I’m running 100 amp service to my shop from my breaker panel at my house. If I’m correct the subpanel at the shop has to be grounded at the house panel. Is that right?
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Replies
yes, and nowhere else.
SamT
yes, and nowhere else.
That's NOT correct.
NEC 250-32 states that every building gets a ground electrode system, whether or not they are on the same service. The only exception is if the building is served by only branch circuit. Since he's putting in a subpanel, this is not the case.
So the subpanel gets connected to a ground electrode at the shop. He also needs to do one of the following (250-32(b)(1) or 250-32(b)(2)):
1.) Run a grounding conductor back to the main panel (4 wires total from sub to main). In this case, the ground and neutral in the sub are not bonded together
2.) Alternately, If there are no continuous metallic paths bonded to the grounding system in both buildings, and there's no GFCI in the common AC service, he can just run three wires (2 hots + one neutral). In this case, the neutral and ground buses in the subpanel are bonded together, and to the ground electrode.
BarryO,
Unless an equipment-grounding conductor is run with the feeder-circuit for grounding purposes.
Grounded neutral conductors serving another building from a main building: Sec. 250-32(b)(2). When a common AC grounded service supplies one or more buildings/structures, you must separately ground each panelboard at each building or structure. You must ground the grounded neutral conductor at the panelboard in the second building/structure, and size it based on the rating or setting of automatic overcurrent protection device (OCPD) in the feeder circuit, as specified in Table 250-122. As per Sec. 225-33(a), the disconnecting means for each supply shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, a group of separate enclosures, or in or on a switchboard. If necessary, Sec. 225-32 and 250-32(d) allows you to locate the disconnecting means elsewhere on the premises.
Grounded conductor used as a neutral and equipment-grounding conductor: Sec. 250-32(b)(2). When you use a feeder-circuit from the service equipment in the main building to serve a panelboard in another building, special requirements apply to the grounded neutral conductor. This is true if you use the grounded conductor as a neutral plus an equipment-grounding conductor. The same rules apply to the second/third buildings, or any buildings installed on the premises and supplied from one common service by a feeder-circuit.
SamT
Unless an equipment-grounding conductor is run with the feeder-circuit for grounding purposes.
Yes, in that case, the grounding conductor is a metallic path bonding the ground electrode systems of the two structures. If you bond the grounding and the grounded (neutral) conductors in the subpanel, you can get "sneak" neutral current flowing over the gorunding conductors.
Good show, yes thats what I thought. I will have an electrical contractor double check my work before hitting the switch anyway. Thanks for the facts.
OK. correct me if i'm wrong, (easily possible.)
What we usually call the neutral is in fact the grounded neutral conductor?
What we usually call the ground is the equipment grounding conductor?
If we ground rod the sub and ground rod the main, aren't we also setting up a stray current possibility due to the possiblity of different parts of the earth being at different potentials and the fact that the neutral is a continuous metalic path to the main ground buss?
I have read the two excerps from the NEC in my previous post several times and I am still confused. Maybe Dawg sees this and that is what decided him to get an electrician out.
If we use a four wire feeder to the sub, do we need to ground rod the sub? If so, do we bond the ground and neutrals in the sub?
It seems to me that we would have to since to do otherwise could put the sub's ground wire at the earth potential of the main, but the subs hots would be referenced to the earth potential at the sub's ground rod.
If true, why do we ever use a four wire feeder if we have to ground rod the sub and the neutral is bonding the two ground rods together and the ground wire is in parallel to the neutral with both ends bonded together?
Sincerely,
SamT
What we usually call the neutral is in fact the grounded neutral conductor?
Yes.
What we usually call the ground is the equipment grounding conductor?
Yes.
If we ground rod the sub and ground rod the main, aren't we also setting up a stray current possibility due to the possiblity of different parts of the earth being at different potentials and the fact that the neutral is a continuous metalic path to the main ground buss?
The earth shouldn't be at significantly different potentials, most of the time. It's "grounded" by definition, after all. Occasionally there will be a potential difference, such as when there's a lightning strike nearby, as the charge dissipates into the ground. Or when there's a power company high voltage line nearby with a bad/corroded ground line and some return current ends up trying to flow through the earth (this can be very bad).
If we use a four wire feeder to the sub, do we need to ground rod the sub?
Yes. The idea behind a ground rod at every building is to make sure the grounding conductor really are at the local ground, so it is always safe to touch them, and anything they're connected to.
If so, do we bond the ground and neutrals in the sub?
NO. If you do that, the main-to-sub grounding and neutral wires are doing exactly the same thing. Some of the neutral current will flow through the grounding wire; this is bad. We never want a situation set up where neutral current flows through the grounding conductor system, except in fault conditions. With no current flowing, the grounding system will all be at the same potential.
It seems to me that we would have to since to do otherwise could put the sub's ground wire at the earth potential of the main, but the subs hots would be referenced to the earth potential at the sub's ground rod.
No, the main and subs ground are at the same potential, as they are bonded together and no current flows through the bond. What this does mean is that, at the sub, the neutral is not at ground, but a little bit above ground due to the current flowing through it. That's OK, since if a short occurs, the fault current still has a return path : through the branch grounding conductors back to the sub, and through the main-to-sub grounding conductor back to the main. So we can be confident the breaker will trip.
If true, why do we ever use a four wire feeder if we have to ground rod the sub and the neutral is bonding the two ground rods together and the ground wire is in parallel to the neutral with both ends bonded together?
Well, as stated above, in the four wire case, the grounds and neutrals are not bonded together at the sub.
