Two Circuits-Shared Common - Electrical - DIY Chatroom Home Improvement Forum


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Electrical

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Display Modes
Old 04-22-2008, 08:40 PM   #1
Newbie
 
Join Date: Apr 2008
Posts: 6
Rewards Points: 10
Default

Two Circuits-Shared common


Hello, I have a question regarding the possibility of common wire overload. I have had a large garage for several years that has metal conduit throughout. Two 20 amp -120 volt circuit breakers lead to one double metal box that included one circuit for one outlet, and the other circuit powers a lighting circuit. My concern is that only three wires came into the box from the main panel- One Red, One Black, and One common (the box is grounded). The two circuits had always shared the common wire without problem. I have recently removed the outlet and expanded that circuit with another lighting circuit of approximately 1300 watts assuming that the “hot” wire and separate circuit would easily meet that demand. Now I am questioning if the shared common between the two circuits could become a problem if both circuits are nearing capacity.

Advertisement

oc250r is offline   Reply With Quote
Old 04-22-2008, 09:01 PM   #2
Member
 
joed's Avatar
 
Join Date: Mar 2005
Location: Welland, Ontario
Posts: 8,376
Rewards Points: 3,546
Blog Entries: 4
Default

Two Circuits-Shared common


The two hot must be on different legs of the incoming service. This way the neutral only carries the differnce in current not the sum. It's called a multiwire branch circuit.

Advertisement

joed is offline   Reply With Quote
Old 04-22-2008, 09:14 PM   #3
Registered User
 
Termite's Avatar
 
Join Date: Apr 2008
Posts: 6,520
Rewards Points: 2,000
Default

Two Circuits-Shared common


What you have is called a multiwire branch circuit...Actually you have two of them. As you stated, they're separate circuits that share a neutral.

The shared neutral is fine as long as the two circuit breakers are not on the same phase. There's an A and a B phase in your house. Two breakers pulling power from the same phase essentially double the juice going through the neutral, which ain't good. With the breakers on separate phases, the neutral doesn't know that it is even being shared, and things work fine.

In most panels, breakers that are on top of one another are on adjacent phases. The only way to be sure is to look at the buss bar under the breakers and see which way the buss bars run. You'll have two...An A and a B.

Simple enough, avoid having the two phase wires (black and red) landing on breakers that are on the same phase. If they're both on A phase or both on B phase, your neutral is waaaay overloaded. If not, you're ok.

You're really heads-up for recognizing a potential problem.
Termite is offline   Reply With Quote
Old 04-22-2008, 09:15 PM   #4
Registered User
 
Termite's Avatar
 
Join Date: Apr 2008
Posts: 6,520
Rewards Points: 2,000
Default

Two Circuits-Shared common


JoeD snuck in and beat me to it while I was typing!
Termite is offline   Reply With Quote
Old 04-23-2008, 06:06 PM   #5
Newbie
 
Join Date: Apr 2008
Posts: 6
Rewards Points: 10
Default

Two Circuits-Shared common


Thank You for your help,

The breakers are one on top of the other. I pulled them and they are on different bars
oc250r is offline   Reply With Quote
Old 04-23-2008, 06:29 PM   #6
Licensed Electrician
 
jrclen's Avatar
 
Join Date: Feb 2008
Location: central wisconsin
Posts: 982
Rewards Points: 500
Default

Two Circuits-Shared common


On a multi wire circuit, properly connected to each polarity of your service, the most current the common or neutral wire will carry is the ampacity of one of the circuit breakers. If the other breaker is carrying current, the load on the neutral will actually decrease. For instance, if both circuit breakers are conducting 20 amps of current, the neutral will be conducting 0 amps. If one circuit is carrying 20 amps and the other is carrying 15 amps, the neutral will carry 5 amps. The neutral only carries the unbalanced load between the 2 circuits. It will never be more than the rating of one of the circuit breakers. But can be less.

Advertisement

jrclen is offline   Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help if it's not too late!! Cyndi Electrical 255 10-10-2008 10:18 PM
How many circuits needed? helpless handyman Electrical 6 04-21-2008 02:57 PM
12/3 Romex to Feed 2 Circuits? DaveHembree Electrical 22 09-28-2007 09:24 AM
Power circuits under leaky bathtub? Beren Electrical 1 07-23-2007 10:02 AM
One neutral for two circuits?? TronCarter Electrical 13 01-14-2007 09:21 AM




Top of Page | View New Posts