Sub panel ground help!

[camaro0991]

Hey all, this wekeend, I am installing a 100 amp sub panel in my garage. Currently, I have 3, 2 gauge direct burial aluminum wires that will be connected to my electrical box in my houses basement and ran back to the garage (roughly 200 feet). 2 hots, and a neutral. Then I will have 2 grounding rods outside of the building driven into the ground. Does this sound correct? The way it was explained to me I didn't need to run a 4th wire as the grounding rods would be acting as a ground. Thanks!

 

[trbomax]

In MI you need a 4th lug in the panel and the ground must be hardwired back to the service panel. The ground and nuetral must be separated in the sub.

 

[FreddiFiche]

Nope...you NEED four wires. The Ground must be carried to any sub panel, whether it is in the same building or a different building. The Ground rods at the sub panel can be used, bonding them to the ground conductor carried from the main panel.

The sub panel MUST have a isolated ground and neutral.

Anything less is dangerous, and will fail inspection in any corner of the country.

Sorry for the harshness, but the bad advice you have been given is fairly common....but it just doesn't cut mustard.

 

[Stuart in MN]

Where do you live? If you're in the US, the national electric code requires you to run four conductors (two hots, a neutral and a ground) to any subpanels (there may be exceptions in some states that are still using older versions of the NEC.)

Also, it sounds like you have three separate wires, not a single multiconductor cable...is that correct?

 

[Speedy Petey]

The Ground rods at the sub panel can be used, bonding them to the ground conductor carried from the main panel.

Grounding electrode(s) at a detached structure with a panel MUST be used, not "can" be used.

 

[camaro0991]

Where do you live? If you're in the US, the national electric code requires you to run four conductors (two hots, a neutral and a ground) to any subpanels (there may be exceptions in some states that are still using older versions of the NEC.)

Also, it sounds like you have three separate wires, not a single multiconductor cable...is that correct?

PA. Yes I have 3 seperate 2 gauge wires. Its a 200 foot run so we needed to use 2 guage.


What wire thickness does the ground wire need to be...I'm hoping not another 2 gauge wire.

 

[Speedy Petey]

The way it was explained to me I didn't need to run a 4th wire as the grounding rods would be acting as a ground. Thanks!

Who ever explained this was quite wrong.
Ground rods NEVER took the place of a grounding conductor. They both serve VERY different purposes.

In older codes, with some very specific requirements, it was allowed to use the NEUTRAL also as the grounding conductor by bonding the neutral to the panel box, just like in a main panel. This exception to the code was removed a while ago and four individual conductors being required regardless of the situation or installation.

 

[FreddiFiche]

Grounding electrode(s) at a detached structure with a panel MUST be used, not "can" be used.

Yep...in my effort to reply, I was more answering from the perspective that he already had them, so the shouldn't be returned. If there was no mention, I would have been more insistent.

 

[Speedy Petey]

PA. Yes I have 3 seperate 2 gauge wires. Its a 200 foot run so we needed to use 2 guage.


What wire thickness does the ground wire need to be...I'm hoping not another 2 gauge wire.

The ground only needs to be #8cu or #6al.
Also, in many places #2al is not OK to use for 100A. You must use larger wire or use a 90A feeder breaker in the main panel.
The 100A main can be used at the sub-panel since it is only acting as a means of disconnect.

 

[Speedy Petey]

Yep...in my effort to reply, I was more answering from the perspective that he already had them, so the shouldn't be returned. If there was no mention, I would have been more insistent.

 

[camaro0991]

Wow thanks all for the replies, im an idiot when it comes to wiring. Ther person whose actually doing this told me I was wrong too so i just wanted to make sure.

NOW, finding direct burial cabel the day before ima bout to do this, especially 200 feet might not be all that fesable. (remote area). I have a seperate, pre existing 50 amp sub panel already in the garage, fed by underground cable i have no access to, in a different location than where this new box is going. If I dissconect this existing panel at both sides, theoretically I could use that pre existing feed as a ground, and turn that existing panel into a junction box, and run the ground in conduit to the new panel correct? ( i have a really goofy setup its hard to explain)

obviously id be dissconnecting the existing wire from the breaker in the house and grounding it, as id rather not burn my house down lol.

 

[hidollartoys]

The ground is required to be in the same conduit as the conductors.

 

[n2ocamaro]

The ground only needs to be #8cu or #6al.
Also, in many places #2al is not OK to use for 100A. You must use larger wire or use a 90A feeder breaker in the main panel.
The 100A main can be used at the sub-panel since it is only acting as a means of disconnect.

The 90A breakers are hard to find. At least in my area they are. None of the big box stores had them and my local electrical supply did not either. I had to drive 30 miles to get one. Then my inspector (30 year master electrician) told me I could have used a 100A since it is a residential application.

 

[pattenp]

That's a new one to me. I don't where in the NEC that says that for a branch circuit. I think 100A would be okay on #2 Al if it's a service.

