common neutral branch circuits
- Thread starter david
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Re: common neutral branch circuits
Ron, I do remember this conversation from the old board, and these types of conversations are what keeps us lively.
I have to let the better half have the phone line now and it is my bed time, so I'll respond tomorrow.
Roger
Ron, I do remember this conversation from the old board, and these types of conversations are what keeps us lively.
I have to let the better half have the phone line now and it is my bed time, so I'll respond tomorrow.
Roger
- Location
- Illinois
- Occupation
- retired electrician
Re: common neutral branch circuits
Ron,
I see no need to limit the load on the 1/0 to 20 amps. I see a need to limit that load to 150 amps. In the case of the dimmer, I think that the conductor should be sized at 2/0. The eight 20 amp breakers on the ungrounded conductors will limit the maximum current on the grounded conductor to 160 amps. Each of the ungrounded conductors are protected at their ampacity as is the one common grounded conductor.
Don
Ron,
I see no need to limit the load on the 1/0 to 20 amps. I see a need to limit that load to 150 amps. In the case of the dimmer, I think that the conductor should be sized at 2/0. The eight 20 amp breakers on the ungrounded conductors will limit the maximum current on the grounded conductor to 160 amps. Each of the ungrounded conductors are protected at their ampacity as is the one common grounded conductor.
Don
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: common neutral branch circuits
Not to get off track here but I have seen many office qubical's that will come with 3 #12awg's and one #10awg neutral for the feeds to the panel. and have seen a UL label saying it is listed assy. so we need to ask UL if this is allowed? or do they allow things that otherwise the NEC might prohibit?
Sound's like a hipacridical role if they do.
Not to get off track here but I have seen many office qubical's that will come with 3 #12awg's and one #10awg neutral for the feeds to the panel. and have seen a UL label saying it is listed assy. so we need to ask UL if this is allowed? or do they allow things that otherwise the NEC might prohibit?
Sound's like a hipacridical role if they do.
david
Senior Member
- Location
- Pennsylvania
Re: common neutral branch circuits
210.3 Rating.
Branch circuits recognized by this article shall be rated in accordance with the maximum permitted ampere rating or setting of the overcurrent device. The rating for other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes. Where conductors of higher ampacity are used for any reason, the ampere rating or setting of the specified overcurrent device shall determine the circuit rating.
210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall be permitted as multi-wire circuits. A multi-wire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panel board.
Please keep in mind I am making a distinction between common neutral circuits and ?multi- wire circuits? Also keep in mind at this point I am distinguishing between branch-circuits and feeder circuits.
Other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes. A common neutral circuit would not be an individual branch circuit. For the sake of argument if a common neutral circuit was recognized by article 210 (I do not yet agree that article 210, Branch circuits recognizes common neutral circuits) then the circuit determined by the ampacity, is limited by 210.3. In other words three 20 amp branch circuits fed from the same phase would be a 60 amp circuit and would not be allowed by 210.3. On the other hand three 20- amp multi-wire branch circuits according to 210.4 would be classed as a 20-amp circuit or three 20-amp circuits.
210.3 Rating.
Branch circuits recognized by this article shall be rated in accordance with the maximum permitted ampere rating or setting of the overcurrent device. The rating for other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes. Where conductors of higher ampacity are used for any reason, the ampere rating or setting of the specified overcurrent device shall determine the circuit rating.
210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall be permitted as multi-wire circuits. A multi-wire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panel board.
Please keep in mind I am making a distinction between common neutral circuits and ?multi- wire circuits? Also keep in mind at this point I am distinguishing between branch-circuits and feeder circuits.
Other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes. A common neutral circuit would not be an individual branch circuit. For the sake of argument if a common neutral circuit was recognized by article 210 (I do not yet agree that article 210, Branch circuits recognizes common neutral circuits) then the circuit determined by the ampacity, is limited by 210.3. In other words three 20 amp branch circuits fed from the same phase would be a 60 amp circuit and would not be allowed by 210.3. On the other hand three 20- amp multi-wire branch circuits according to 210.4 would be classed as a 20-amp circuit or three 20-amp circuits.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: common neutral branch circuits
Where does 210.3 not allow a 60 amp circuit. it states:Where conductors of higher ampacity are used for any reason, the ampere rating or setting of the specified overcurrent device shall determine the circuit rating.
and 210.4 allows it to be installed as multiple circuits.
A multiwire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard.
Where does 210.3 not allow a 60 amp circuit. it states:Where conductors of higher ampacity are used for any reason, the ampere rating or setting of the specified overcurrent device shall determine the circuit rating.
and 210.4 allows it to be installed as multiple circuits.
