I have installed a new sub-panel box replacing an old federal pacific box. The old pf “sub-panel” used the supply metal conduit as ground. In my newly installed sub-panel I have run a #4 copper ground wire from a new ground rod( close by my sub-panel) to my equipment ground bar which I screwed (bonded) to my panel box- local code will not let me use the metal conduit between my main panel and my new sub-panel as sufficient ground. I removed the tie bar between the neutral and ground buses and bonded the ground bus to the panel box with the green screw supplied. Do I need a jumper wire from my bonded equipment ground bar to my ground bus even though they are now already continuous? Do I need to insulate my metal conduit that runs from the main to my sub-panel so that there is no longer any continuity between this conduit and my new sub-panel? The old rigid metal conduit has me kind of confused.I’m used to working with 4 wires(2hot,neutal,grd) running to sub-panels . Thanks for your help.
Replies
As I understand your situation you don't need a jumper and there is no need to isolate the conduit.
A jumper could increase the cross-section of the bond between the panel enclosure and the ground bar and could be a good idea if you had used a sheet metal screw to attach the ground bar to the panel instead of the manufacturers screw or other fine thread screw. The differences in contact area and conductivity are substantial. Not that I have seen problems with a lot of panels on this point. It is more a theoretical issue but one easily corrected.
The conduit being grounded is different from the conduit being used as a ground path for other fixtures. The conduit should be grounded so that any fault within the raceway will have a sufficiently low impedance path to generate the current needed to trip the breaker and kill the power. Many think that the breaker tripping is a terrible thing. Most electricians view it as doing its job. Better that the circuit should trip and disconnect itself from power, alerting you to a problem, than it slowly cook its way to ignition in a wall or attic.
In terms of theory the conduit makes a grand grounding conductor with a large cross-section and potentially a very low impedance. The problem being that conduits are more subject to physical and often chemical abuse and have mechanical connections at least every ten feet. Any one of those connections could be insufficiently tightened, vibrate free or otherwise fail without anyone knowing it. Too often I have seen the vital ground connection running through the conduit broken when the conduit couplings loosen or the conduit disintegrates into rust. The outcome of someone inadvertently bridging that gap and completing the current path is seldom pretty and lacking a ground path stray current will seek ground through other paths. Like uncle Bruno.
If you doubts about how good the connection to the conduit is, concentric knockouts and loose fittings in walls are both common causes of faults, you might consider adding a grounding bushing at one or both ends of the conduit and a jumper from this bushing to the ground bar. In commercial work, particularly with three phase, I might be concerned with the possibility of circulating currents associated with having connections at both ends but seldom is this an issue with the conditions and distances common to residences.
Thank you very much for your insight and clarification. I may just go ahead and add a ground bushing to the entering conduits in my new sub-panel to be on the safe side. One other question. In the old sub-panel and existing conduit several hots off individual breakers were run to supply individual circuits with one white wire acting as the neutral for three circuits within the same conduit. Will the new code still allow this wiring configuration as long as I pull a new wire to use as ground? thanks so much for your help.
Assuming this is a normal single phase, two hots and a neutral in the middle, residential power and not three phase, three hots and a neutral, you should only share a neutral with two hots not three. The two hots should not be on the same leg, should have 240v between them to prevent overloading of the neutral.
Is it possible that the conduit feeds a 240v load and a separate 120v load? It would be OK in that case. If it feeds three 120v loads or circuits you might need to pull in another neutral in addition to the new ground. As it's in conduit this is not, usually, difficult. Just make sure the shared neutral splits the 240v. Of course it might be just as easy to pull in two additional neutrals so each circuit has its own. This reduces the chance of problems.
Thank you so much for the help. I believe it did feed a 240 and 120. I have disconnected the 240 load which is no longer needed. I believe I will pull the additional neutrals as you suggested to be safe. I appreciate the help and your timely advice.
Hey Dave,
After reading your post I don't follow you on a couple things.Is this subpanel in a separate structure ? Or just in the same residential structure as the main panel? Are you using the existing feeder conductors,without a ground wire ,to feed the subpanel? I ask because it seems that you're saying that your new ground rod is what grounds your subpanel,when actually the metal conduit is a far better ground(under normal conditions as 4Lorn1 points out).Sometimes people think you can create a ground wherever you need one by driving a new rod,when in reality the earth makes a very poor conductor for carrying fault current back to the main panel.If you did isolate your conduit from the panel,you might not be able to get a breaker to trip.
Did you take a panel with two neutral busses and make one a neutral bar and one a ground bar? I ask because the green screw provided won't go into a factory equipment ground bar.
Barry
Barry,
Thanks for your interest in my problem. Actually this is a new sub-panel in the same building-about a fifty year old workshop structure. The feeder wires come from the main through 1 1/4 emc to my new sub-panel-no ground wire. I put a new ground rod at my sub-panel because local "experts" told me that the old conduit would no longer be considered a reliable bonded or continuous path. In my new sub-panel I screwed(bonded) a new equipment ground to my panel and then ran my #6 copper from it to my ground rod. I used the screw to make one bus a ground bus. I was thinking of also using a bond bushing where the 1 1/4 enters my sub-panel and have a jumper from the bushing to my equipment ground bar in the sub-panel. Hope this clarifies my situation.Thank you for any advice.
