Basic Electrical Questions

Old 05-18-16, 05:03 PM
Thread Starter
Join Date: May 2016
Location: USA
Posts: 10
Upvotes: 0
Received 0 Upvotes on 0 Posts
Basic Electrical Questions

I know these may sound like dumb questions but I'm just trying to learn something here and get a better grasp on a few things. Hope the questions make sense.

1. I have a 120V 20 amp lighting circuit coming out a panel that I want to connect 8, 4', 112W light fixtures on. It will be 2 rows of 4 lights ea. One row running down the length of a 40' building and the other running back. I can run the conduit in a U shape with power coming in one end of the U and ending at the other (approx. 110' total). Or, I can run it as two, individual 40' parallel rows and then run the 20' power line across the top splicing in those two 40' parallel rows together. It would be in the shape of the Pi symbol. I know the first way would make it a 110' run with power going in one end and ending at the other and the second would be like two 40' runs with a 20' line feeding both. Would either way make a difference in voltage drop or would they be the same or would it even matter? Sure hope someone understands what I'm trying to explain and ask.

2. I have been told that if you run a MWBC that the ground doesn't count as a CCC because in a MWBC there is no current on the neutral because the 2 circuits cancel each other out because they are on different phases. But, when you get to the panel you connect that neutral to the same neutral bar that the regular neutral circuits are tied to which do carry current. Wouldn't it then put current on the MWBC neutral?
Old 05-18-16, 05:19 PM
ray2047's Avatar
Join Date: Mar 2006
Location: USA
Posts: 29,711
Upvotes: 0
Received 15 Upvotes on 13 Posts
I have been told that if you run a MWBC that the ground doesn't count as a CCC because in a MWBC there is no current on the neutral
Was that a typo. Neutral and ground are two different things. The ground is strictly to clear faults and normally doesn't carry current. The neutral does carry current. On a MWBC that curent is the difference between the amps on the two legs of the MWBC. If leg one is 10 amps and leg two is 8 amps the neutral carries 2 amps, 10a-8a=2a.
they are on different phases
Actually it is split phase. More correctly different legs of a single phase.
Old 05-18-16, 05:19 PM
Join Date: Mar 2010
Location: USA
Posts: 4,454
Upvotes: 0
Received 267 Upvotes on 243 Posts
The neutral of an MWBC carries no current when the loads on the two MWBC legs are equal.

When everything is working properly (no unwanted touching of wires e.g. faults) no current flows on the ground wire ever.
Old 05-18-16, 06:14 PM
Join Date: May 2015
Location: USA
Posts: 3,106
Received 145 Upvotes on 118 Posts
To answer your first question, the second way will have somewhat lower voltage drops to some of the lights. As a practical matter, with 1000 watts of load on a 20 amp circuit with reasonable length runs, it won't matter. Do it whichever way is easiest for you.

The second way would probably be easier if you wanted two separate switches, one for each row of lights.
Old 05-19-16, 04:45 AM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 13,935
Received 181 Upvotes on 159 Posts
A neutral from a MWBC does not count as a CCC for ampacity adjustments. It does count towards conduit and box fill.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: