B. Adams wrote:
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
no-no.)

2. Should the building have its own ground rod? There are no
connections between house and building except for the underground feed.

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
trimmers.
--
Tom Horne