Neutral Sharing Question


  #1  
Old 01-17-11, 04:18 PM
M
Member
Thread Starter
Join Date: Jan 2011
Location: USA
Posts: 10
Upvotes: 0
Received 0 Upvotes on 0 Posts
Neutral Sharing Question

First, thanks to everyone that replies to posts. This is a great forum.

I have a question that I want to confirm before I proceed with any further work.

I'm wondering about shared neutrals in general, and have a specific example to demonstrate:

I just bought the house and want to understand it's electrical system better. From my breaker I have a 12-3 cable feeding a junction box. In the breaker panel the black wire terminates in a 20amp breaker, and the red wire terminates in a 15amp breaker.

At the junction box, the black wire is tied to 12-2 and the red wire is tied to 14-2. The neutrals in the junction box are all wire nutted together.

So my question is (I think): Can a 20amp circuit and 15amp circuit share a neutral?

Thanks.
 
  #2  
Old 01-17-11, 05:10 PM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: United States
Posts: 14,336
Received 878 Upvotes on 741 Posts
What you describe is VERY rare, and in my book, a bad idea but would be legal as far as the code is concerned. Here's why:

The 12/3 wire is under fused on the 15 amp leg since #12 can carry up to 20 amps. The 15 amp breaker is correct since there is #14 wire on that circuit. The neutral will never carry more than 20 amps which is fine because it is #12 from the splice to the panel.

The biggest problem I see is somebody can come along later in life and decide that you have #12 on a 15 amp breaker and choose to up it to 20 amps without knowing there is #14 wire else ware on the circuit. The other issue is the code since 2008 requires multiwire circuits (those sharing a neutral) to be two pole breakers. This circuit may have been installed before that code so it would be grandfathered in, but I could see somebody putting it on a two pole 20amp for safety sake.
 
  #3  
Old 01-17-11, 06:51 PM
M
Member
Thread Starter
Join Date: Jan 2011
Location: USA
Posts: 10
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks for the response Tolyn.

As always with this type of thing, the answer has spawned a few additional questions.

1. Based on your answer, I can assume that sharing neutral's among circuits with different wire gauges is legal (though not recommended), so long as the "hots" are properly sized in the panel?

2. If I were to shut off the 15amp breaker in the panel, would the 15amp circuit truly be safe to work on? Or could the shared neutral with the 20amp circuit possibly allow voltage and current on the 15amps circuit?

I see that you live in the Twin Cities. Saint Paul, myself.

Thanks for the help. Interesting stuff.
 
  #4  
Old 01-18-11, 05:53 AM
Justin Smith's Avatar
Member
Join Date: Dec 2010
Location: Cressona, Pa, USA
Posts: 2,272
Upvotes: 0
Received 1 Upvote on 1 Post
It may be a good ides to put a "14awg down the line" tag on the Red wire for safety sake.
 
  #5  
Old 01-18-11, 06:41 AM
M
Banned. Rule And/Or Policy Violation
Join Date: Dec 2007
Location: Chicago
Posts: 904
Upvotes: 0
Received 0 Upvotes on 0 Posts
For this to work safely, the two ungrounded ("hot") conductors have to be on opposite phases.

The reason is the the loads are on the same phase, the amperage on the grounded ("neutral") conductor is additive, i.e. if there is a 10A load on one un-grounded conductor and a 12A load on the other, 22A is present on the shared grounded ("neutral") conductor.

In a typical 240V residential panel board the bus is designed so that the phase alternates at each breaker in the vertical direction, the result is that any two vertically adjacent breaker positions are on opposite phases.

As a result, the grounded conductors of a multi-wire circuit originating from a two-pole breaker will automatically be on opposite phases.

(This one reason why current code requires that such circuits originate at single disconnect, such as two pole breaker).

If two single pole breakers are used, care must be taken to ensure that the selected breaker positions are on opposite phases.

Also, keep in mind that a single disconnect guarantees if one ungrounded conductor is deenergized the other will be as well - if two separate breakers are involved, there is the possibility that someone will be working on an energized circuit they assume has been deenergized.
 
  #6  
Old 01-18-11, 08:43 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 9,785
Upvotes: 0
Received 45 Upvotes on 43 Posts
Originally Posted by MetroCP
1. Based on your answer, I can assume that sharing neutral's among circuits with different wire gauges is legal (though not recommended), so long as the "hots" are properly sized in the panel?
Yes, and the hots must be on opposite legs.

2. If I were to shut off the 15amp breaker in the panel, would the 15amp circuit truly be safe to work on? Or could the shared neutral with the 20amp circuit possibly allow voltage and current on the 15amps circuit?
Both breakers must be off for the shared circuit to be safe. Since 2008, code requires that the breakers must be handle-tied to ensure that both legs switch off together. As there is no such thing as a 15/20A double-pole breaker you would need to use an actual handle-tie kit compatible with your breakers to secure the two single-pole breakers together.
 
  #7  
Old 01-18-11, 06:11 PM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: United States
Posts: 14,336
Received 878 Upvotes on 741 Posts
Originally Posted by MetroCP
1. Based on your answer, I can assume that sharing neutral's among circuits with different wire gauges is legal (though not recommended), so long as the "hots" are properly sized in the panel?

