240V residential AC: what's the difference between common and neutral

gil3000

Member
Location
US
Occupation
electrician
I'm sorry to bother you guys, but I am confused and frustrated. I have researched this for days and simply cannot find a straight answer. I'm trying to differentiate between common and neutral. I'm preparing for a NEC exam. I'm asking about basic USA household wiring (240VAC, 120VAC, 24VAC)-- non-industrial. For example, I'm not referring to "common", as it is used in a split capacitor, 3 and 4 way switches, etc. I'm 60, and have worked with residential wiring for 25 years.

Please try to explain it to me like I'm a dummy! When can these terms be used interchangeably and when can they not be used.

My current understandings:

1) All neutral wires are common wires, not all common wires are neutral wires. Neutrals have white or grey insulation and must be grounded, whereas a common, depending on the application, may have insulation of various other colors. and may be a hot lead (ex. split capacitor).

2) A common is the lead that is common to all devices from the same control voltage source. It usually has no contacts, switches, etc between it and loads. A common must go to more that one of something. So if a main panel has only one 120VAC single pole 15 amp breaker that goes to only one single outlet (not duplex outlet)-- there is no common and only a neutral that technically could not be called a common.

3) Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions. It carries current away from the device back to the electrical panel and ultimately back to the ground or source. It completes the circuit. It is (in this simple context) also called the grounded conductor.

4) If two separate 120VAC phases share a neutral, then the neutral is also a common.

5) The neutral wire is specifically designed to return current in an electrical circuit, while the common wire can refer to various return paths in different contexts, including switches or ground connections.

6) The power company says that for the power lines coming from the pole to the service entrance, the return is a neutral and not a common. Yet the neutral serves two phases-- so isn't that a common as well?

7) I've read this: "In a 120V system, there is no neutral, but we all call the white wire a neutral (need three wires to have a neutral)." Is this true??

OMG, thank you in advance for your time. My name is Gil...
 
Since you posted this in the NEC forum, let me point out that the NEC has a definition for "neutral" but no definition for "common." In fact I rarely see the term "common" used in this forum. I think it's fair to say that NEC-wise, there is no "common" conductor.

On the other hand, we can take your point (2) above as a definition of "common" and then try to answer your questions. I'll leave that to others.

The 2017 NEC definitions of "neutral" are excerpted below for reference.

Cheers, Wayne

2017 NEC said:
Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.

Neutral Point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct-current system.
 
I would forget the word common completely. Wayne's Article 100 definitions are more than adequate to describe when you have a neutral.
 
Since you posted this in the NEC forum, let me point out that the NEC has a definition for "neutral" but no definition for "common." In fact I rarely see the term "common" used in this forum. I think it's fair to say that NEC-wise, there is no "common" conductor.

On the other hand, we can take your point (2) above as a definition of "common" and then try to answer your questions. I'll leave that to others.

The 2017 NEC definitions of "neutral" are excerpted below for reference.

Cheers, Wayne
Thanks, Mr Wayne. I did look up the definitions of neutral and common in article 100 definitions of the NEC yesterday, and also saw that there was no definition of common!

Thanks!!!
 
I would forget the word common completely. Wayne's Article 100 definitions are more than adequate to describe when you have a neutral.
Yes, Sir. I see your point and agree. I guess common will just apply for 3/4 way switches and split caps.etc.

Hey-- thank you for your time...
 
I would forget the word common completely. Wayne's Article 100 definitions are more than adequate to describe when you have a neutral.
This is my first post. If I posted under the wrong heading, please correct me...
 
I would forget the word common completely.
One place where common and neutral kind of go together is in POCO distribution. You will often find them running a common conductor that is both primary and secondary neutral conductor.

Most of the time NEC applications are referring to "grounded conductor" which sometimes is not also a "neutral conductor". Though NEC has revamped definitions to now include things like the grounded conductor of a high leg delta system as being considered a "neutral" though technically is not the true neutral point of the voltages in such a system. Before that change that conductor in such a system was simply the grounded conductor to the NEC. To some degree I think it still should be that way. Those that get confused are likely confused no matter what on this, simply calling it the grounded conductor was more accurate IMO.
 
