Panel Questions

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  #1  
Old 09-24-05, 09:35 AM
bigblock
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Panel Questions

I have run 90 ft of 1" SCH 40 PVC to a sub into my garage. The charts in Lowes/HD show #4 is good for 95 amps and #6 is good for 75 amps. I have a few questions on what I should be using. I have a 40 amp 220v compressor that needs power right now, maybe more power will be needed later. I bought a 125A subpanel that will have the single 220v 40 Amp breaker, I just need power for that.

1. Can I use a 70 amp breaker with #6 THHN single stranded copper? Or does the breaker need to be smaller?

2. If I do use #6 wire, what's the largest feeder breaker I can use?

3. Since #4 would be at 1" conduit fill max, What size feeder can I use for that?

4. I talked with the inspector and he said I could not use a 100amp feeder with #4 wire, he said I could only upsize the breakers in the main panel. Is this true?

5. Could I use the smaller breaker to pass inspection and then go ahead and install the 100amp? I don't know why it would be okay to do in the main panel, but not the subpanel. Seems like an inspector intepretation issue.
 
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  #2  
Old 09-24-05, 01:36 PM
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Join Date: Jan 2004
Location: Oregon
Posts: 1,219
I think that you have a bit of reading to do. I suggest that you get a couple of books on wiring and read them.

The issue is that there are _many_ details to electrical wiring, and right now I suspect that you don't even know all of the questions to ask. I will answer each of the points in your post, but I strongly suggest that you not get started until you know more. The fact that you've already installed a conduit that is potentially undersized for your application should give you pause to step back and do some more planning.

1) You are misreading the table. According to the NEC, you must use the temperature column for both the conductors and the terminations...and while you almost certainly have 90C wire, you also almost certainly have 75C terminations. Thus you must use the 75C column. If you are using _copper_ conductors, then #6 has an ampacity of 65A and #4 has an ampacity of 85A.

2) If you use #6 wire, and your _calculated load_ is 65A or less, then you may use a 70A breaker. Note that this does not mean that you can use this wire to carry 70A. It means that the wire can carry up to 65A but may be protected with a 70A breaker.

3) The ampacity of the #4 conductors is 85A, with a maximum 90A protection. Note that you do not need to pull _four_ #4 conductors. Your ground only needs to be a #8. You may even be able to reduce the size of your neutral, and if you can demonstrate lower maximum loading on your neutral, you could reduce its size. But in a 1" SCH 40 PVC you can just barely fit 3 #3 wires plus 1 #8, which is good for 100A.

4) The NEC has a specific exception for main panels in residential use only that lets you use #4 conductors with 100A protection. This does not mean that the NEC lets you push 100A through those wires. It only means that in residential service the dynamic aspects of the load (you _rarely if ever us the full capacity_) permit you to safely protect an 85A conductor with a 100A breaker. The NEC does not permit this sort of use for subpanels. _Some_ inspectors will permit this sort of use; I personally believe that they are wrong to do so.

5) No. It might be an inspector interpretation issue, but that does not make it okay to go around the inspector. _All_ electrical systems have a chance of failure; even ones installed perfectly to code. There are liability issues involved with willfully skirting inspection requirements.

-Jon
 
  #3  
Old 09-24-05, 01:51 PM
bigblock
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Originally Posted by winnie
I think that you have a bit of reading to do. I suggest that you get a couple of books on wiring and read them.

The issue is that there are _many_ details to electrical wiring, and right now I suspect that you don't even know all of the questions to ask. I will answer each of the points in your post, but I strongly suggest that you not get started until you know more. The fact that you've already installed a conduit that is potentially undersized for your application should give you pause to step back and do some more planning.

-Jon
Jon,

Thanks for the info. I have a few books already but they don't really cover the areas I need help with. That's why I'm here.

The load in my garage will never exceed above 50 amps. The compressor motor is rated at 22 amps(they recommend using a 40A) and maybe a seperate 110v welder @ 20Amps. That's about all I will ever use out there.

I just wanted to run a little bit bigger cable to prevent from only having a single 220v outlet in the garage. That's the only reason I'm putting in a panel.

I think 6 GA will be more than enough for now. I'll use a 60 amp feeder and it should all be well within the voltage drop loss I have calulated. Unless you think I can use a 70 amp. I will most likey never have 2 tools running at the same time, so don't think this should be an issue.
 
  #4  
Old 09-24-05, 04:18 PM
ollie
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Bigblock
I think #6 is fine for what you need. Use a 50 or 60A breaker in the house. I don't know how many wires your planing to pull but if it was me I'd pull 4 (two hot, one neutral and a ground) your going to have to pull a ground anyway. This will give you the option for 120V in the future. Four THHN/THWN will work with your 1" PVC.
The 125 Amp panel is over kill. If you can trade it in for a 60 amp disconnect switch rated for "service entrance use", that is the neutral buss is insulated from the box and the grounding buss is bonded to the box, if it's fused use 40A fuses.
I take it the building is not attached to you house so your also going to have to drive a ground rod and attach it to the grounding buss. Good Luck
Ollie
 
  #5  
Old 09-24-05, 06:22 PM
bigblock
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Originally Posted by ollie
Bigblock
I think #6 is fine for what you need. Use a 50 or 60A breaker in the house. I don't know how many wires your planing to pull but if it was me I'd pull 4 (two hot, one neutral and a ground) your going to have to pull a ground anyway. This will give you the option for 120V in the future. Four THHN/THWN will work with your 1" PVC.
The 125 Amp panel is over kill. If you can trade it in for a 60 amp disconnect switch rated for "service entrance use", that is the neutral buss is insulated from the box and the grounding buss is bonded to the box, if it's fused use 40A fuses.
I take it the building is not attached to you house so your also going to have to drive a ground rod and attach it to the grounding buss. Good Luck
Ollie
I know the 125amp panel is overkill, I just like to be able to add a few circuits later if needed. The 100amp panel was really small and only had 6 openings, 2 220v circuits would eat up 4 of those slots. So, that's the only reason I went with the 126amp panel. I really only need the 40 amp right now. The panel was cheap too, $20 with a nice closeable door. I was planning on running 4 cables through the conduit, with the ground and neutral seperated in the box.

Anything else I'm missing before I plunk down $130 for 500ft of wire?

thanks, jason
 
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