Wiring a sub panel?

motorcity11

I have a 3 wire 200amp service with no grounds in the panel.
(mfg building) L1, L2, and N 120/240. I would like to run a 100amp sub panel. I am using #3 wire + #8grd. I will connect L1 and L2 to a 100amp breaker at the 200amp panel, and connect the N and ground together at the Neutral bus bar.(is that ok so far?) It will all run down a 1"EMT conduit to the 100amp Sub. I will attach L1, and L2 to each line lug, and the N to the Neutral bus bar, and the grd to a seperate grd bus bar. I will install a grd rod at the 100 sub with #6 to the grd bar. Is this a good install...thanks...I probably don't need the grd rod...but will it cause any safety violations if I do install it?

Thank you.

joed

If the subpanel is in the same building NO ground rod is to be used. The only ground rods are to be at the main disconnect.

motorcity11

Thanks Joed.
The main is in another building which is attached. The conduit is piped thru the cinder block. The building has a roll up steel door as the only access.

Other than the rod, this is a stable system?

thanks for your help

motorcity11

Tennis anyone?

J. V.

You got it. Make sure you do not install the bonding screw into the neutral bus bar. This will bond the N bus bar to the enclosure. No rod needed in attached structures.

For convenience you may want to consider a main breaker (not required). Since access to the main is not readily accessible.

motorcity11

I hate to beat this post to death ...if the neutral from the main is common to the ground at the main, and then neutral from main connected to the neutral of the sub, and EMT was used for the run to the sub, wouldn't the neutral at the sub then be grounded to the panel?

I checked the sub neutral to EMT with a OHM meter and it showed continuity...I'm missing something here. Can I have a explanation to why we then isolate the neutral in a sub.

Thanks

junkcollector

The nuetral is isolated because if it was bonded to ground at the sub, there would be a parallel path back to the main panel. In other words, the ground wire (or EMT, in your case) would be carrying some of the neutral current. The ground is not suppose to carry current except in a fault situation.

InPhase277

The ground and neutral connect at a single point, the first means of disconnect. The purpose of this is so that fault current has a path to complete the circuit. Remember, a short circuit is still a circuit. The actual earth has nothing to do with grounding a fault. Current flows from the hot through the fault and back along the ground path (EMT) to the neutral bond at the main. When you check continuity at your sub, you will read a connection because of the tie at the main.

At the sub, we keep them separate because we don't want any current on the ground except fault current during shorts.

ScottR

Quite right.. Neutral is bonded to the grounding conductor at the main, but only at the main. Therefore the grounding conductors in your sub are bonded to neutral, via the main panel.

If you bonded in both places, you would have some current on the bonding (ground) wire in your feeder under normal conditions. And if the neutral connection was broken, you would have full normal current on your bonding wire. Ground faults would also utilize both the feeder neutral and the bonding wire as a path.