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Old 05-27-2008, 06:37 AM
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Electrical question on wire size

I am wiring a 100 amp sub-panel in my new workshop.
What size wire should I use? The wire length from the main house panel will be 60 feet. My 24X48 shop will have a water heater, A/C and 5hp compressor. I only have a small 110V welder.

Reading all that I could find in Google searches said that only #4 is required but some recommend #3. I have already run 1" PVC conduit underground.
Pretty sure local codes would only require #4.
What about the type of wire? THHN or THWN?

Should the green be #6 or #8 in either case of the feed wire size?

Thanks,
Roger

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Last edited by roger1; 05-27-2008 at 07:01 AM.
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Old 05-27-2008, 07:31 AM
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Always go one size larger and have a better safety factor.
The ground wire should not be any smaller then the lardest current carrying conductor in the conduit. Just pull 4 conductors of the same size and put green tape on the ground (both ends) before you put them in the conduit.
Here is a chart I just got from 66GMC.
Not sure about thge difference between power trans ., and chassis?
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Old 05-27-2008, 07:52 AM
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As an 'economic' suggestion - don't ask opinions on wiring - go to the latest NEC (National Electrical Code). They are the US source for codes, types of wires and sizes. Then you need your local town's interpretation. If you don't have that as well, you decide to sell your property and the home loan inspector is a PITA, you could end up pulling out the wrong stuff and rewiring at your cost. Some of mine caused a problem a few years ago - I didn't have enough staples holding it in place!!! I just was in one of the local "big box" stores and for sure you don't want to buy any more copper wire then you absolutely need as the price is out of reason.

Ground wires are the same AWG size as the conductors

Dave W
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Old 05-27-2008, 08:33 AM
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#4 wire is rated for 100 amps for a residential service feeder and 70 amps for a branch circuit. But since this is residential and #4 is approved for 100 amps, #4 is fine. 60' is not enough to worry about significant voltage drop. THHN or THWN are fine, both are moisture, heat and flame resistant. #8 for the ground based on the #4 wire.
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Old 05-27-2008, 08:37 AM
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I have wired a lot of things in my life and there is one thing that I do not do and that is give out wiring advice over the internet..Too many ways to slip up and get it wrong..

Sam
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Old 05-27-2008, 09:53 AM
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Quote:
Originally Posted by OneMoreTime
I have wired a lot of things in my life and there is one thing that I do not do and that is give out wiring advice over the internet..Too many ways to slip up and get it wrong..

Sam

Sam
Thanks for the back up - I too have wired much but internet forums, while informative can get you in trouble and in this case could be permanent (deadly) and/or expensive (fire, coffin, redo correctly)

He needs to use the NEC and his local code period, and this should be the end of the discussion. Too many things can go wrong if done by an amateur.

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Old 05-27-2008, 10:05 AM
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if you look on the wire box's in the big box stores, you will see the NEC stated requirement. don't go smaller but you could go bigger. as stated above, local codes are also nessasary.. you could call a local building inspector for the codes if you don't know who to ask in your town. codes are diffrent in every state so asking on internet forum will likely lead to the wrong information, as the NEC is the national requirement, not the state or town/ county requirement
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Old 05-28-2008, 06:24 AM
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Thanks to everyone who replied.

I did find some new information late yesterday.
San Angelo goes by 2002 National Electric Code and publishes their requirements for wire sizes on their web-site. For residential use, their requirement for 100A is #4.
Since my run is only 60 feet and is underground (temperature is not an issue), I will go ahead and run the #4.
I also talked to a local electrician who agreed to do a pre-inspection for me before the city comes to do their inspection. He told me to use 3 - #4s with a #8 ground wire. He also said my 1" conduit is fine. This is the same electrician I used to bring in the 400A service to my house last year.
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Old 05-28-2008, 06:58 AM
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Many times you can call the city building dept. and talk to a inspector. They will give the best advice, remember they are there to protect you from yourself. The better rapor you have with them the better the inspection will go. You want them on your side helping. Tom F
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Old 05-28-2008, 07:13 AM
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Quote:
Originally Posted by roger1
Thanks to everyone who replied.

