Max Length for ground wire

[Cutty]

Is there a maximum length for a ground wire that is being ran from a 100 amp sub panel?

The last place had a really short foundation and I just drilled a hole in the floor and pounded my a new one through the floor. It was about 3 feet away form the sub-panel.

New place sits several feet off the ground, old method is not an option. I also do not want the neighbors seeing me pound in a new ground rod, outside, on the opposite side of the house as the meter.

I have about 40-50 feet from where the new sub-panel is going to the ground rod that is in place.

 

[cashmunny]

Hmmm, that's a good question. The length of a ground wire can have implications for electrical noise in sound systems and stuff like that. Not sure about this though. Someone more knowledgable needs to chime in here.

I can just say that resistance is proportional to the length and inversely proportional to the cross sectional area of the wire. So if you increase the length of a ground wire by a factor of 4 , you have to increase the wire diameter by a factor of 2 in order to maintain the same resistance. But what the maximum resistance for a ground wire run is I don't know.

 

[vicious bee]

40 or 50 feet I wouldn't worry about at all. Use #8 copper. I don't know at what distance you have to make it bigger but 40 or 50 feet is not long enough for you to need to upgrade.

 

[PharmaCan]

Your ground wire should be run back to the main panel and attached to the ground bus in the main panel.

PC

 

[Cutty]

Well I only have the 2/2/4/ Al wire. If I had a 2/2/2/4 wire laying around I would do just that.
I had the option when buying the wire the first time but most everything I have read grounds the sub-panel out at the sub side with it's own ground rod, to a minimum depth of 4' (100AMP),
not the main side.

In effect I would be grounding out at the main but instead of connecting it in the box, I would just add another lug to the in-place ground rod and attach my ground there.

I don't want it explained to me how the fire started by a state inspector. Most likely at that point I will be in one of those little rooms.

If I take a 40'-ish length of #6 cu and run it form the ground bar of my sub and connect it to the ground rod servicing my main I can avoid fires, right?

 

[PharmaCan]

First of all, the ground isn't for avoiding fires, proper sizing of the wires and breakers and using proper wiring techniques does that. The ground wire is a safety thing, mainly to keep people from getting zapped.

In any event, nothing says the ground has to be bundled with the other cables. You could just as easily run your ground wire back to the main panel as to the ground rod.

If using Al cable, be sure to use some De-Ox on your connections.

PC

 

[vicious bee]

PharmaCan advice is good advice. Around here we call the Al wire antioxident NoLox.

 

[madpenguin]

You can't ground to a separate grounding electrode as PC already said. You must run it back to your main disconnects neutral terminal. Also, The NEC does state that the grounding conductor must be run in the same raceway as the grounded and ungrounded conductors. All conductors of the same circuit must be in the same enclosure/raceway.

Putting the NEC aside, The grounding conductor doesn't carry current under normal conditions so if you really wanted to, I suppose you could over look that rule and run your grounding conductor separate, but it wouldn't be code compliant. In order to be code compliant, you'd have to ditch that SEU cable and use SER instead.

Also keep your grounds and neutrals separate in your sub...... You would need to size your grounding conductor according to table 250.122, so for 2-2-4 SEU, you would have to use at least a #8 CU or #6 AL green insulated conductor.

Technically, #2 AL for your ungrounded conductors is only good for 75A according to the 2008 NEC. You can throw a 100A breaker on there if you want, but I personally, would not.

 

[madpenguin]

most everything I have read grounds the sub-panel out at the sub side with it's own ground rod, to a minimum depth of 4' (100AMP),
not the main side.

That would be correct if you were on an earlier code revision, the sub-panel was in a separate structure than the main and there were no other conductive paths back to the mains structure.

Also, I think 3' is the minimum depth for a grounding electrode but that's if you bury it completely horizontal. You can also drive it at a 45 degree angle. But grounding electrodes are 8' long so if you drive it vertical, then the minimum depth would be 8', right?

In effect I would be grounding out at the main but instead of connecting it in the box, I would just add another lug to the in-place ground rod and attach my ground there.

