient temperature correction factors (Table 310.16) and I was refering to the temp limitations of "Termination provisions" as stated in 110.14(C)(1)(b). Two different issues.
you are right .but we are talking service feed not connected equip.
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Old 12-06-2009, 03:06 PM   #12
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Default Re: 125 amp service what wire size to use

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you are right .but we are talking service feed not connected equip.
Connected equipment also includes panels and disconnects.
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Old 12-06-2009, 10:27 PM   #13
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Default Re: 125 amp service what wire size to use

it still goes back to ambient temp ratings.for example,the wire terminals on a pannel will state the lugs will only use 60 or 75*c wire.there fore a wire of 90*c is not to be used.then you use the correction factor tables to find a suitable wire size.table 310.16.
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Old 12-07-2009, 02:00 AM   #14
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Default Re: 125 amp service what wire size to use

Let me see if I can explain this "conductor tremination temperature" vs 'insulation temperature rating" vs "ambient temperature" in a better way.

Table 310.16 has three columns, 60 deg C, 75 deg C and 90 deg C both for copper and for aluminum. These columns are based on the Insulation type and reflect the temperature capacity of the respective insulation types. These temps have nothing to do with ambient temp. Ambient temperature correction factors are at the bottom of the table and reflect a "correction factor" for temperatures that are associated with where the conductors are physically located.

In utilizing conductors it will be necessary to terminate them. Article 110.14(C) instructs that the maximum termination temperature for any device or equipment determines which column in table 310.16 that should be used for selection of ampacity values.

Article 110.14 specifies that for terminations of 100 amps or less limits you to the ampacities specified in the 60 deg C column regardless of the insulation temp rating. For terminations of greater than 100 amps the ampacities must be derived from the 75 deg C column. For example, #8 THHN has a table ampacity of 55 amps but the termination of 100 amps or less requires the ampacity to be derived from the 60 deg C column. This means the actual ampacity is 40 amps. Likewise, #1/0 THHN has a table ampacity of 170 amps but for terminations over 100 amps the actual ampacity is derived from the 75 deg C column and is 150 amps.

As far as I know, the distribution lugs in a wireway, junction box or CT cabinet are possibly the only 90 deg C rated terminations. I do not believe that any panelbord, circuit breaker, loadcenter.disconnect switch, etc has a termination temperature rating greater than 75 deg C.

The 90 deg C column can be used for the ambient temp correction, conductor proximity adjustment or both. Once the correction factors have been applied to the table 310.16 conductor ampacity, it will be necessary to compare the results with the termination temperature limitations as defined in 110.14(C). 110.14(C) will take precident.
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Old 01-31-2010, 04:24 AM   #15
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Default Re: 125 amp service what wire size to use

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Originally Posted by hidollartoys View Post


In utilizing conductors it will be necessary to terminate them. Article 110.14(C) instructs that the maximum termination temperature for any device or equipment determines which terminations of 100 amps or less limits you to the ampacities specified in the 60 deg C column regardless of the insulation temp rating. For terminations of greater than 100 amps the ampacities must be derived from the 75 deg C column. For example, #8 THHN has a table ampacity of 55 amps but the termination of 100 amps or less requires the ampacity to be derived from the 60 deg C column. This means the actual ampacity is 40 amps. Likewise, #1/0 THHN has a table ampacity of 170 amps but for terminations over 100 amps the actual ampacity is derived from the 75 deg C column and is 150 amps.


I don't read it that way and it doesn't jibe with the examples given in the NFPA's commentary following the section. Maybe you are working off an old version of the NEC. I have the 2008 version.

I see zero in there limiting 100 amps or less circuits to the 60 degree column as you state, and the NFPA itself in its commentary uses the 8 AWG example with 75 degree termination equipment and gives a conductor ampacity of 50 amps from the 75 degree column, contrary to your example above.

Further, we know that it is totally fine to run #3 AWG for 100 amp feeders and using your statement above that would not be allowed as 3 AWG would be limited to the 60 degree column and 85 amps. The next size up breaker is 90 so no go there either. What I mean is that there is no way to 100 (like with 2 romex where the 60 degree rating required to be used because it is bundled is 95 amps so you can run 2/3 romex on a 100 amp breaker) under the next size breaker exception. So there you have it, for 100 amp feeds, you either run 3 AWG in conduit or 2/3 romex.

