Wire size for a 125A Sub-Panel

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  #1  
Old 04-03-06, 02:59 PM
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Wire size for a 125A Sub-Panel

I am installing a 125A subpanel from my existing 200A subpanel. I have a 70' run. I am planning on installing two #2 for hot, one #2 for common, and a #6 for ground. I can put this on a 100A breaker at the 200A panel. Am I good for wire size or do I need to go larger or what?? Thanks for all the help
 
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  #2  
Old 04-03-06, 03:20 PM
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If your local electrical inspectors permit the use of NEC table 310.15(B)(6) for residential sub panel feeders, then #2 copper is acceptable for 125A. (residential main feeder rule)

If your load calculations come in at less than 115A, then #2 copper may be protected with a 125A breaker. (round up rule)

Otherwise you will need to use #1 copper for your conductors.

Voltage drop should not be a significant issue at this distance.

-Jon
 
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Old 04-03-06, 03:28 PM
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Since I have not yet bought the wire, then I may as well get #1. Phoenix uses the NEC so it will be ok. But existing conduit is in the attic and is RNC (pvc Sched 40) 1.25". Looks a little tight for #1..... Fill chart says I am ok..... What size ground would I then use??? #4 ???? And can I use insulated wire for ground THHN..... I have some already... Thanks
 
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Old 04-03-06, 03:45 PM
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Since you are using a 100 amp breaker to feed this panel you can most likely use 310.15(B)(6) which will allow #4cu for the hots and neutral. A #8 ground is all that is needed.

Just double check with your local code office to see if this chart is used in your area. It is in mine.
 
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Old 04-03-06, 04:54 PM
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I plan to use at least #2. I may want to expand its usage later, you never know. I know I cant go behyond the 125A, so if I want to plan for later and still use a 100A breaker for now, should I go #1 or will #2 work ok???? If its in the NEC then my inspector said he will accept it.... Ground size for #2 is #6?? What ground for #1 ??????/

thanks again
 
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Old 04-03-06, 05:05 PM
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That would be a big whoops on my part.

I saw the 125A panel number but not the 100A breaker number. My above assessment thus changes to:

If your local electrical inspectors permit the use of NEC table 310.15(B)(6) for residential sub panel feeders, then #4 copper is acceptable for 100A. (residential main feeder rule)

If your load calculations come in at less than 85A, then #4 copper may be protected with a 90A breaker. (round up rule) (This is if the above statement doesn't apply.)

Otherwise you will need to use #3 copper for your conductors protected by the 100A breaker.

Voltage drop should not be a significant issue at this distance.

The minimum size for the equipment grounding conductor would be #8 copper. It can be _green_ insulated THHN, bare, or _larger_ that #6 copper and marked with green tape at the ends.

-Jon
 
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Old 04-03-06, 05:39 PM
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OK.... Heres my plan ...... Feed the 125A box with #2, ground #4 since I have it. For now, use an existing 100A breaker that I have. If I ever want to go up to 125A service, then I am hearing that the #2 will meet/exceed that....... Did I get it??

Thanks all
 
  #8  
Old 04-03-06, 05:50 PM
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Pretty close.

What you propose would work just fine at 100A.

Furthermore, you could change to a 125A breaker if either of the conditions that I mentioned in my first post are present.

I suggest that you spend some time working load calculations prior to buying the wire. Just play with the numbers some, figure out what the maximum unbalanced neutral current would be, what sort of 240V loading you would have on this panel, etc.

If the maximum unbalanced neutral current that you could envision is less than 85A (say you have 8 20A 120V circuits, with the rest being 240V circuits), then you could use the #4 that you have for the neutral, run #2s for the 'hot' conductors, and run a #6 for the ground. This would save a few bucks, and give an easier pull in the conduit. But you do lose a bit of later expansion capability, because it limits the number of 120V circuits you can install.

-Jon
 
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Old 04-03-06, 06:15 PM
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I think I am OK....... I will have 6 20A lighting and outlet circuits, 2 dedicated GFCI circuits, 2 240V pool pumps (20A each I think) and a 60A A/C circuit.

Thank you all..........
 
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Old 04-03-06, 06:25 PM
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Given those numbers, I change my suggestion to a _strong suggestion_.