A multiwire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard.
david
Senior Member
- Location
- Pennsylvania
Re: common neutral branch circuits
210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(2)
Multioutlet Branch Circuits. Conductors of branch circuits supplying more than one receptacle for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit.
Once the rating of a branch circuit is determined then and a common neutral circuit turns out to be supplying more than one receptacle or cord and plug portable loads. The circuit conductors would have to be sized according to the circuit rating.
In other words a 60-amp circuit, three 20-amp breakers fed from the same phase sharing a common neutral both the hot and grounded conductors would have to have a 60-amp rating.
[ August 29, 2003, 06:16 AM: Message edited by: david ]
210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(2)
Multioutlet Branch Circuits. Conductors of branch circuits supplying more than one receptacle for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit.
Once the rating of a branch circuit is determined then and a common neutral circuit turns out to be supplying more than one receptacle or cord and plug portable loads. The circuit conductors would have to be sized according to the circuit rating.
In other words a 60-amp circuit, three 20-amp breakers fed from the same phase sharing a common neutral both the hot and grounded conductors would have to have a 60-amp rating.
[ August 29, 2003, 06:16 AM: Message edited by: david ]
david
Senior Member
- Location
- Pennsylvania
Re: common neutral branch circuits
Hurk, Sorry I got booted off earlier:
It is true if you oversize conductors for any reason ,voltage drop harmonics, conduit fill the amp rating is how you determine the circuit rating not the conductor size.
What I said was Other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes.
Then I concluded that a circuit sharing the neutral with more than one phase was not an individual branch circuit. I tried to show you where given permission to consider a traditional Multi-wire circuit as more than one circuit but that permission is not extended to a common neutral circuit.
A multi wire circuit has a specific definition given in the code. Notice the NEC handbook commentary after 210.4
NEC commentary:
?The term multiwire branch circuit is defined in Article 100 as a branch circuit that consists of two or more ungrounded conductors that have a voltage between them and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor"
[ August 29, 2003, 06:09 AM: Message edited by: david ]
Hurk, Sorry I got booted off earlier:
It is true if you oversize conductors for any reason ,voltage drop harmonics, conduit fill the amp rating is how you determine the circuit rating not the conductor size.
What I said was Other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes.
Then I concluded that a circuit sharing the neutral with more than one phase was not an individual branch circuit. I tried to show you where given permission to consider a traditional Multi-wire circuit as more than one circuit but that permission is not extended to a common neutral circuit.
A multi wire circuit has a specific definition given in the code. Notice the NEC handbook commentary after 210.4
NEC commentary:
?The term multiwire branch circuit is defined in Article 100 as a branch circuit that consists of two or more ungrounded conductors that have a voltage between them and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor"
[ August 29, 2003, 06:09 AM: Message edited by: david ]
david
Senior Member
- Location
- Pennsylvania
Re: common neutral branch circuits
210.11 Branch Circuits Required.
A) Number of Branch Circuits. The minimum number of branch circuits shall be determined from the total computed load and the size or rating of the circuits used. In all installations, the number of circuits shall be sufficient to supply the load served. In no case shall the load on any circuit exceed the maximum specified by 220.4.
220.4 Maximum Loads.
The total load shall not exceed the rating of the branch circuit, and it shall not exceed the maximum loads specified in 220.4(A) through (C) under the conditions specified therein.
If you have a load of 60 amps on a circuit neutral then the rating of the circuit would have to be at least 60 amps. I think this further supports my thinking that a common neutral circuit fed from three 20 amp breakers of the same phase, would have to have a rating of 60 amps.
210.11 Branch Circuits Required.
A) Number of Branch Circuits. The minimum number of branch circuits shall be determined from the total computed load and the size or rating of the circuits used. In all installations, the number of circuits shall be sufficient to supply the load served. In no case shall the load on any circuit exceed the maximum specified by 220.4.
220.4 Maximum Loads.
The total load shall not exceed the rating of the branch circuit, and it shall not exceed the maximum loads specified in 220.4(A) through (C) under the conditions specified therein.
If you have a load of 60 amps on a circuit neutral then the rating of the circuit would have to be at least 60 amps. I think this further supports my thinking that a common neutral circuit fed from three 20 amp breakers of the same phase, would have to have a rating of 60 amps.
Re: common neutral branch circuits
David, I don't see any of your argument that could be defined as anything but cloudy.
As far as
Are you going to require the cable be trashed and replaced to match the constantly changeing harmonic loading?
I know this isn't quite the same as a common grounded conductor but is the same in actualality as far as the grounded conductor carrying more than any one ungrounded conductor.