For 125 amps, you should be able to use number 2 for a feeder and you absolutely can use number 2 if it's the service entrance feed under 310(15)(B)(6). In other words, if this line is carrying 100 percent of the building's load you can use number 2 AWG. If it's a feeder of a subpanel you can use number 2 as well as you can go up a size to from the rated 115 amps of #2 (75 degree C column) to the 125 breaker under the next size up exception.

Last edited by MrMark; 01-31-2010 at 05:35 AM.
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Old 01-31-2010, 10:05 AM   #16
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Default Re: 125 amp service what wire size to use

My code is 2005 and if should refer to page 35, Section 110.14 (A)(1)(a) & (b). Not generally an issue because most wiring is oversized. It is however an issue if you are terminating to equipment that is not rated at the temp of the wire that is chosen. The actual terminal and type of clamping mechanism where the wire is landed is the real limiting factor. Most all panels and switch gear rated for 100 amps or more are manufactured with terminations that are compatable with the ampere rating of the specified use. There are situations, especially in branch circuits, where the panel is rated at a higher temp than the connected load (IE recept, switch, disconnect) so the circuit has to be derated based on the temp rating of the final termination device.
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Old 01-31-2010, 10:30 AM   #17
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Default Re: 125 amp service what wire size to use

THE 90 DEGREE COLUMN CANNOT BE USED TO SIZE CONDUCTORS!!!! There are no terminals rated to used over 75 degrees, plus any wet location (Which a conduit outdoors or underground is defined by the NEC as a wet location) is limited to 75 degrees, based on that a copper conductor would have to be #1 AWG THWN,THW rated for 130 amperes, and aluminum would 1/0 rated for 120 amperes 6( In this case one is allowed to upsize to the next higher standard overcurrent device). These values were taken from table 310.16 NEC 2005 edition,table 310.15(B)(2)(a) is not relevant to this discussion as it is for DWELLING services, it is not allowed for subfeeds & outbuildings & non-residential services.

#4 AWG THWN = 85A, #2 AWG THWN = 115A .

EDIT: If NM cable was part of the discussion, it is limited to 60 degree ampacity see NEC article 334.80, sizing conductors is not always cut & dried which causes trouble when one relies strictly on the temp rating of conductors.

Last edited by Norcal; 01-31-2010 at 10:36 AM.
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Old 01-31-2010, 12:33 PM   #18
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Default Re: 125 amp service what wire size to use

What is the pro/con of copper feed vs. aluminum? I'm doing the same thing underground and other than the corrosion propensity of aluminum, its per foot cheaper, anything else to consider seeing I'm going 200 feet and cost isn't a show stopper?
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Old 01-31-2010, 01:10 PM   #19
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Default Re: 125 amp service what wire size to use

Ok, so to sum all this up cause I am confused here...

What wire is needed for 125amp 60*/75* for copper and aluminum??

I was told it is #2 for copper and 1/0 for aluminum. That is off the table 310.15(B)(6)

Last edited by nate379; 01-31-2010 at 02:56 PM.
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Old 01-31-2010, 02:34 PM   #20
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Default Re: 125 amp service what wire size to use

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Ok, so to sum all this up cause I am confused here...

What wire is needed for 125amp 60*/75* for copper and aluminum??

I was told it is #2 for copper and 1/0 for copper. That is off the table 310.15(B)(6)
You are correct if this is a house and it is the service entrance, but Norcal seems to think this is commercial. I'm not sure why. BTW, you have a typo.

Could you explain the "125 amp 60*/75*, please. That writing found on the termination device, I assume, is typical electrical equipment pitiful specifications. Why is everything having to do with electrical such low quality crap? At least when compared to other fields.


In the example I gave above using 75 degree terminations I found that 2 AWG is also fine for 125 amp feeder circuits when using the next size up rule for breakers. But Norcal states that 1 AWG is required.

I don't do this for a living but I did stay at a Holiday Inn Express last night.
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