Neutral Sharing Question
#1
01-17-11, 04:18 PM
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.
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
01-17-11, 05:10 PM
Group Moderator
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.
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
01-17-11, 06:51 PM
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.
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.
#5
01-18-11, 06:41 AM
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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.
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
01-18-11, 08:43 AM
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?
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?
#7
01-18-11, 06:11 PM
Group Moderator
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.
I See that you live in the Twin Cities. Saint Paul, myself.
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
01-20-11, 11:04 AM
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.
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
01-20-11, 01:23 PM
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.
Does each hot leg alternate left to right, or top to bottom?
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 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.
#12
01-21-11, 04:52 PM
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.
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
01-21-11, 05:53 PM
Member
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?
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?
Last edited by CasualJoe; 01-21-11 at 06:01 PM. Reason: add info