To me the notion of a common belongs to electronics or hobby applications where generally voltage and current is low, and there is some functional convenience to use a single wire for one side of multiple circuits, perhaps fed from different sources. Such a thing is not so safe for mains electricity, which is why you don't find it in the NEC.

'Neutral' implies a zero-sum vector relationship to all other conductors from a source of supply (or as further clarified by the NEC definition). A mains neutral is generally required to be grounded by the NEC, and most of the rules that distinguish how we treat the neutral in NEC wiring are technically rules that apply to grounded conductors.
 
One place where common and neutral kind of go together is in POCO distribution. You will often find them running a common conductor that is both primary and secondary neutral conductor.

Most of the time NEC applications are referring to "grounded conductor" which sometimes is not also a "neutral conductor". Though NEC has revamped definitions to now include things like the grounded conductor of a high leg delta system as being considered a "neutral" though technically is not the true neutral point of the voltages in such a system. Before that change that conductor in such a system was simply the grounded conductor to the NEC. To some degree I think it still should be that way. Those that get confused are likely confused no matter what on this, simply calling it the grounded conductor was more accurate IMO.
"Most of the time NEC applications are referring to "grounded conductor" which sometimes is not also a "neutral conductor"."

Yes, Sir. I am by no means "Mr Commercial" when it comes to electricity, but I did notice that.

Thank you very much for taking the time to respond!!
 
To me the notion of a common belongs to electronics or hobby applications where generally voltage and current is low, and there is some functional convenience to use a single wire for one side of multiple circuits, perhaps fed from different sources. Such a thing is not so safe for mains electricity, which is why you don't find it in the NEC.

'Neutral' implies a zero-sum vector relationship to all other conductors from a source of supply (or as further clarified by the NEC definition). A mains neutral is generally required to be grounded by the NEC, and most of the rules that distinguish how we treat the neutral in NEC wiring are technically rules that apply to grounded conductors.
I see what you're saying. I probably understand it good enough, but I just didn't want to be tripped up on the exam over me misinterpreting a word like "common". Great answer, Sir. And thank you!
 
"Most of the time NEC applications are referring to "grounded conductor" which sometimes is not also a "neutral conductor"."
It would be simpler if the NEC just kept the two terms grounded conductor and neutral separate. We already have a decent definition of a neutral so if meets that definition lets just call it a neutral and not a grounded conductor.
 
It would be simpler if the NEC just kept the two terms grounded conductor and neutral separate. We already have a decent definition of a neutral so if meets that definition lets just call it a neutral and not a grounded conductor.
How can you keep them separate when they are the same most of the time? Most of the mentions actually apply to grounded conductors regardless of whether they are neutral, so that's why 'grounded conductor' is used more often. When the rule actually applies specifically to neutrals (e.g. when counting CCCs) then 'neutral' is used.
 
Well, what about a 120/24v transformer used for HVAC for example. It is "common" ....no pun, to refer to one of the 24-volt wires as the "hot" wire and the other one as a "common". I don't think I ever herd anyone call it a neutral.
 
It would be simpler if the NEC just kept the two terms grounded conductor and neutral separate.
Don't they do that already? Can you provide an example of a requirement/permission for grounded conductors that should really only apply to grounded conductors that are neutrals, or a requirement/permission for neutral conductors that should really apply to all grounded conductors?

Cheers, Wayne
 
Don't they do that already? Can you provide an example of a requirement/permission for grounded conductors that should really only apply to grounded conductors that are neutrals, or a requirement/permission for neutral conductors that should really apply to all grounded conductors?