Dave,
There's no harm in adding the ground rod,I just want others to see that the metal conduit is the critical path that any fault current would take in opening the circuit.Read xJohn's description of what the ground rod actually does.Some of the theory about limiting voltages to ground and reference potentials isn't easy to understand.
I think somehow you might have been lead astray about a new ground rod being better for grounding a sub-panel than the conduit.What you should probably do is go back and doublecheck the tightness of all the setscrews on the couplings and connectors(you said EMT right?) and that the locknuts are tight in the panels.A common weak point in a ground path like yours is the panel knockouts; eccentric or concentric K.O.'s have to be treated carefully so they don't tear out.That's why I ALWAYS pull an equipment grounding conductor.Leaving the ground wire out, because the Code allows you to use metal conduit as a ground ,is foolish.It's one of the things in the NEC I would change.If everything is tight I wouldn't worry about the bond bushings.We only use them on conduit runs that have a mix of PVC and metal conduit,and if we have to use metal conduit ahead of the overcurrent protection on a service.
I guess I'm not sure what you're calling the ground bus that you used the green screw on.Most panels don't come with a ground bar,you have to buy it separately and install it,and that's what I think you did with your #6 grounding electrode conductor.Most panels that I install have two neutral bars,one on each side of the panel,that are tied together by a buss piece hidden by plastic insulating material.If the panel is used as a main panel you install the green screw through the neutral bar and into the panel case.This is the factory means of bonding the neutral and ground if the panel is the service equipment and first disconnecting means.If there is a main CB or fusible switch ahead of the panel you bond the neutral there and throw the green screw away.I'm not trying to get on your case Dave,I don't know your level of experience.But if you have the neutral grounded in the main panel,and then reground it with the green screw in a subpanel,you set up a parallel circuit with neutral current flowing where you don't want it to.Possibly a dangerous situation.
Barry
Edited 11/26/2003 10:31:25 PM ET by IBEW Barry
Barry
Thanks very much for the info. I may have used the incorrect terminology. My new sub-panel came with two neutral busses. I removed the tie bar and used the screw to make one a ground bus bonded to the panel. Ideally, I would have liked to run a ground wire through the conduit to my new panel-that is what I have always done and was familiar with but I can't do with the existing old work. I sure do appreciate your insight and advice.
Dave
>>Sometimes people think you can create a ground wherever you need one by driving a new rod,when in reality the earth makes a very poor conductor for carrying fault current back to the main panel.
I really appreciate it when you and 4Lorn1 explain this stuff (especially when I realize you've covered an issue I've actually understood {G}!)
Am I correct in understanding that the confusion here arises from using the same word "ground" for 2 different functions - the "equipment grounding electrodes" (which we commonly refer to as the "ground wires") serve to facilitate tripping of the breaker in the case of ground faults, whereas the "system ground" between the main panel and the driven rod or metal plumbinng inlet serves to assure zero potential between the neutrals and equipment grounding electrodes and the real ground?
And I'm curious as to how you guys feel about Fed Pac Stblok boxes.
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> Am I correct in understanding that the confusion here arises from using the same word "ground" for 2 different functions ....
Yes, exactly. You need a metal-all-the-way path to the service entrance where ground and neutral are bonded to ensure that the breaker will trip if a hot accidentally contacts a box or a pipe, etc. You need ground rods driven in the dirt and bonded to the system ground to prevent you from getting a shock between true earth ground and anything that's bonded to the system ground. Resistance through the dirt is almost always too high to trip a breaker, or even give you more than a dull glow from a lightbulb.
-- J.S.
Bob,
As you pointed out people sometimes confuse the role of the ground rod in the electrical system.Grounding potential theory isn't easy to understand,but it's importance is brought home when we run into a miswired system.Wandering or mysteriously fluctuating voltages,and people getting shocked under normal conditions often result.
Since you ask I don't know of an electrician that doesn't dislike Federal Pacific panels and circuit breakers.If I were a home inspector I would point them out to potential home buyers as being old,without good track records,and candidates for upgrading.I don't know how your realtors would feel about telling clients that,but it's the truth,they just don't trip reliably.
And by the way,my post in another thread that offended you was a Chris Rock joke from his HBO special.Sorry.
Barry
Edited 11/26/2003 10:54:24 PM ET by IBEW Barry
Edited 11/26/2003 10:54:55 PM ET by IBEW Barry
RE FEd PAcs - they certainly have generated more than their share of crticism.
I know 2 electricians who work on NEC drafting committees who say they haven't seen a higher incidence of failure with Fed Pacs as with any other system.
As an HI, I always inform my clients that they are widely criticized and that there are many people who feel they do not function well, and that they should consider an upgrade.
Re the Chris Rock line - just goes to show that context is critical.
In fact, wasn't he in that movie where he takes overy the body of a fat old white guy and tries to do black standup routines in a black comedy club?
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I'm matching tool donors to a church mission to Haiti - we're shipping a bus converted to a medical facility in January (we hope) and can fill it with clothes, tools and all sorts of stuff needed in that poorest of all countries. A few hand tools or power tools can provide a livelihood for an otherwise destitute family. Please email me if you have tools to donate.