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

[TX+ MASTER#4544]

2023 NEC
The simple answer to what is a neutral is found in the Code book.
Article 100 Neutral Conductor
Article 100 Neutral Point

Not very good answers, right?

A much better definition is found in 220.61(A) ".......neutral load shall be the maximum unbalance of the load....."

So, it takes 3 conductors to have a maximum unbalance load. Or 4 conductors from a Y system: 3 conductors and the neutral conductor.

Example using romex, a red and black and a white wire (12-3 WG). The white wire is a neutral conductor and meets the definition of needing 3 conductors to call it a neutral.

Example: If the red wire carries 15 amps and the black wire carries 11 amps, then the unbalance load is 4 amps, thus it is a neutral conductor.

A 12-2 WG (1white and 1 black wire) romex wire, the white wire is not a neutral, it is a white ground wire.
It returns current back to the source, so it is a current carrying conductor.

A common conductor is just what it says, it could be the switch leg of a 3 way switch or the hot for the switch (common).
You will find it on a small control transformer that may have different voltages to select from (not a door bell transformer)

My advice is to stop trying to label every thing a "common".

And think that you will need at least 3 wires to call it a neutral, 2 hot conductors and one white wire.
There's a formula for calculating the neutral current in a 4 wire circuit.

220.61(B) explains in detail if the neutral is permitted to be reduced in size or if there shall be no reduction of the neutral or grounded conductor.
Notice that a groundED conductor will normally be a neutral. (most cases)

A groundING conductor is not a neutral. An EGC is not a neutral nor a grounding electrode conductor. It is for bonding continuity.

Thanks for reading
Comments accepted
TX+MASTER#4544

 

[electrofelon]

I Think of "common" as just being used for 3 ways. A helper of mind uses "common" to refer to a neutral/grounded conductor and it drives me crazy.

 

[infinity]

A 12-2 WG (1white and 1 black wire) romex wire, the white wire is not a neutral, it is a white ground wire.
It returns current back to the source, so it is a current carrying conductor.

Why would you call it a white ground wire? And please explain why it is not a neutral conductor.

 

[kwired]

Common seems to be common on small multi tap transformers
I see them often. Here's a pic of one.
View attachment 2574998

Blue is generally the L2 conductor in 24VAC HVAC controls. Though L1 is usually red and not yellow, but my guess is they used yellow because they already used red for one the primary leads.

Since it is AC source you could swap secondary leads and things would still work the same though.

ADD: I guess the one called common on that label is the common lead of the primary. HVAC tends to call the L2 secondary control conductor Common everywhere through the control system is what I was initially thinking of.

 

[infinity]

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.

I agree. And the question from the OP is can we be interchanging the words neutral and common. The answer is no. That doesn't mean that the word common doesn't existing in electrical parlance it's just not applicable to the title of this thread, 240V residential AC: what's the difference between common and neutral.

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[jaggedben]

Why would you call it a white ground wire? And please explain why it is not a neutral conductor.

Neither the NEC definition of a neutral or the vector sum notion of a neutral apply in a 2 wire system. You can't have a vector sum of zero if you can't have a sum (because you only have one operand).

 

[jaggedben]

Blue is generally the L2 conductor in 24VAC HVAC controls. Though L1 is usually red and not yellow, but my guess is they used yellow because they already used red for one the primary leads.

...

Never seen red on the secondary of a 24V transformer, but I haven't really been working with them long. But all the new ones seem to be like the one posted earlier, sometimes with brown instead of yellow. Yeah they use red for the 240V-to-common tap on the primary so it makes sense not to have two red wires.

 

[wwhitney]

Neither the NEC definition of a neutral or the vector sum notion of a neutral apply in a 2 wire system. You can't have a vector sum of zero if you can't have a sum (because you only have one operand).

True, but the NEC definition of neutral refers to a current carrying conductor connected to the neutral point of a system. So if you have a voltage system on the premises with a neutral point, then a conductor connected to it is a neutral conductor, even if it is part of a 2-wire circuit.

Cheers, Wayne

 

[jaggedben]

True, but the NEC definition of neutral refers to a current carrying conductor connected to the neutral point of a system. So if you have a voltage system on the premises with a neutral point, then a conductor connected to it is a neutral conductor, even if it is part of a 2-wire circuit.

Cheers, Wayne

Yes that's why I said system, not circuit.

(Although, when counting CCCs for derating purposes, perhaps it's simpler for 2-wire circuits to just forget what kind of system they're connected to.)

 

[wwhitney]

Yes that's why I said system, not circuit.

OK, so tracing back the quotes, TX+ MASTER#4544 was not discussing a 2-wire system, so infinity's language criticism was on point.

Cheers, Wayne

 

[kwired]

Never seen red on the secondary of a 24V transformer, but I haven't really been working with them long. But all the new ones seem to be like the one posted earlier, sometimes with brown instead of yellow. Yeah they use red for the 240V-to-common tap on the primary so it makes sense not to have two red wires.

Pigtail off a transformer may or may not be red, but the L1 of the control system is usually designated as the "R" terminal on most equipment and typically has field installed conductors that are red, for simplicity reasons, one could really use any color they want or even all same color with just number identifiers, which will cause a ton of confusion for some HVAC tech's that know what goes where on stuff they run into the most but are not so up on actual circuit logic.

 

[jaggedben]

Pigtail off a transformer may or may not be red, but the L1 of the control system is usually designated as the "R" terminal on most equipment and typically has field installed conductors that are red, for simplicity reasons, one could really use any color they want or even all same color with just number identifiers, which will cause a ton of confusion for some HVAC tech's that know what goes where on stuff they run into the most but are not so up on actual circuit logic.

I guess traditions are traditions but personally I like having the low voltage stuff be totally different colors. Blue and yellow (or orange) has suited me well.

 

[kwired]

I guess traditions are traditions but personally I like having the low voltage stuff be totally different colors. Blue and yellow (or orange) has suited me well.

With HVAC is usually is also 16, 18 or possibly 20 AWG though so that also helps differentiate from larger power conductors when they are in the same space.

With some multistage heat pumps you may run a 9 or 10 conductor control cable at times - going to have most the readily available colors in that cable alone.