Find out the nameplate full load amp ratings of the pumps and the AC. The trip rating of the circuit breaker is only a maximum, and the actual device will draw less than this amount. You need to figure out exactly how much these items will be using. How many square feet will the 120V circuits service? Any other interesting/heavy loads to consider?

ACs and pumps are big power consumers, and given your location they could be running continuously for several hours at a time.

-Jon
 
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Old 04-03-06, 09:55 PM
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Hers what I need power for:
A/C - 27A
2 Pool [email protected] 11A - 22A
1 Furnace (gas) 9A
6 house circuits 35A
1GFCI 5A

Total 98A

Looks like I better pull the #2

I could pull the A/C circuit from the 200A panel and not put it one this subpanel. Comments
 
  #12  
Old 04-03-06, 11:05 PM
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>Here's what I need power for:
>A/C 27A
>2 Pool Motors @11A 22A
>1 furnace (gas) 9A
>6 house circuits 35A
>1 GFCI 5A
> Total 98A

I get about 75A total. Definitely under 80A assuming that it all runs at the same time.

> Looks like I better pull the #2

I think it could be a good idea. How many spaces in this panel?
Don't run #2 only to shortchange yourself with an 8-space panel.


> I could pull the A/C circuit from the 200A panel and not put it
> one this subpanel. Comments.
Bad idea. The A/C will benefit from using the heavier conductor for 70' rather than a #8. Heavier conductors have less voltage drop (good for startup) and less power wasted on heating the conductors (good for the environment; less work for A/C).
 
  #13  
Old 04-04-06, 04:57 AM
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Originally Posted by Hooter
Hers what I need power for:
A/C - 27A
2 Pool [email protected] 11A - 22A
1 Furnace (gas) 9A
6 house circuits 35A
1GFCI 5A
Is the gas furnace a 120V or 240V load?
When you say 6 house circuits 35A, did you calculate that you are supplying 8400VA of load (by square footage) and then divide by 240? Or did you get the 35A number in some other fashion? Is the 35A number a 240V or 120V value.
Same question for the GFCI?

Originally Posted by Hooter
I could pull the A/C circuit from the 200A panel and not put it one this subpanel. Comments
I would wire the A/C so that it has the shortest, fattest wires that make sense. If it is very near to this subpanel, then I'd use the subpanel because the feed to the subpanel will be nice and thick (low resistance). If it is between the subpanel and the main panel, then I'd wire directly to the main panel.

There are other options which you might consider, eg. running two 70A subpanels, 1 for the 240V loads, and 1 for the 120V loads.

-Jon
 
  #14  
Old 04-04-06, 07:12 AM
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The new subpanel will be a 20 space.
The gas furnace is 110V.
The footage of the house that it will supply (bedrooms and 1 bath) is 1190 sq. ft.
I got the amps from the calcs on another blueprint that ahd an average of 5A per outlet circuit.
The GFCI circuit will have 4 outlets.
 
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Old 04-04-06, 09:12 AM
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> The area of the house that it will supply (bedrooms and 1 bath) is 1190 sq. ft.

Please correct me if I don't understand, but is this same space already served by the 200A panel?
 
  #16  
Old 04-04-06, 09:29 AM
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Well YES and NO !! This area (not built yet) is serviced by the 200a panel in the enginners calcs. Since the 200A panel is a little far away (I will have outlet runs of 200' if I use the 200A panel) I decided the best bet is a subpanel that is much closer. I am not really adding any load beyond the engineers calcs, just moving the point of access to the power (new 125A subpanel). Hope that clears that up. My house has a 400A Main with two 200A subs. My amperage use calcs out out 285A between the 2 subs, so I still have a lil "wiggle" room.
 
  #17  
Old 04-04-06, 10:02 AM
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> This area (not built yet)

Okay, that clears it up. This panel does not have to go by floor area. It can be sized for the specific loads that it serves.

However, looking at the square footage and distance, I would not go with any less than 125A to reduce the future expense of running yet another subpanel.
 
  #18  
Old 04-04-06, 01:08 PM
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hooter,

I think that you have a good setup here.