Roger
David, I don't see any of your argument that could be defined as anything but cloudy.
As far as
what are you going to do if you have an additive harmonic load on the neutral above the ampere rating of the OCPD and you used a super neutral cable?
Are you going to require the cable be trashed and replaced to match the constantly changeing harmonic loading?
I know this isn't quite the same as a common grounded conductor but is the same in actualality as far as the grounded conductor carrying more than any one ungrounded conductor.
Roger
- Location
- Illinois
- Occupation
- retired electrician
Re: common neutral branch circuits
David,
I don't agree. The rating of the branch circuit does not change just because you are using a common neutral. The rating of each circuit is still the rating of the OCPD that protects the ungrounded conductor. The use of a common grounded conductors for multiple ungrounded conductors does not change the rating of the circuit.
While I agree that the code does not extend specific permission for a common neutral circuit, it also does not specifically prohibit it. In the Chapter 3 articles, the 2005 code will no longer have the xxx.10 sections (uses permitted). The code position will be that unless the installation is prohibited by the xxx.12 sections (uses not permitted), it will be permitted.
Don
David,
I don't agree. The rating of the branch circuit does not change just because you are using a common neutral. The rating of each circuit is still the rating of the OCPD that protects the ungrounded conductor. The use of a common grounded conductors for multiple ungrounded conductors does not change the rating of the circuit.
While I agree that the code does not extend specific permission for a common neutral circuit, it also does not specifically prohibit it. In the Chapter 3 articles, the 2005 code will no longer have the xxx.10 sections (uses permitted). The code position will be that unless the installation is prohibited by the xxx.12 sections (uses not permitted), it will be permitted.
Don
david
Senior Member
- Location
- Pennsylvania
Re: common neutral branch circuits
Ok let me flip this around on you.
A contractor utilizes this common neutral circuit by taking three 20 amp breakers fed with # 12 ungrounded conductors fed from the same phase and uses a # 12 common neutral. You are saying this circuit is recognized by article 210; I have no code reference that is specific to call this a violation. According to you I have to pass it.
I don?t want to make my argument sound to simplistic but I am saying that the only common neutral circuit recognized by 210 is a ?multi-wire circuit
How do you determine the rating of this hybrid common neutral circuit. If your correct and it is a 20-amp circuit determined by one of the 20 amp breakers protecting this circuit than the min size wire I can ask for is a # 12.
Normally I would agree that the rating of a branch circuit is determined by the size of the over current protection. You would have to agree that the over current protection in normal branch circuits would limit the current on the conductors in the circuit. What about a combined inrush current on this circuit?s grounded conductor?
I am saying you have permission to use a common neutral circuit outside for lighting as specifically allowed in 225.7 but I don?t agree that permission is given in article 210. I am saying that a multi-wire circuit was given specific approval and defined in article 210 for branch circuit if this is not the case why did they bother to define this circuit.
Please explain to me what is going to happen if the neutral is lost at the source to this circuit. Would you look at that as a hazard?
I am not alone in my thinking on this please check out this link that was given to me. http://ceenews.com/ar/electric_common_neutrals_gym/index.htm
[ August 29, 2003, 01:21 PM: Message edited by: david ]
Ok let me flip this around on you.
A contractor utilizes this common neutral circuit by taking three 20 amp breakers fed with # 12 ungrounded conductors fed from the same phase and uses a # 12 common neutral. You are saying this circuit is recognized by article 210; I have no code reference that is specific to call this a violation. According to you I have to pass it.
I don?t want to make my argument sound to simplistic but I am saying that the only common neutral circuit recognized by 210 is a ?multi-wire circuit
How do you determine the rating of this hybrid common neutral circuit. If your correct and it is a 20-amp circuit determined by one of the 20 amp breakers protecting this circuit than the min size wire I can ask for is a # 12.
Normally I would agree that the rating of a branch circuit is determined by the size of the over current protection. You would have to agree that the over current protection in normal branch circuits would limit the current on the conductors in the circuit. What about a combined inrush current on this circuit?s grounded conductor?
I am saying you have permission to use a common neutral circuit outside for lighting as specifically allowed in 225.7 but I don?t agree that permission is given in article 210. I am saying that a multi-wire circuit was given specific approval and defined in article 210 for branch circuit if this is not the case why did they bother to define this circuit.
Please explain to me what is going to happen if the neutral is lost at the source to this circuit. Would you look at that as a hazard?