I did find some new information late yesterday.
San Angelo goes by 2002 National Electric Code and publishes their requirements for wire sizes on their web-site. For residential use, their requirement for 100A is #4.
Since my run is only 60 feet and is underground (temperature is not an issue), I will go ahead and run the #4.
I also talked to a local electrician who agreed to do a pre-inspection for me before the city comes to do their inspection. He told me to use 3 - #4s with a #8 ground wire. He also said my 1" conduit is fine. This is the same electrician I used to bring in the 400A service to my house last year.
May I chime in? Over 20 years as an electrician....

Its always best to check with the local jurisdiction for the proper install, and what this inspector told you is ok. Using 1" pvc might be alright, but depending on the number of 90's in your pipe run using small pipe like that can be a real pain to pull the wire through. I would recommend at least 1 1/4" pipe myself. Cost will be barely more but the labor might prove to be much easier. Also, I totally disagree with the ground wire size.I would go with a #4 as a ground wire due to there being much less resistance in the larger wire. Remember you want any stray voltages to go to ground and less resistance to the electron flow is what you are looking for. Also, since the 100A panel is a subpanel, the ground wires and the neutral wires in the 100A panel need to be kept separate and terminated onto separate bars. Insulate the neutral bar from having direct contact with the panel tub (housing) so that the neutrals and grounds are truly separate. You do not want two points of potential. You want stray voltages to go to ground and not to the neutrals. And NO ground rod off the subpanel, thats what the ground wire coming from the 400A panel is for.

As far as the wires sizing for the 100A panel, #4 is acceptable in residential applications, #3 is commonly used in residential and always used in commercial applications, is hardly anymore money, but gives you more wire to carry heavy loads.

ALSO, I did not read where anyone mentioned that all these numbers are concerning COPPER wire and not aluminum. If you use aluminum you will want to go 2 wire sizes larger and increase the size of the pipe accordingly. Make sure everything is good for CU (copper), or AL/CU. If you mix metals you will need to use some sort of anti oxidant to keep the metals from corroding.

This electrical stuff ain't no biggie, its the fire caused by improper installations thats the big deal.....
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Old 05-28-2008, 10:35 AM
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Quote:
Originally Posted by ztoy
May I chime in? Over 20 years as an electrician....

Its always best to check with the local jurisdiction for the proper install, and what this inspector told you is ok. Using 1" pvc might be alright, but depending on the number of 90's in your pipe run using small pipe like that can be a real pain to pull the wire through. I would recommend at least 1 1/4" pipe myself. Cost will be barely more but the labor might prove to be much easier. Also, I totally disagree with the ground wire size.I would go with a #4 as a ground wire due to there being much less resistance in the larger wire. Remember you want any stray voltages to go to ground and less resistance to the electron flow is what you are looking for. Also, since the 100A panel is a subpanel, the ground wires and the neutral wires in the 100A panel need to be kept separate and terminated onto separate bars. Insulate the neutral bar from having direct contact with the panel tub (housing) so that the neutrals and grounds are truly separate. You do not want two points of potential. You want stray voltages to go to ground and not to the neutrals. And NO ground rod off the subpanel, thats what the ground wire coming from the 400A panel is for.

As far as the wires sizing for the 100A panel, #4 is acceptable in residential applications, #3 is commonly used in residential and always used in commercial applications, is hardly anymore money, but gives you more wire to carry heavy loads.

ALSO, I did not read where anyone mentioned that all these numbers are concerning COPPER wire and not aluminum. If you use aluminum you will want to go 2 wire sizes larger and increase the size of the pipe accordingly. Make sure everything is good for CU (copper), or AL/CU. If you mix metals you will need to use some sort of anti oxidant to keep the metals from corroding.