I See that you live in the Twin Cities. Saint Paul, myself.
Greatings to a fellow Minnesotan!

The others covered it pretty well, but I just wanted to mention that the neutral would also need to be sized to the largest breaker/fuse. Which in this case would be #12 for 20 amp.
 
  #8  
Old 01-20-11, 11:04 AM
M
Member
Thread Starter
Join Date: Jan 2011
Location: USA
Posts: 10
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks for the help so far, everyone.

Just to summarize:

I have a 12-3 romex. The black hot terminates at a 20amp, single pole breaker, and feeds #12 wire. The red hot terminates at a 15amp, single pole breaker, and feeds #14 wire. The neutrals for the 12-3 romex, the #12 and #14 runs are all wire nutted in a box.

This is acceptable IF the 20amp and 15amp breakers are on seperate legs in the breaker panel. To completely deactivate the #12 and #14 runs, both single pole breakers must be off.

Correct?

Also, I have attached a photo of my breaker panel. Does each hot leg alternate left to right, or top to bottom?

Last question: If the 20amp and 15amp circuits are both in use, how could the neutral not exceed 20amp? Why wouldn't the neutral amps be additive if the circuits are on opposite phases?

Thanks again.
 
  #9  
Old 01-20-11, 01:23 PM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 9,785
Upvotes: 0
Received 45 Upvotes on 43 Posts
Originally Posted by MetroCP
This is acceptable IF the 20amp and 15amp breakers are on seperate legs in the breaker panel. To completely deactivate the #12 and #14 runs, both single pole breakers must be off.
correct.

Does each hot leg alternate left to right, or top to bottom?
Left to right

how could the neutral not exceed 20amp? Why wouldn't the neutral amps be additive if the circuits are on opposite phases?
The neutral current is the sum of the hot currents when they are on the same phase (leg). In this case you could have 20A + 15A = 35A on the neutral.

The neutral current cancels out and is the difference of the hot currents when they are on opposite phases (legs). In this case you cannot exceed 20A on the neutral: 20A - 0A = 20A. If both hots were active: 20A - 15A = 5A on the neutral.
 
  #10  
Old 01-20-11, 06:43 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: United States
Posts: 9,872
Received 185 Upvotes on 166 Posts
Am I the only one who sees a backfed main breaker in the picture with no retaining clip? Also, no grounding conductors?
 
  #11  
Old 01-21-11, 09:41 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 9,785
Upvotes: 0
Received 45 Upvotes on 43 Posts
Good catch Joe. There doesn't seem to be a ground in sight!
 
  #12  
Old 01-21-11, 04:52 PM
M
Member
Thread Starter
Join Date: Jan 2011
Location: USA
Posts: 10
Upvotes: 0
Received 0 Upvotes on 0 Posts
Let me clarify a couple of things, then ask a question about the last response.

The house is a duplex. From the main panel, there are two smaller panels. The picture I posted is one on of the two smaller panels. Above the panel and inside a metal enclosure are the grounds that are run with the romex. They are all tied together and bolted to the metal enclosure. The metal enclosure is connected to the panel. The main panel is grounded with #4 to my water service.

1. Am I in need of a ground bus bar inside the panel? If so, what needs to be grounded, and can I install one?

2. What is a "back fed main breaker?" Is that an acceptable situation?

You guys are extremely helpful. Can't thank you enough so far.
 
  #13  
Old 01-21-11, 05:53 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: United States
Posts: 9,872
Received 185 Upvotes on 166 Posts
Above the panel and inside a metal enclosure are the grounds that are run with the romex. They are all tied together and bolted to the metal enclosure. The metal enclosure is connected to the panel. The main panel is grounded with #4 to my water service.

1. Am I in need of a ground bus bar inside the panel? If so, what needs to be grounded, and can I install one?

2. What is a "back fed main breaker?" Is that an acceptable situation?
You say there is a metal enclosure above the panel with ground wires tied together and bolted to the metal enclosure (good), but the panel is bottom fed (confusing). Are those ground wires on the line side or load side of the panel? You say the metal enclosure is connected to the panel, would that be by metal conduit? If so, you need bonding bushings on the fittings because they are installed in concentric knockouts. A #10 grounding conductor should be run from the bonding bushings to a ground bar in the panel. You must insure you have an approved grounding path from the ground bar in the main panel to each of the two subpanels. This is a main lug panel, but the main lugs aren't being used. A 2 pole 50 amp breaker has been installed on the bus and is being backfed with the 50 amp feeder to feed the panel, this is your backfed main. A backfed main must have a retaining clip or bracket installed on it to keep it from accidentally coming unplugged from the bus. Square D should show this in their Digest. In addition to the #4 ground wire to where the water service enters the building, today's code also requires a minimum #6 copper ground wire to a ground rod with resistance of 25 ohms or less. Most electricians simply install a 2nd ground rod 6 feet from the first one to assure a good ground and to avoid having to prove the resistance to the inspector.
 

Last edited by CasualJoe; 01-21-11 at 06:01 PM. Reason: add info
 

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