Cheers, Wayne
A neutral is a grounded conductor but a grounded conductor is not always a neutral. You see the NEC has made it really simple. :rolleyes:
 
Common seems to be common on small multi tap transformers😅
I see them often. Here's a pic of one.
IMG_4401.jpeg
 
I'm sorry to bother you guys, but I am confused and frustrated. I have researched this for days and simply cannot find a straight answer. I'm trying to differentiate between common and neutral. I'm preparing for a NEC exam. I'm asking about basic USA household wiring (240VAC, 120VAC, 24VAC)-- non-industrial. For example, I'm not referring to "common", as it is used in a split capacitor, 3 and 4 way switches, etc. I'm 60, and have worked with residential wiring for 25 years.

Please try to explain it to me like I'm a dummy! When can these terms be used interchangeably and when can they not be used.

My current understandings:

1) All neutral wires are common wires, not all common wires are neutral wires. Neutrals have white or grey insulation and must be grounded, whereas a common, depending on the application, may have insulation of various other colors. and may be a hot lead (ex. split capacitor).

2) A common is the lead that is common to all devices from the same control voltage source. It usually has no contacts, switches, etc between it and loads. A common must go to more that one of something. So if a main panel has only one 120VAC single pole 15 amp breaker that goes to only one single outlet (not duplex outlet)-- there is no common and only a neutral that technically could not be called a common.

3) Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions. It carries current away from the device back to the electrical panel and ultimately back to the ground or source. It completes the circuit. It is (in this simple context) also called the grounded conductor.

4) If two separate 120VAC phases share a neutral, then the neutral is also a common.

5) The neutral wire is specifically designed to return current in an electrical circuit, while the common wire can refer to various return paths in different contexts, including switches or ground connections.

6) The power company says that for the power lines coming from the pole to the service entrance, the return is a neutral and not a common. Yet the neutral serves two phases-- so isn't that a common as well?

7) I've read this: "In a 120V system, there is no neutral, but we all call the white wire a neutral (need three wires to have a neutral)." Is this true??

OMG, thank you in advance for your time. My name is Gil...

I'm sorry to bother you guys, but I am confused and frustrated. I have researched this for days and simply cannot find a straight answer. I'm trying to differentiate between common and neutral. I'm preparing for a NEC exam. I'm asking about basic USA household wiring (240VAC, 120VAC, 24VAC)-- non-industrial. For example, I'm not referring to "common", as it is used in a split capacitor, 3 and 4 way switches, etc. I'm 60, and have worked with residential wiring for 25 years.

Please try to explain it to me like I'm a dummy! When can these terms be used interchangeably and when can they not be used.

My current understandings:

1) All neutral wires are common wires, not all common wires are neutral wires. Neutrals have white or grey insulation and must be grounded, whereas a common, depending on the application, may have insulation of various other colors. and may be a hot lead (ex. split capacitor).

2) A common is the lead that is common to all devices from the same control voltage source. It usually has no contacts, switches, etc between it and loads. A common must go to more that one of something. So if a main panel has only one 120VAC single pole 15 amp breaker that goes to only one single outlet (not duplex outlet)-- there is no common and only a neutral that technically could not be called a common.

3) Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions. It carries current away from the device back to the electrical panel and ultimately back to the ground or source. It completes the circuit. It is (in this simple context) also called the grounded conductor.

4) If two separate 120VAC phases share a neutral, then the neutral is also a common.

5) The neutral wire is specifically designed to return current in an electrical circuit, while the common wire can refer to various return paths in different contexts, including switches or ground connections.

6) The power company says that for the power lines coming from the pole to the service entrance, the return is a neutral and not a common. Yet the neutral serves two phases-- so isn't that a common as well?

7) I've read this: "In a 120V system, there is no neutral, but we all call the white wire a neutral (need three wires to have a neutral)." Is this true??

OMG, thank you in advance for your time. My name is Gil...
 
Common on a low voltage transformer like that refers to the wire that
Common seems to be common on small multi tap transformers😅
I see them often. Here's a pic of one.
View attachment 2574998
Note how the common on the primary side is used for all primary voltages and the other wires to taps on the winding are colored according to the voltage relative to the common. This sense of common is simply a reference to which everything else is measured, but it's not a neutral.

As I mentioned above, the notion of 'common' is more ...er, common in low voltage.
 
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