That doesn't make sense to me. "in place ground rod"? Forget about the separate ground rod, buy a separate grounding buss bar and fasten it to the metal of the sub panel enclosure. All grounds go there. Neutrals will float on your isolated neutral buss bar. Keep them separated.

 

[PharmaCan]

You can't ground to a separate grounding electrode as PC already said. You must run it back to your main disconnects neutral terminal. Also, The NEC does state that the grounding conductor must be run in the same raceway as the grounded and ungrounded conductors. All conductors of the same circuit must be in the same enclosure/raceway.

Silly me - I assumed he would only drill one set of holes. LOL

That doesn't make sense to me. "in place ground rod"? Forget about the separate ground rod, buy a separate grounding buss bar and fasten it to the metal of the sub panel enclosure. All grounds go there. Neutrals will float on your isolated neutral buss bar. Keep them separated.

What the OP is talking about is running the sub-panel grounding cable to the ground rod that is currently in use for the main panel, which of course makes no sense since he would be almost to the main with the cable and then he wants to make a left turn and take it to the ground rod outside, just to be illegal I guess.

PC

 

[Cutty]

just to be illegal I guess.

That got me laughing! Yeah, want to be safe as I can be while still being illegal.

Here's the driving factor, I tore out some drywall just to run this 2/2/4 and it is all closed back up, mudded, smoothed out and painted again. Now if I HAD to I would rip out that section again, not a big deal in the scheme of things. But then again, I have the wire running under the house. If I can take a ground wire and run it to the same ground rod the main is using then awesome. If not, well I am gonna err on the side of caution and do it right.

Amps can kill they say, and I want to keep mine in check

Final ruling?

 

[WeedChuck]

how old is your home? Houses that were plumbed with galvanised and copper plumbing during construction can have the grounding lines attach to a water spicket pipe.

 

[Cutty]

yeah...no such luck here. Newer home. PVC.

 

[madpenguin]

Well, even running 2-2-4 SEU plus a separate grounding conductor thru the same hole is illegal. A cable assembly is consider as a "raceway", therefore, anything but the use of SER is illegal.

If you were using conduit and ran your 2 hots and a neutral inside the conduit but then taped/strapped the grounding conductor to the outside of the conduit, your in the same boat. The conduit is considered as the "raceway".

Conductors of the same circuit need to be ran in the same raceway. It's pretty cut and dry as far as the NEC is concerned. Since you already mudded your shit back up, go ahead and run a #8 green insulated copper conductor where ever you can run it but please take it back to your main disconnects neutral terminal. If your grounds and neutrals are separated at your main panel, then that likely means you have an outside disconnect. If that's the case, then you terminate your grounding conductor onto your isolated grounding buss in your main panel. Either way you slice it, take it back to the location of the Over Current Device that is protecting the feeders to your sub panel. That is in your main panel, not outside on a ground rod.

Running the grounding conductor separate isn't really a huge deal. It's not going to burn your house down in a ground fault situation. You just have this solo green wire running around that leads someone to say WTF is that for? If it were in the same conduit or cable assembly, you'd know what it was for....

Just make sure you terminate your sub properly with regards to the floating neutrals and grounds attached to the frame of the panel as well as taking the main grounding conductor back to the main panel. Do all that and you'll be just fine.

 

[PharmaCan]

That got me laughing! Yeah, want to be safe as I can be while still being illegal.

Here's the driving factor, I tore out some drywall just to run this 2/2/4 and it is all closed back up, mudded, smoothed out and painted again. Now if I HAD to I would rip out that section again, not a big deal in the scheme of things. But then again, I have the wire running under the house. If I can take a ground wire and run it to the same ground rod the main is using then awesome. If not, well I am gonna err on the side of caution and do it right.

Amps can kill they say, and I want to keep mine in check

Final ruling?

LOL - Yeah, if I'd patched all the drywall already I'd be looking for alternatives too.

Final ruling - I'd listen to what the crazy penguin has to say. He's pretty much on top of things.

...and don't forget to use De-Ox or something similar on all your connections with the Al wire.

