Recently I had a breaker box installed in an outbuilding (Homeline, 70 amps
max, with 2 20 amp breakers). Power is supplied via an underground cable
from the main fusebox in the basement of my house. The distance is around
100 ft and the cable is 12-2 w/ground. I wired the buidling myself
(four circuits, 2 per breaker). The circuits supply 1 light and 1
receptacle in one room and 1 light and 1 receptacle in the other. However,
as there was no ground bar, I tied all the grounds with the neutrals on the
neutral bar. A bonding screw was included but not used. My questions:
1. Does the NEC permit this? Is it safe? (I know that tieing
neutral and ground together on a subpanel in the same structure is a
2. Should the building have its own ground rod? There are no
connections between house and building except for the underground feed.
You need to add a "grounding electrode." (probably 2 ground rods if you
really want to get picky.) If I understand what you said correctly, you
should add the bonding screw in order to ground the metal panel box.
What kind of breaker do you have in the house to protect this circuit?
Obviously what you have done is wrong. I am not sure if the grounding rod
is an acceptable fix or not.
However, a 100' 12/2 line is pretty long. As long as you just have 100w
lights and clocks on it you should be okay, but a significant load will
exceed your voltage drop allowance real fast.
If you are not familiar with it, the link below should be helpful.
I think 12/2 is a bit small for a 100' subpanel feed to an outbuilding. I
would have suggested 10/3 as the cable of choice for a light load such as
It is a code violation and a potential safety hazard to have the grounds and
the neutrals connected together in a subpanel. Buy an appropriate ground
bar for your panel and install in the subpanel and relocate all of your
ground wires to it.
You do need to install at least one ground rod for your outbuilding and
connect it using at least a #6 wire to the ground bar in your sub panel.
No the US NEC does not permit that. Since there is an Equipment
Grounding (bonding) Conductor (EGC) in the cable you must use it to
ground all non current carrying parts of the electrical installation
back to the Service Equipment in the house. You need to install an add
on EGC buss bar in the Garage's panel. These are available wherever
SquareD equipment is sold. You then put all the EGCs on the add on buss
bar and keep all of the neutrals on the built in buss bar. The reason
for not running the neutrals and EGCs on the same buss bar is that in
the event of an open neutral between the garage and the utility's
transformer the voltage on all exposed metallic parts of the garage's
electric system will go dangerously high if the EGCs are not separate
from the neutrals.
The separate building does need it's own grounding electrode system.
The minimum is a driven rod, eight feet or more in length, which
measures less than 25 ohms resistance to earth after installation. The
code requires that a second rod be added unless the first measures <
than 25 ohms. Once you install a second rod the code is satisfied so
most electricians install two rods to be done with it. The two rods
must be at least six feet apart but more is better. Best practice would
be to have driven the two rods through the bottom of the wire trench at
ten and twenty feet from the building respectively and connect them back
to the panel with bare number 2 AWG run in the bottom of the trench.
Since it sounds like the trench is long back filled it is too late for
that. Just drive the two rods at least six feet apart and connect them
back to the garage panel with bare number four copper wire. Using an
acorn clamp on each rod run the wire from the furthest rod back to the
nearest rod through it's acorn clamp and back to the add on buss bar in
the garage panel. That bare copper wire is called the Grounding
Electrode Conductor (GEC). It must be protected by conduit if it is
exposed to severe physical damage such as from lawn mowers or power
As I wrote in my original post, a bonding screw came with the breaker box
but was not used. The instructions read: "When it is required to bond box
to neutral plate, use long screw enclosed. Insert through hole in neutral
plate and thread into hole in box." Does the NEC require it, in addition
to adding a ground buss?
You *don't* need to install another buss. (I'm assuming you are running
a 240V feeder to the garage, and you are using the white wire for one of
the hot legs.) In this application, your little panel is considered
"service equipment", and it can have a common ground and neutral buss.
But the metal box has to be grounded, and that's what the bonding screw
accomplishes. If you are running 120V to the panel in the garage, and
the white wire is neutral, then you should install a ground buss in the
panel and leave out the bonding screw -- but I don't know why your would
go to the expense of using a breaker panel in the outbuilding if you
just ran a 12/2 120V circuit.
I don't know that the equipment grounding conductor in a 12/2wg cable is
sufficient for a grounded neutral wire. You really should have used
bigger cable. But maybe it's OK. Your whole installation doesn't make
a lot of sense to me.
Thanks for your input. It's 120V, not 220, and it's to a small utility
buidling. Also, the load is generally very light, just a couple of 100w
bulbs. Occasionally a space heater, which pulls 11 amps.