The 90A breakers are hard to find. At least in my area they are. None of the big box stores had them and my local electrical supply did not either. I had to drive 30 miles to get one. Then my inspector (30 year master electrician) told me I could have used a 100A since it is a residential application.

 

[Falcon67]

How much power are we really talking here anyway? I have 960 sq/ft on 130' run of 2-2-2-4 aluminum. Two AC units, welder, 240V compressor, mill, drill press, lathe, lotsa lights, etc. All on a 70A breaker. No problems. The shop panel is a 100A 20 slot, the main out there makes a great disconnect.

 

[Stuart in MN]

That's a new one to me. I don't where in the NEC that says that for a branch circuit. I think 100A would be okay on #2 Al if it's a service.

It's allowed for a residential service entrance, but since this is for a subpanel it falls under a different part of the code. It's one of those quirks of the NEC that is often missed.

 

[Norcal]

That's a new one to me. I don't where in the NEC that says that for a branch circuit. I think 100A would be okay on #2 Al if it's a service.

Here is a copy & paste from the 2008 NEC 310.15(B)(6)

(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders. For individual dwelling units of onefamily,
two-family, and multifamily dwellings, conductors,
as listed in Table 310.15(B)(6), shall be permitted as
120/240-volt, 3-wire, single-phase service-entrance conductors,
service-lateral conductors, and feeder conductors
that serve as the main power feeder to each dwelling unit
and are installed in raceway or cable with or without an
equipment grounding conductor. For application of this section,
the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by
branch circuits or by feeders, or both, all loads that are part
or associated with the dwelling unit. The feeder conductors
to a dwelling unit shall not be required to have an allowable
ampacity rating greater than their service-entrance conductors.
The grounded conductor shall be permitted to be
smaller than the ungrounded conductors, provided the requirements
of 215.2, 220.61, and 230.42 are met.

That 30 year "Master Electrician" must have picked up his Lic. in a Cracker Jack box as a prize. A subpanel or a outbuilding does not meet the criteria listed to allow undersizing the wire.

Bold by me.

 

[Speedy Petey]

It's allowed for a residential service entrance, but since this is for a subpanel it falls under a different part of the code. It's one of those quirks of the NEC that is often missed.

It is not "missed", it is a difference of interpretation.

MANY inspectors and electricians would use <the old> 310.15(B)(6) for feeders as well as services, since it DOES say it is for feeders. It is the way the section is worded that gets misinterpreted.

MANY areas still allow this regardless of code cycle.

 

[Speedy Petey]

That's a new one to me. I don't where in the NEC that says that for a branch circuit. I think 100A would be okay on #2 Al if it's a service.

This is not a branch circuit, it is a feeder.

 

[Speedy Petey]

The 90A breakers are hard to find. At least in my area they are. None of the big box stores had them and my local electrical supply did not either. I had to drive 30 miles to get one. Then my inspector (30 year master electrician) told me I could have used a 100A since it is a residential application.

That's a shame.
I don't rely on home centers for anything electrical. They are a fall back for me for certain items.
The three electrical supply houses local to me all carry 90A breakers in stock.

 

[camaro0991]

The ground is required to be in the same conduit as the conductors.

there is no conduit...its direct burial cable. it shouldnt matter where the ground is as long as its grounded.

 

[Speedy Petey]

there is no conduit...its direct burial cable. it shouldnt matter where the ground is as long as its grounded.

It certainly DOES matter. It must be run with the other conductors.

 

[pattenp]

Ditto.. I knew that.

This is not a branch circuit, it is a feeder.

 

[Jefe]

The 90A breakers are hard to find. At least in my area they are. None of the big box stores had them and my local electrical supply did not either. I had to drive 30 miles to get one. Then my inspector (30 year master electrician) told me I could have used a 100A since it is a residential application.

Guys, you're already on the internets; nothing is hard to find. Let the UPS guy do the driving:

http://www.amazon.com/s/ref=nb_sb_n...ords=90+amp+breaker&rh=i:aps,k:90+amp+breaker

I only mention it here because I've seen "hard to find" come up before and it surprises me.

 

[Speedy Petey]

I only mention it here because I've seen "hard to find" come up before and it surprises me.

You have to remember, there are still people that think if it can't be had from a home center big box then it is "hard to find".

 

[Falcon67]

I want it NOW and I don't wanna pay freight!

 

[n2ocamaro]

That's a shame.
I don't rely on home centers for anything electrical. They are a fall back for me for certain items.
The three electrical supply houses local to me all carry 90A breakers in stock.

Yeah, from now on I am always going to my local supply house first. Very knowledgeable and up to date on codes. You usually can not find anyone working in electrical at the home centers and if you do, they do not know much.

Guys, you're already on the internets; nothing is hard to find. Let the UPS guy do the driving:

http://www.amazon.com/s/ref=nb_sb_n...ords=90+amp+breaker&rh=i:aps,k:90+amp+breaker

I only mention it here because I've seen "hard to find" come up before and it surprises me.

Well aware. I needed it that weekend to finish.