PC

 

[vicious bee]

It makes no difference whether you connect your ground wire to the ground bar in the panel or the ground rod outside. They are both ground. The ground rod outside is more at ground potential than the ground bar in the panel. Both are good. As far as the ground in the same raceway.JEEEESSS!! he doesn't care about the NEC he just wants it safe. Just so it's grounded at the same ground as your first outside panel.

 

[madpenguin]

he doesn't care about the NEC he just wants it safe.

That has to be the most conflicting statement I think I've EVER heard. Please use your head before you open your mouth. The NEC is 100%, unequivocally, absolutely and only about minimum safety standards.

There is a reason why all conductors of the same circuit have to be in the same raceway. And it's NOT because someone says "Hey.... What's that lone green wire for?" as I previously alluded to.

You guys don't want to listen to me then don't listen to me. I don't care. If you think you know better than me then you think you know better than me. I don't care. I'm offering code compliant advice, nothing more, nothing less.

Every revision of the NEC it gets bigger and bigger. This is strictly due to the fact that every code cycle more and more people die from electrical related accidents. I used to have a copy of the very first NEC. If I'm not mistaken, it was released in 1890 something. It was literally a pamphlet. Every time someone dies, a revision or amendment is made to the NEC in the hopes that that same type of death will be less likely to happen.

But again, listen to me or don't.... I don't care.

 

[madpenguin]

It makes no difference whether you connect your ground wire to the ground bar in the panel or the ground rod outside.

That is also oh so wrong. The ground fault will clear in the quickest amount of time and will also have the least resistive path back to the xfmr if the grounding conductor is terminated in the same enclosure as the OCPD.

But again..... Don't listen to me. I'm actually full of shit.

 

[vicious bee]

I saw nothing about a ground fault. If he's using a regular circuit breaker it would ground faster on the ground rod, but a ground fault will work just fine with the ground tied to the ground rod. The reason you put the ground in a raceway with the other conductors,as you know, is so that in an grounded condition you don't touch it and get shocked. There's nothing wrong with what you said, but you quoted ,"all conductors of the same circuit have to be in the same raceway". Isn't a raceway generally EMT. He's not running EMT. He said he was going to run an insulated ground. The reason I said run it to the ground rod is I believe he said he was going to run it under the house. It would be easier to hide and probably easier to run. I'm not trying to get in a pissing match with you and I shouldn't have been so snarky "sorry". What I said though was perfectly safe. He's not an electrician. If he stays out of the electrical panel and ties the ground to the ground rod It's probably safer for HIM. Would he be better off with an electrician. Not if the electrician figures out what he's doing. No he would not. Better to give advice that he can use HIMSELF than quoting the NEC. No one said you were full of shit. Your recitation of the NEC is a thing of beauty. Sorry could not help the last line.

 

[madpenguin]

No, I'm not trying to be pissy either. I'm just sticking with what I have in my signature and giving advice based upon the 2008 NEC...

Article 100 defines a raceway as "An enclosed channel of metal or nonmetalic materials designed expressly for holding wires, cables....". Not going to type the rest but it does mainly refer to emt, imc, rmc, ent et. all.... It's a general and broad definition but a cable assembly (romex) is considered as a raceway as I've bolded above.

As for a ground fault, that's the only reason why you want a separate grounding conductor in the first place. Other wise, ditch the grounding conductor and just use the return neutral for any ground faults.....

Yes, he could attach the grounding conductor to the outside grounding electrode. But now you've just created not only a longer path for ground fault current to return to it's source, but you've just also created a higher resistive path for it to take as well. The whole purpose for having a dedicated grounding conductor is to give ground fault current the least resistive path back. Power will always return to it's source, so if you terminate at the neutral buss in the main panel, it facilitates the OCPD to trip immediately as well as funneling the ground fault back out the service drops neutral.

Yes, I'm kind of splitting hairs here. But the proper way to do this is to terminate the grounding conductor in the same enclosure as the breaker.

You guys are always free to do what you want, I'm just trying to help.