Ground wire for a sub-panel question

[Ken Vick]

Here is a question for the electrical experts here.  When I moved into my Lewiston, NY house, the shop (i.e. garage) had inadequate power for my shop tools.  I had an electrician run a cable from my main to a new 100 amp sub-panel.  He had the bright (actuall, not) idea of reducing the expense of the cable by using a 3 wire cable (2 hots and a common) but not including a ground wire.  For the ground, he used a length of insulated wire that he ran through the wall into the adjacent basement and attached it to a metal water pipe.  The Main box is some 30 feet away and also grounded to the water supply so the electrician figured that he would use the water pipe in place of a wire all the way back to the main breaker box.  I should say that the electrician was moonlighting and I had drawn the permit.  When I had the electrical inspected awhile ago, the inspector definitely didn't like this arrangement and said in case of an electrical fire could compromise my insurance coverage.  In the meantime, the electrician has flown the coup.  I want to correct this problem.  Rather than pulling out the 3 wire cable, can I  just run a separate ground wire between the boxes?  Does it have to be insulated?  The wire would be run in the ceiling joists above the basement but is exposed.  What would you do?  Thanks, Ken

[Alan S]

Can you ask the inspector?  That may be the best way to find out what he would accept.  Some inspectors are more amenable to this than others.

[GSENT]

The National electric code requires that any subpanel be wired with separate neutrals and grounds.  You MUST use 4 wire and do not bond the neutral buss to the box.
Your main breaker to the house is considered the main service, so any panel wired into the system after the main will need to have separate neutrals and grounds.
Some houses locate the main breaker outdoors, some indoors.  Whatever the case, this is the only place neutrals and grounds can be bonded together.
Hope this helps.

[Ken Vick]

I guess my specific question is whether the ground wire has to be in the cable with the 2 hots and common.  Can you run a separate ground wire outside this cable from the main breaker box to the sub-panel?  And is it likely I can get a single wire conductor that can be mounted in an exposed position (up in the cieling joists area of an open ceiling)?  I am thinking this might be easier and cheaper than going with a 4 conductor cable to replace the 3 conductor cable I now have.  I hope that clarifies things.  Thanks.  Ken

[Robert Adams]

I guess my specific question is whether the ground wire has to be in the cable with the 2 hots and common.  Can you run a separate ground wire outside this cable from the main breaker box to the sub-panel?  And is it likely I can get a single wire conductor that can be mounted in an exposed position (up in the cieling joists area of an open ceiling)?  I am thinking this might be easier and cheaper than going with a 4 conductor cable to replace the 3 conductor cable I now have.  I hope that clarifies things.  Thanks.  Ken

Sometimes an inspector will let you add it like you want but it's not supposed to be done that way. I forget how the code is worded but it's something like along the same raceway or conduit. It's one I don't worry about cause I use 4 conductor or conduit. 
    
       Best answer is ask your inspector how he wants it done as they are the final say not the code book.

[TDKPE]

Sometimes an inspector will let you add it like you want but it's not supposed to be done that way. I forget how the code is worded but it's something like along the same raceway or conduit. It's one I don't worry about cause I use 4 conductor or conduit. 
    
Best answer is ask your inspector how he wants it done as they are the final say not the code book.

There are words in the NEC somewhere about adding an equipment grounding conductor to an existing ungrounded circuit to bring it up to snuff, but since this isn't an existing circuit from back in the day (it's a new installation), I'm guessing he/she isn't having it, and wants it done by the numbers. 

To save some money, the contractor could have used aluminium SER cable (1/0 for 100A).  But regardless, as Robert Adams said, ask the inspector what he/she wants.  I'm guessing the inspector is from a service, not from the town (one of several pre-approved by the town or county, but not employed by), as that's how it's done up here, and I've found them to be quite reasonable and easy to talk to. 

Also to save money, consider how much power you actually need.  A 100A 120/240V feeder will give you 200A of 120V power.  That's a lot of juice.

[rwe2156]

If this was a licensed electrician you assume he knows the local code but from experience I can tell you don't bank on it.

Intuitively (and that doesn't mean common sense is code) ground is ground but I would use a ground rod, not the water pipes. However, neutral is not just anything and it needs to be tied to the same source as the hots.

So would someone explain how you get a separate neutral without pulling it from the main panel?

I think you're alright except I would redo the ground.

Best bet is call another electrician and have them take a look.

[RJT123]

In our locale, if the sub panel is going into a separate structure it needs a ground rod. I believe I have 4 wires coming from the house, but would need to check to be sure.

[TDKPE]

Up until the 1996 code cycle, a 3-wire feeder was permissible for an outbuilding, unless there were any metallic pathways between the structures, in which case a 4-wire feeder was required. Starting with the '96 cycle, only a 4-wire feeder is permissible. A grounding electrode system was always required for outbuildings, as far back as my old code books go.

But a grounding electrode system (usually a pair of ground rods, at least back-when) is NOT a substitute for an equipment grounding conductor back to the service equipment (usually the main panel), sized to the overcurrent protection ampacity (Table 250-95), where all the neutrals and grounds are tied together, along with the building's ground electrode conductor (from the ground rods, or other system). The earth can't carry enough current at 120V under the very best conditions to even think about tripping a puny 15A breaker, let alone some high-amp feeder. That's not what the grounding electrode system is for, and it's even stated in so many words in 250-1 (fpn) and 250-51.

[Roly]

In the code it states all conductors for a circuit (including grounding and bonding) shall be in the same conduit, raceway,cable, etc.  This would prevent you from adding a separate ground wire outside of the existing cable.  I don't have a current code book but it would in the 300. ?   section.  Roly