Are you suggesting that the breaker box was not necessary? The guy who did
the work was not a licensed electrician, btw, just a neighborhood handyman
who had done work for me before. I hired him to replace the old cable
after the building lost power. It turned out the old underground cable had
been spliced with pvc tape and had burned in two! There was already an old
rusty fuse box in the building and he replaced that with the new breaker
box. Who did the original work I do not know, but it sure was a hell of a
Do you know what kind of cable he used? It should have been UF cable
(underground feeder). It looks like regular NM-B cable, but it has a
tough PVC jacket that is weather-proof and sunlight resistant. UF costs
a lot more than NM, and it's very difficult to work with.
I think all you *really* needed was a deep double-gang switch box for
your light switches. You also need to use GFCI outlets. A ground rod
woulda still been a good idea but I don't think necessary.
You have a 120V service box with multiple circuits; you need to install
a ground rod.
I'm not going to suggest it I'm going to state it as fact. By
installing that circuit as a feeder that supplies overcurrent protective
devices he has made work for you with no benefit. If he had simply
installed a multi wire branch circuit to the out building he would have
doubled the available power without triggering the requirement for a
grounding electrode system. Let me suggest that you install a two pole
switch assembly in place of the breakers now in use and add a ground
bar. This will turn the underground wire into a branch circuit and
relieve you of the requirement to install a grounding electrode system.
NO, at least not if the circuit from the house was not suppling Over
Current Protective Devices. The exception that allows you to avoid
installing a grounding electrode system in a separate building applies
to buildings supplied by a single branch circuit with an Equipment
Grounding (Bonding) Conductor run with the circuit conductors. The US
NEC specifically says that a multiwire branch circuit is a single branch
circuit for purposes of that exception. The definition of branch
circuit is "the circuit conductors between the final overcurrent device
protecting the circuit and the outlet(s)." By using the twelve gage
underground conductors to supply over current protective devices the
local handyman has turned it into a feeder vis.. "Feeder. All circuit
conductors between the service equipment, the source of a separately
derived system, or other power supply source and the final
branch-circuit overcurrent device." The quoted materials is copyright
2002 National Fire Protection Association.
IIRC, the minimum size for feeders is 30A, which means that the #12 cable
is undersized, and the installation cannot meet code with the subpanel in
You need to either remove the subpanel, and turn this into a multiwire
branch circuit, or replace the feeder cable with a minimum of 10/3 UF.
Given the distance involved, I would use at least #8 to compensate for
voltage drop, if not #6.
-- Welcome My Son, Welcome To The Machine --
Bob Vaughan | techie @ tantivy.net |
You did say you were working from memory but I would be interested to
know what national code language sets a minimum size for a feeder. If
it is a local amendment or code I'm not interested.
Feeders smaller than 30A might not make any sense, but I can't think of
anything inherently unsafe about them, so this might be one of those
cases where the NEC has a solution in search of a problem. I wouldn't
dig up a buried cable just to satisfy this part of the code unless an
inspector told me I had to. (I might dig it up because I realized too
late that 20A wasn't big enough, or because the voltage drop is
unacceptable, but not just to satisfy this point in the code.)
You're right! There it is in 215-2(a)(2) in plain black and white. I
really appreciate your taking the time to find that reference. I was
obviously unfamiliar with it. Now there is absolutely no good reason
that the OP would want to keep that panel in the out building. If he
combines both circuits in the out building into one branch circuit he
could then supply it with the twelve gage two wire feeder but what would
be the point of that. His best course of action is to remove the panel
from the out building and substitute a large J box with a switch mounted
in a raised cover to function as the building disconnecting means. That
switch should be rated for the entire ampacity of the twenty ampere
circuit. At least then he will not be obliged to install a grounding
Inserting that screw is the same as connecting your grounds and neutrals
together. NO. Do not insert the green bonding screw in the neutral bar and
screw it into the threaded hole for your subpanel. This is only done in
your main panel. I'm beginning to wonder if you should be doing this type
of work. From what you have posted so far it seems as though you have
shortcomings in your knowledge concerning basic wiring.
No the US NEC Does not require or permit the installation of the bonding
screw in this case. To install it would be a violation of 250.24 (A)
250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a
grounded ac service shall have a grounding electrode conductor connected
to the grounded service conductor, at each service, in accordance with
250.24(A)(1) through (A)(5).
(5) Load-Side Grounding Connections. A grounding connection shall not be
made to any grounded circuit conductor on the load side of the service
disconnecting means except as otherwise permitted in this article.
NO. In a subpanel, the NEC requires that the neutral NOT be bonded. Bonding is
*only* for service entrances.
Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?
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