This electrical stuff ain't no biggie, its the fire caused by improper installations thats the big deal.....
Thanks for your reply.
Yes, I will be using all copper and the ground will be separated as you say. I have 2 - 90s and a 45 in the conduit run.

I would like to know more of your reasoning for not agreeing with the #8 for the ground wire. Table 250-122 of the NEC states that for 100A, #8 wire is required.
I assume that your recommendation of using a bigger wire must be for safety reasons you've witnessed though experience rather than what code says.
From my understanding, the ground wire doesn't carry the full load except for extremely short bursts which trips the breaker. Since the bursts are short, heat isn't built up in the wire, thus the reduction in wire size needed.
I am confused at your reference to resistance and stray voltages. Wouldn't the strays be of low power making the resistance difference negligible?
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Old 05-29-2008, 10:30 AM
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I guess I am still wavering on whether to go ahead and get the #3 instead of the #4.
I have already installed the 1" PVC conduit. ( I know, I should have put in 1 1/4.) I am going to buy the wire tomorrow.

Do you think I can pull 3 - #3s and a #8 through the 1 inch OK?
It's only 60 feet total with 2 - 90s and a 45. I'll figure on using lots of lube and stagger the wires on the connection to the pull tape.

Thanks,
Roger
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Old 06-03-2008, 05:15 PM
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Quote:
Originally Posted by roger1
Thanks for your reply.
Yes, I will be using all copper and the ground will be separated as you say. I have 2 - 90s and a 45 in the conduit run.

I would like to know more of your reasoning for not agreeing with the #8 for the ground wire. Table 250-122 of the NEC states that for 100A, #8 wire is required.
#8 is not required, its the minimum allowed. Going larger by one size reduces the resistance to electron flow which is what you want. #8 will work, but the cost difference is so minor that using #4 is what I usually use, at my cost as to the cost difference.
Quote:
Originally Posted by roger1
I assume that your recommendation of using a bigger wire must be for safety reasons you've witnessed though experience rather than what code says.
From my understanding, the ground wire doesn't carry the full load except for extremely short bursts which trips the breaker. Since the bursts are short, heat isn't built up in the wire, thus the reduction in wire size needed.
I am confused at your reference to resistance and stray voltages. Wouldn't the strays be of low power making the resistance difference negligible?
No. You would be better served having a wire where any stray voltages, high or low, can pass through easier than the other alternatives, like breakers or equiptment. Kind of like water that flows through the point of least resistance.

Quote:
Originally Posted by roger1
I guess I am still wavering on whether to go ahead and get the #3 instead of the #4.
I have already installed the 1" PVC conduit. ( I know, I should have put in 1 1/4.) I am going to buy the wire tomorrow.

Do you think I can pull 3 - #3s and a #8 through the 1 inch OK?
It's only 60 feet total with 2 - 90s and a 45. I'll figure on using lots of lube and stagger the wires on the connection to the pull tape.
Pipe is too small. Go with 1 1/4". Check your local code but I believe the new code requires a larger pipe when using pvc.
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Old 06-09-2008, 04:26 PM
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Quote:
Originally Posted by ztoy
#8 is not required, its the minimum allowed. Going larger by one size reduces the resistance to electron flow which is what you want. #8 will work, but the cost difference is so minor that using #4 is what I usually use, at my cost as to the cost difference.


No. You would be better served having a wire where any stray voltages, high or low, can pass through easier than the other alternatives, like breakers or equiptment. Kind of like water that flows through the point of least resistance.



Pipe is too small. Go with 1 1/4". Check your local code but I believe the new code requires a larger pipe when using pvc.



if your going bigger than #8 then why not use #6 as your ground ?
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Old 06-09-2008, 04:53 PM
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I went with 3 - #4s and a #8.
I checked with 2 local electricians and they said that would be fine.

No problem pulling the wires through the 55 foot run of 1" conduit with 2 90s and a 45. Used lube and a helper pushing while I pulled.
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