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mr500 04-15-2008 10:49 PM

Sizing Wire
 
Im running a new 100amp sub about 40 ft from house. In you guys' opinion, should I go with THWN #2 or #3 for my hots. As well, What about the neutral and EG? I'm kinda confused about the 90/75 degrees that rate this type wire. Also I understand about having to conduit this in from panel to panel underground. Asking this because I was going with 2-2-4-6 al mobile home feeder but the place I was going to buy it from went out of business and I WONT listen to the guys at HD and Lowes. TIA.

May have answered my own question with the NEC Table 310.15.B.6
Posted it up as well. Says I can use up to #4 for 100amp. Whats your guys' opinion on that?? So If I use #4 does that not mean I can use #8 for the EQ ground??

So in a nutshell I will run 2 #4 for hot, 1 #4 for Neutral and 1 #8 for equipment gnd. out to sub. Does this all compute OR am I way off base?


Mike

Stubbie 04-15-2008 11:49 PM

Can't use that table it is for main service entrance conductors or main power feeders that supply an entire dwellings electrical. Sub panels do not qualify.

You must use 310.16 for you wire ampacity.

No voltage drop concerns require #3 awg copper but #2 is easier to find for a DIYer.

Wrong table for the equipment ground you need to use T250.122.

mr500 04-16-2008 12:04 AM

2 Attachment(s)
OK are these correct ?? So if i can find #3 I can use that if not use #2 OK.. On the neutrals I was going to go with same size as my HOTS.

The gnd will have to be #8 no matter what I feed with ??

So 2 #3's for hots, one #6 for Neutral and one #8 for eq gnd. Would this do me and let me have power. I luv power lol

All of which will be THWN buried 2' in conduit!! Correct? One last thing, what is the temp rating all about? IS that the ground temp or what? just like to be informed????

Stubbie 04-16-2008 10:45 AM

S
Quote:

o 2 #3's for hots, one #6 for Neutral and one #8 for eq gnd. Would this do me and let me have power. I luv power lol
Neutral should be full sized...same as your hots. For over 60 to 100 amp overcurrent protection a #8 copper for your equipment ground.

jrclen 04-16-2008 10:46 AM

The temperature listing you see in the ampacity tables is the rating of the insulation. Most lugs and terminals have a maximum 75 degree rating, so that is the column to use for conductor types listed in the 90 or 75 degree column. The 90 degree column is used as a starting point for any required derating such as in table 310.15 B 2 a, for the wire types listed in that column. Make sure you use the correct side for either copper or aluminum.

Many jurisdictions (including mine) will make you show your load calculations to justify reducing the neutral conductor size on feeders. I would check that before pulling it in.

The equipment ground is sized per the size of the over current protection devise on the circuit. 100 amps requires a #8 minimum. As Stubbie said, see table 250.122.

See table 300.5 for burial depths. You mentioned conduit. 18" for PVC or 6" for rigid or intermediate metal.

Stubbie 04-16-2008 02:07 PM

Quote:

Many jurisdictions (including mine) will make you show your load calculations to justify reducing the neutral conductor size on feeders. I would check that before pulling it in
.

John

Thats exactly why I don't like homeowners fooling with reducing neutrals. There is no guarantee that the calculation will be verified.

mr500 04-16-2008 05:19 PM

OK Thanks guys. I will keep the Neutrals same size as my main hots. In this case #3 IF I can find it. Also #8 for equipment ground.

One last thing I have #6 bare, will that be OK to run out my panel down to the ground electrode outside ????

TexasSparky 04-16-2008 09:31 PM

Quote:

Originally Posted by mr500 (Post 116879)
OK Thanks guys. I will keep the Neutrals same size as my main hots. In this case #3 IF I can find it. Also #8 for equipment ground.

One last thing I have #6 bare, will that be OK to run out my panel down to the ground electrode outside ????

Yes #6 will be fine.

Cow 04-16-2008 09:58 PM

Quote:

Originally Posted by Stubbie (Post 116783)
SNeutral should be full sized...same as your hots. For over 60 to 100 amp overcurrent protection a #8 copper for your equipment ground.

What about 250.122(B)? Honestly, I never really thought about it until now, since #2 is so common for 100 amp circuits. I actually ran one today, two #2's, #4 neutral, and a #8 ground. Hope the inspector doesn't catch it...:jester:

mr500 04-16-2008 10:40 PM

I just found a place that has the 2-2-4-6 direct burial mobile home feeder. Its a little cheaper being its aluminum. Is there a preference among you pros out there?

TexasSparky 04-16-2008 10:45 PM

I don't like any direct burial. If anything ever happens to it your S O L.

mr500 04-16-2008 11:12 PM

Happens to it? Such as? Just curious is all. :eek:

jrclen 04-16-2008 11:12 PM

Quote:

Originally Posted by mr500 (Post 116953)
I just found a place that has the 2-2-4-6 direct burial mobile home feeder. Its a little cheaper being its aluminum. Is there a preference among you pros out there?

I was hoping you weren't going to do that. :laughing: For aluminum you will need a #1 wire.

jrclen 04-16-2008 11:22 PM

Quote:

Originally Posted by Cow (Post 116935)
What about 250.122(B)? Honestly, I never really thought about it until now, since #2 is so common for 100 amp circuits. I actually ran one today, two #2's, #4 neutral, and a #8 ground. Hope the inspector doesn't catch it...:jester:

The conductors aren't being upsized due to voltage drop or any reason related to proper circuit operation. So I wouldn't apply that here. Good point though. Are you talking about a service with aluminum 2-2-4-8? We are talking about a copper feeder with 2-2-2-8. I doubt the inspector will even blink at your wire size if it is a service. That is very common. :thumbsup:

jrclen 04-16-2008 11:24 PM

Quote:

Originally Posted by mr500 (Post 116965)
Happens to it? Such as? Just curious is all. :eek:

There are probably a million miles of direct burial wire in use right this minute. Some guys just don't like it. Some guys don't like Miller Lite either. :laughing:


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