Grounded neutral conductors serving another building from a main building: Sec. 250-32(b)(2). ...........You must ground the grounded neutral conductor at the panelboard in the second building/structure
You said ..."The idea behind a ground rod at every building is to make sure the grounding conductor really are at the local ground" and "What this does mean is that, at the sub, the neutral is not at ground"
I'm soooooo confused.
Sigh!
SamT
I said, "at the sub, the neutral is not at ground", in the 4-wire case you were asking about. NEC 250-32(b)(2) that you're making reference to applies to the 3-wire case. These are two mutually exclusive schemes, operating on slightly different principles. You can either follow 250-32(b)(1) for the 4-wire case, or 250-32(b)(2) for the 3-wire case. In 250-32(b)(1), the neutral is not grounded at the sub, in 250-32(b)(2), it is.
Thanks Barry. I'll sure be glad when I can go back to work and afford to purchase the code books.
SamT
LOLOLOLOL I am a bit confused. So give me the short version.
OK so the end result if I run 3 wire to the shop. Do I ground at the shop or run a separate ground back to the main panel.
OK so the end result if I run 3 wire to the shop. Do I ground at the shop or run a separate ground back to the main panel
If you run 3 wires, at the shop you bond the grounding and grounded (neutral) bus bars in the subpanel, and to a ground rod (2, actually, if there's no other ground). You can only use the 3-wire scheme if there are no phone wires, plumbing pipes, or other type of metal connection between the 2 buildings.
Otherwise, you run 4 wires. The grounding and grounded (neutral) bus bars in the subpanel are not bonded, and the grounding bus is connected to ground rods.
Now thats a bit clearer and more like what I learned.
I didnt know that other wires would make a difference. I plan on running security, telephone, coax, and LAN cable to the shop but in a seperate trench.
So I guess I'm using 4 wires. I have budgeted for that.
I have an older house (built in 1960), and a subpanel was added in the shop building, which is about about 30 yards from the house. Three-wire cable (2 hots + neutral) was used to connect the main panel in the house with the subpanel in the shop. The ground and neutral bars are bonded in the supanel. I know metal water pipes run between the shop and the house, since the shop also serves as the well/pump house supplying the main house with water.
What risks am I taking with this situation? Should I isolate the ground and neutral bars in the subpanel? Should I replace the 3-wire cable connecting the main and sub with 4-wire? (I would really hate to have to do this -- it is very hard and rocky ground, making trench digging a real undertaking.)
I would very much appreciate your advice.
vlperk
If the situation is as you described, it is not code compliant.
The problem is that the metal water piping, beiing bonded to the electrical ground at both buildings, is in parallel to the neutral conductor running between the buildings. Thus, a good deal of the neutral current may, in fact, be running over that pipe. That's not supposed to happen.
Are you sure the pipe between the buildings is metal? My house is the same era. Above-ground pipe in both house and pump house is galvanized steel (or it was, until the house plumbing got replaced with PEX). But the underground run is black polyethelene.
BarryO, you were right! I just got back from digging in the back yard, and the underground water supply pipe is, in fact, plastic, not metal. (Thanks for the exercise; I needed it. :-)
Am I right in assuming that since: (1)there are no other metal connections between the house and shop, (2)the house and shop are each grounded by a ground rod, and (3)the electrical line running to the shop is a 3-wire cable; then the neutral and ground at the shop subpanel should be bonded to each other?
Thanks very much for your help.
vlperk
'sounds like you're OK.
Thanks a million!
vlperk
What we usually call the neutral is in fact the grounded neutral conductor?
At least in NEC speak, if I remember correctly, there ain't no such thing as "neutral" or at least I don't recall it ever appearing in the NEC. Seems to me that what is commonly referred to as the "neutral" is referred to in NEC as "grounded circuit conductor" - which makes some sense as the "neutral" ain't really "neutral" in regard to electromotive force. It would have been nice, however, if they could have come up with some words less easily confused than "grounding" and "grounded"...
Regardless of how the code terms the two conductors, I've had an easier time communicating with my inspector and with forum participants when I use the term "Equipment Grounding Conductor" or "EGC" for the ground wire, which is always green if insulated or bare copper, and simply "neutral" for the conductor that is always white or taped with white.
Is there a general consensus among electricians that the ground is accurately termed "EGC"?
Actually the NEC does use neutral.
I am looking in the index for the 99 NEC.
Under NEUTRAL is has (also see Conductors, Grounded)
But you see entries for
Bare
Bonding to service equipment
Common
Conductors
Continuity
Definition
Equipment grounding
Feeder Load
Grounding of AC and DC systems
Identification
Ranges and Dryers
Uninsulated where permitted
There are about the same number of entries under Conductors, Grounded
However, in sec 100 where there is suppose to be a definition there isn't one for neutral.
But there is one for grounded coductor - "a system or cirucit condutor that is intentionally gorunded" - which does not really say anyting <G>.
Yes. You have to run 4 wires from the main panel to the subpanel: two hots, one neutral, and one ground. Unlike in the main panel, the bus bars for the neutral and the ground have to be separate in the subpanel, with neutrals all going to one bus bar (and insulated from the panel enclosure), and the grounds going to the other bus bar.
So the grounding of your entire system goes back to the main panel, where it's physically connected to the earth.
Most panels can be converted from a main to a subpanel by following the manufacturer's instructions for isolating the neutral bus bar. Often you only need to remove a screw or strap. Sometimes you need to purchase an inexpensive conversion kit.
When you buy the cable, of course, ask for 4 wire cable. They make cable specifically for this.
Don't forget, if your cable is aluminum (which it probably will be), your connections have to be rated for aluminum and you have to apply the special "goop" to the connections.
I didnt know about the goop for the aluminum wire. I'm sure its to prevent corrosion. Ill make sure to get it. Thanks for the info.