Sub panel ground wire size ?

Posted by on April 7, 2009, 7:32 am

I'm going to put a subpanel in a garage. I'm going to run #4 copper
off a 70 amp breaker for the hots and neutral. It's about 70 feet and
#6 would probably do but I picked up a partial roll of #4 cheap. But
what I don't know is what the rules are for the ground wire size?

Posted by on May 4, 2009, 3:36 pm

On Apr 7, 7:32=A0am, jamesgan...@gmail.com wrote:

> I'm going to put a subpanel in a garage. =A0I'm going to run #4 copper
> off a 70 amp breaker for the hots and neutral. =A0It's about 70 feet and
> #6 would probably do but I picked up a partial roll of #4 cheap. =A0But
> what I don't know is what the rules are for thegroundwiresize?

I never found anyone with an answer for this. How do you figure out
the ground wire size for a subpanel?

Posted by RicodJour on May 4, 2009, 4:58 pm

On May 4, 3:36=A0pm, jamesgan...@gmail.com wrote:

> On Apr 7, 7:32=A0am, jamesgan...@gmail.com wrote:

> > I'm going to put a subpanel in a garage. =A0I'm going to run #4 copper
> > off a 70 amp breaker for the hots and neutral. =A0It's about 70 feet an=

> > #6 would probably do but I picked up a partial roll of #4 cheap. =A0But
> > what I don't know is what the rules are for thegroundwiresize?

> I never found anyone with an answer for this. =A0How do you figure out
> the ground wire size for a subpanel?

Did you look? I DAG'd
http://www.google.com/#hl=3Den&q=3Dwhat+size+ground+wire+subpanel
and the first hit answers your question and tells you the relevant
code. The Terry Love forum hit near the top is also pertinent, as is
this:
http://www.codebookcity.com/codearticles/nec/necarticle310.htm
R

Posted by on May 5, 2009, 12:15 pm

> On May 4, 3:36=A0pm, jamesgan...@gmail.com wrote:

> > On Apr 7, 7:32=A0am, jamesgan...@gmail.com wrote:

> > > I'm going to put a subpanel in a garage. =A0I'm going to run #4 coppe=

> > > off a 70 amp breaker for the hots and neutral. =A0It's about 70 feet =

and

> > > #6 would probably do but I picked up a partial roll of #4 cheap. =A0B=

> > > what I don't know is what the rules are for thegroundwiresize?

> > I never found anyone with an answer for this. =A0How do you figure out
> > the ground wire size for a subpanel?

> Did you look? =A0I DAG'dhttp://www.google.com/#hl=3Den&q=3Dwhat+size+grou=

nd+wire+subpanel

> and the first hit answers your question and tells you the relevant
> code. =A0The Terry Love forum hit near the top is also pertinent, as is
> this:http://www.codebookcity.com/codearticles/nec/necarticle310.htm
> R

Neither one really answers my question for a 70 amp circuit. I saw it
said #8 for 100amp. I've stumbled across other posts that refer to
tables. I'm guessing it's decided by the supply size breaker, not the
breakers in the sub panel. Given #8 for 100amp I'm guessing maybe I
could use #10 for 70 amp? I've got to go about 70 ft so I don't want
to buy any bigger wire than I have to.

Posted by dpb on May 5, 2009, 12:38 pm

jamesgangnc@gmail.com wrote:
...

> Neither one really answers my question for a 70 amp circuit. I saw it
> said #8 for 100amp....
> ... Given #8 for 100amp I'm guessing maybe I
> could use #10 for 70 amp? ...

Code Sec 250-94 limits minimum size to no less than #8.

Voltage drop tables indicate for 240V you need T rated temperature to
make 75 ft ampacity rating for 70A/2% voltage drop w/ #8 anyway.

I'd pull #6; the initial cost differential is a one-time thing; the
voltage drop savings and peace of mind are forever.

--

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