I am not alone in my thinking on this please check out this link that was given to me. http://ceenews.com/ar/electric_common_neutrals_gym/index.htm
[ August 29, 2003, 01:21 PM: Message edited by: david ]
- Location
- Massachusetts
Re: common neutral branch circuits
I look at this in a different way, what electrical hazard would arise from running three 20 amp ungrounded conductors of the same phase out to loads and returning them to the panel on a 60 amp grounded conductor?
I can think of no hazard from doing this, so if the code stays within it's purpose this should be acceptable.
All that aside wouldn't we be violating 240.8 Fuses or Circuit Breakers in Parallel?
I would say this would be a direct violation of 240.4 and 310.15, as you would be protecting the 12 awg grounded conductor with 60 amps.
I look at this in a different way, what electrical hazard would arise from running three 20 amp ungrounded conductors of the same phase out to loads and returning them to the panel on a 60 amp grounded conductor?
I can think of no hazard from doing this, so if the code stays within it's purpose this should be acceptable.
Bob
All that aside wouldn't we be violating 240.8 Fuses or Circuit Breakers in Parallel?
- Location
- Illinois
- Occupation
- retired electrician
Re: common neutral branch circuits
David,
I agree with Bob, your example is a violation of 240.4 as the 20 amp rated grounded conductor is protected at 60 amps. If you put in a 60 amp rated grounded conductor, this violation goes away.
Bob,
The OCPDs are not connected in parallel when using a common grounded conductor. The line and load of the OCPDs are not directly connected to each other. If the common neutral circuit is a violation of 240.8 then so is a multiwire circuit.
Don
David,
I agree with Bob, your example is a violation of 240.4 as the 20 amp rated grounded conductor is protected at 60 amps. If you put in a 60 amp rated grounded conductor, this violation goes away.
Bob,
The OCPDs are not connected in parallel when using a common grounded conductor. The line and load of the OCPDs are not directly connected to each other. If the common neutral circuit is a violation of 240.8 then so is a multiwire circuit.
Don
- Location
- Massachusetts
Re: common neutral branch circuits
By David
In that case the circuit would simply be "off" no damage or hazard.
Compared to a NEC defined multiwire branch circuit that is clearly allowed, but can do damage and create a hazard if the neutral is lost.(one true neutral for two or three ungrounded conductors of different phases)
Bob
[ August 29, 2003, 01:43 PM: Message edited by: iwire ]
Point taken
By David
Dave, are you asking about loosing the grounded conductor on a circuit that is not a NEC multiwire branch circuit? (one grounded conductor for multiple ungrounded conductors of the same phase)
In that case the circuit would simply be "off" no damage or hazard.
Compared to a NEC defined multiwire branch circuit that is clearly allowed, but can do damage and create a hazard if the neutral is lost.(one true neutral for two or three ungrounded conductors of different phases)
Bob
[ August 29, 2003, 01:43 PM: Message edited by: iwire ]
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: common neutral branch circuits
Maybe I have gotten lost in this lengthy discussion but:
Many panelboard manufacturers make a "column width" panel. This panel has the circuit breakers mounted in an enclosure that can fit into the web of a building support I-Beam. In an effort to keep the narrow panel as uncluttered as possible, the individual branch circuit neutral/grounded conductors are usually terminated in a single pull box located at the top of the column. This common neutral bar is feed by a single conductor (sized equal to the panel bus rating) which is run from the panelboard neutral to the pull box.
How is this common installation different from the Option A grounded conductor run to the lighting contactor panel in the original question?
Maybe I have gotten lost in this lengthy discussion but:
Many panelboard manufacturers make a "column width" panel. This panel has the circuit breakers mounted in an enclosure that can fit into the web of a building support I-Beam. In an effort to keep the narrow panel as uncluttered as possible, the individual branch circuit neutral/grounded conductors are usually terminated in a single pull box located at the top of the column. This common neutral bar is feed by a single conductor (sized equal to the panel bus rating) which is run from the panelboard neutral to the pull box.
How is this common installation different from the Option A grounded conductor run to the lighting contactor panel in the original question?
- Location
- Illinois
- Occupation
- retired electrician
Re: common neutral branch circuits
Jim,
The only difference is that the column width panel application is specifically permitted by 300.3(B)(4).
Don
Jim,
The only difference is that the column width panel application is specifically permitted by 300.3(B)(4).
Don
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: common neutral branch circuits
Would this not apply?
240.4 Protection of Conductors.
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
Would this not apply?
240.4 Protection of Conductors.
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
G
Guest
Guest
Re: common neutral branch circuits
Anyone willing to venture a definition of a common neutral as opposed to a shared neutral?
Anyone willing to venture a definition of a common neutral as opposed to a shared neutral?
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