I would pull the #2 hot conductors, which will let you go up to a 125A supply breaker in the future. Note the conditions for using #2 conductors at 125A that I mentioned above; you may want to pull #1 hot conductors but would need to calculate conduit fill _very_ carefully.

You only need a #8 for the EGC, but need to pull a #6 if you want to change to a 125A supply breaker. You've mentioned that you have a #4 sitting idle, but if conduit fill becomes an issue you won't want to use it. If conduit fill isn't a problem then just use it.

The only decision left is the size of the neutral. You have about 49A of 240V load, which doesn't use the neutral at all. You can size the neutral to carry the maximum unbalanced current possible with your system. You have a total of 8 120V circuits; if these are 20A circuits and you balance them on the two supply legs, then the _maximum_ unbalanced neutral current would be 80A. A #4 conductor can carry 85A, so with the panel as you've described it, you could feed it with 2 #2 hot conductors, 1 #4 neutral conductor, and 1 #6 EGC. This would, however, limit your ability to add additional 120V circuits.

Having seen your load calculation, I'd probably run a full sized neutral.

-Jon
 
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Old 04-04-06, 03:23 PM
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According to a fill chart I saw, 1.25" EMT (Yea I know Im using RNC) will handle 4 #1 or 5 #2. I dont know the difference between the I.D.'s of the two but they must be close. If I can get 3 #1 and a #4 ground, I will...... Otherwise it will be 3 #2 and #4 ground....( I am determined to use that #4)

Thanks
 
  #20  
Old 04-04-06, 03:32 PM
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RNC SCH40, trade size 1-1/4inch, 40% fill for more than 2 wires, 374mm^2 available
1ga THHN/THWN 100.8mm^2
2ga THHN/THWN 74.7mm^2
4ga THHN/THWN 53.16mm^2

3 #1THHN conductors plus 1 #4THHN conductor is essentially the max you can stick into that pipe.

I've never done a pull of that size or that close to maximum fill; I invite anyone with such experience to chime in and tell Hooter how to do this properly.

-Jon
 
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Old 04-04-06, 03:58 PM
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I got an idea... I have some 3/4" RNC here. I am still somewhat open where I COULD run a 3/4" "sister conduit with one #1 in it...... What you think?? Little more work but I hate hard pulls....
 
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Old 04-04-06, 04:08 PM
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According to a fill chart I saw, 1.25" EMT (Yea I know Im using RNC)
tell me if i am reading this right EMT conduct or RNC ??

there is big diffrence there with inside diam.

for 125 amp service system generally i used 1 1/2 inch pvc or emt depending on the run goes.

I got an idea... I have some 3/4" RNC here. I am still somewhat open where I COULD run a 3/4" "sister conduit with one #1 in it...... What you think?? Little more work but I hate hard pulls....
this is not a good idea at all for safey reason !!!


if need more quetion please do drop a line here

merci , marc
 
  #23  
Old 04-04-06, 04:30 PM
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Marc... The 1.25" conduit is existing and it is RNC schedule #40 pvc. What concerns are there in pulling both hots and the ground in one conduit and pulling the neutral in another?? Does it have to do with someone later not understanding the set up , etc or is it against code?? It will be going from one subpanel to another......
 
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Old 04-04-06, 04:50 PM
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> I COULD run a 3/4" "sister conduit

No, for the small conductors that you have, the NEC requires that all conductors for a circuit be in the same conduit/cable/raceway.

How many bends are in the run?

Based on all the total picture including Jon's engineering study and available materials, I think you should go with #4 for the neutral.

On the rest, go with either two #1 hots and a #6 EGC or two #2 and a #4 EGC.

And these conductors had better be soft copper.
You'll need wire lube, and someone to pull while you lubricate and push.

Use plenty of lubricant. Pulled a lubricated cloth through first to wax the inside right before you pull the wires.
 
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Old 04-04-06, 05:38 PM
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I have 2 90 sweeps and a 45 in the mid........ I think I can get em in there.....Thanks
 
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Old 04-10-06, 08:13 PM
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Update !!

OK..... I made the pull with 2 - #1 for hot, #2 neutral and a #2 ground. I know I am maxed for the 1.25 RNC. The pull went very well. I pulled and wife feed and lubed. The last sweep was a lil tuff, but once thru, no problem....... Thanks all
 
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