Subpanel wiring

[Portland]

At my house there is a subpanel in the detached garage that is feed by 6/3 wiring (two hots and a neutral). I have been reading this forum and it seems that my wiring may not be up to current code. The nuetral and bar is grounded by a ground rod driven into the ground outside of the garage. The garage doesn't have water. Question is the garage wiring safe? How come some suggest not bonding the neutral and grounds at a subpanel - I still don't see it when it is bonded at the house.
Thanks

[montytx]

Normally you have a hot and a neutral and a ground. As long as the ground is secure you should be okay. I work on old houses and most of them arent even properly grounded and have been around for 80+ years.

[bkrahmer]

Sorry, Joel, but you are way off base here. Whether you have one hot or two is simply a matter of whether you need 120 or 240 at the panel. But, that is not the main issue. The ground and neutral bar MUST be isolated at the panel. All of the neutrals must be carried back to the main panel. Grounding at the subpanel is recommended if it's in a separate building. Isolate the neutral bar and send it back on the feeder neutral, and you should be fine.

[Portland]

This is a separate building so is the grounding of the nuetral ok?

[giddonah]

no, that's not what he's saying. The ground and neutral busses need to be separate in all subpanels. The ground though, should run back to the main panel if in the same building, and should run back to the main panel (via the feeder cable) and to it's own grounding rod if in a separate building. (I'm starting to get this straight myself, so I might get corrected here).

[bkrahmer]

Exactly right.

[giddonah]

sweet. :wink:

[Sparks]

Portland, your electrical system may be perfectly safe. Here's the key point, if you have just a 3 wire feeder as you say you do, the neutral and ground SHOULD be bonded (connected together via bond screw, jumper wire or strap inside garage panel). A three wire feeder is still allowed even by current codes for separate buildings as long as there are no other metallic paths between main panel in house and sub in garage. This metallic path could be a phone line, a copper water pipe, etc. A separate building always needs a grounding electrode system, usually for garages this is a driven ground rod. A 4 wire feeder is the preferred method, no doubt, but 3 wire feeders were used all the time in the past for outbuildings, no big deal. Nothing unsafe about it if done right.

[montytx]

I guess thats why I have an electrician. Good info.

[Portland]

Thanks to all that replied. OK, so as long as I don't add a water line then all is well. If water is added later - pull the 4th wire; but why the water line is already grounded at the house.

[Sparks]

No, if you added a water line nobody could force you to modify your electrical system as this would probably be grandfathered. The grounding of the water pipe at the house is part of the reason that 4 wire feeders are commonly used now for outbuildings. The intent is to not create a parallel return path for neutral current. If you're running a compressor or anything for that matter in the garage, lets say you're using 7 amps, well 7 amps is being delivered to the garage and then 7 amps is returning back to the house and back to the utility transformer. ( I'm referring to a 120v load, a 240v load usually has no neutral return current as the opposing phase angles cancel each other out). Anyhow, if you bond the neutral to ground in the sub-panel and also have a copper water pipe running to the garage, of course water pipe is grounded at both house and garage, you are allowing neutral current to use 2 paths back to the transformer. In other words, you would be allowing neutral current to flow back to the house panel through the copper water pipe in this example. This creates a few potential hazards. ie. What if for some reason you needed to disconnect that water pipe when current was flowing on it, well then you probably would be the return path, also any current flowing on a residential water pipe is never a good thing. There has been cases of people being shocked by their sink and shower faucets due to issues like this. The fact is that there is an exception in the NEC that allows a 3 wire feeder under certain circumstances. A 4 wire feeder is always preferred. If you were wiring a new garage I would say definitley use a 4 wire feeder and isolate the neutral from the ground. In your case, do not isolate the neutral from the ground, if you did, you would be creating a hazard for anyone that used the garage. If have a 3 wire feeder and you isolate the neutrals from the ground you will have no ground fault protection at all, in other words, your breaker wouldn't even trip if one of your hot wires contacted a ground or a person or whatever. The impedence of the earth alone is way to high to allow fault current to trip a breaker. I'm not speaking of GFCI's here, different topic altogether. Long story short is leave your sub-panel alone, do not isolate anything unless you're rewiring it completely and running a 4 wire feeder. Isolating the neutral and grounds on a panel with a 3 wire feeder is dangerous and you could kill someone.

[Portland]

Thanks, it is finally starting to make sense!

[Sparks]

One more note on this one. Driving a ground rod does not make an electrical installation safe. Ground rods are really only for lightning and utility surge/ spike protection. The most important aspect of a safe electrical installation is ensuring a low impedance path back to the main service for fault current to flow. If in your case, you went ahead and isolated your neutral and grounds, you would have no path for fault current to flow other than the earth itself which would not be effective and would not allow the breaker to trip if the circuit became unintentionaly grounded. If a light fixture shorted to ground for example, normally the breaker would trip and clear the ground fault, if you separated the neutrals and grounds with a 3 wire feeder, that same fixture would not trip the breaker and the fixture would stay energized and the metal frame of the fixture would also be energized just waiting for an unlucky person to grab it. Well, just wanted to make sure that we were clear on this because it's pretty important stuff if your performing electrical work. Knowing when to bond and when not to bond can be the difference between life and death.