24v temperature control cabling using white conductor as "hot"

[tahj3m]

I have recently been doing temperature control work in a commercial setting and the first "rule" I was told was white is "hot" and black is "common" or neutral. This seemed weird at first, but after doing a couple service calls it seems to be standard practice for temp control wiring. is this acceptable by code?

 

[hbiss]

With LV there is no "hot" or "neutral" nor does the NEC specify wire colors so you are free to use whatever colors you like as a standard practice.

-Hal

 

[Silversam]

With LV there is no "hot" or "neutral" nor does the NEC specify wire colors so you are free to use whatever colors you like as a standard practice.

-Hal

In Public Address/Paging Systems Black is always common and whatever the other color is, is the hot leg. Usually most cables will be Black and Red, however....

I did a very large system where the shop got a great buy on some Belden PA cable. Black and White.

How many circuits do you think were wired backwards?

All of them.

How many had to be reopened and reversed?

All of them.

Like Hal said: with LV there is no code, BUT there are standard practices that must be followed.

Sam

 

[hbiss]

With sound and PA, if I have a choice the speaker cable or conductors I specify are red and black. Black is always ground or common. So if the cable you have has white and black conductors, the black again is the common or ground and any other color is the "hot".

This is based on the DC convention where black is negative and red is positive. It's just common practice that you learn (sometimes the hard way) through experience in the trade.

-Hal

 

[don_resqcapt19]

With LV there is no "hot" or "neutral" nor does the NEC specify wire colors so you are free to use whatever colors you like as a standard practice.

-Hal

Some low voltage systems have one side of the power source connected to ground, creating a grounded conductor. If that is the case, it must be identified per the rules in Article 200. The most common case where you would see this is the Class 2 control circuit for a HVAC unit that has a 480 volt power supply....and the rule in Article 200 is almost never complied with.

 

[kwired]

Some low voltage systems have one side of the power source connected to ground, creating a grounded conductor. If that is the case, it must be identified per the rules in Article 200. The most common case where you would see this is the Class 2 control circuit for a HVAC unit that has a 480 volt power supply....and the rule in Article 200 is almost never complied with.

Then we have common uses in HVAC for controls using just about any color for the "return" or "L2" side, quite often it is blue but I have seen it just about anything not used for other purposes, white is usually call for heat signal, and green is usually call for fan signal, all are colors or uses restricted by NEC, unless those restrictions do not apply to class 2 circuits, have to check into that a little more before saying anything definite. Same color codes are used pretty much industry wide on 24 volt controls and does not matter what the primary voltage of the control transformer is, or if the secondary is grounded or not.


After looking at 200.7 and at 250.20 which was referenced in 200.7, I see what you are getting at when you mentioned a HVAC unit with a 480 volt power supply.

I will add that the 480 volt HVAC unit is likely a listed unit, and, for compliance with 250.20, I don't know that I have seen one with ungrounded control secondary. But also when reading 200.7 it doesn't really say that white is reserved for grounded conductor use, it just says such a system must be grounded when 250.20 says it must be grounded.:?

 

[don_resqcapt19]

...
I will add that the 480 volt HVAC unit is likely a listed unit, and, for compliance with 250.20, I don't know that I have seen one with ungrounded control secondary. But also when reading 200.7 it doesn't really say that white is reserved for grounded conductor use, it just says such a system must be grounded when 250.20 says it must be grounded.:?

I have no idea what exactly 200.7(B) is trying to tell us, but it is my opinion that 200.6 applies to the grounded conductor of a Class 2 control circuit.

 

[kwired]

I have no idea what exactly 200.7(B) is trying to tell us, but it is my opinion that 200.6 applies to the grounded conductor of a Class 2 control circuit.

Well we can go back to 200.1 and ask if this is premises wiring, but I think it is.

The rules of art 200 go back a long way, but so does the practice of using a white conductor as a non grounded conductor in HVAC controls, so I really don't have an answer at this point.

It is also common practice to use black as negative conductor in a DC circuit, and sometimes it is grounded sometime it is not, but if it were grounded it should probably be white to comply with art 200.6.

I have not carefully read through art 725 yet to see if something is there that relieves the requirements of art 200.

OK after a quick look at 725 I did find this in 725.3: "Only those sections of Article 300 referenced in this article shall apply to Class 1, Class 2, and Class 3 circuits." Although it doesn't apply to our situation of applying art 200, I find it at least worth mentioning that other than the mentioned articles there you can forget about chapter 3 when running a 725 control circuit.

 

[petersonra]

I have no idea what exactly 200.7(B) is trying to tell us, but it is my opinion that 200.6 applies to the grounded conductor of a Class 2 control circuit.

how do you get that out of 725.3?

 

[kwired]

how do you get that out of 725.3?

Don never mentioned 725.3, I did. But 725.3 never mentions anything in chapter 2.

 

[petersonra]

Don never mentioned 725.3, I did. But 725.3 never mentions anything in chapter 2.

you are right. I was kind of thinking 725.3 more or less exempted you from the rest of the code, but it really only exempts you from most of article 300.

 

[don_resqcapt19]

... I find it at least worth mentioning that other than the mentioned articles there you can forget about chapter 3 when running a 725 control circuit.

It doesn't say that you can forget about Chapter 3, it says you can forget about Article 300.

 

[kwired]

It doesn't say that you can forget about Chapter 3, it says you can forget about Article 300.

Oops, my bad - you are correct.