Split A/C unit

[Dennis Alwon]

If the signal wire is switching a 240 volt signal there would be backfeed from the other side of the 240 volts.

I don't believe it is part of the 240V system. Obviously it would be required to be on a switch if it were. I was of the understanding that it is was just a communication wire -- I didn't think that would have 120V on it.

 

[GoldDigger]

I don't believe it is part of the 240V system. Obviously it would be required to be on a switch if it were. I was of the understanding that it is was just a communication wire -- I didn't think that would have 120V on it.

I have seen a wiring diagram of one mini-split in which the signaling wire is fed as an offset from one of the 240V phase conductors rather than being ground or neutral referenced. The offset signal voltage is small (10V?) but since there is no indication of whether this is current limited or to what value, the signaling wire must also be interrupted by the disconnect. Otherwise hazardous voltages will still be present in the disconnected unit.
That does not mean that the OP's system is wired that way, but it cannot be ruled out, especially if the manufacturer says that it must be switched!

 

[Dennis Alwon]

I have seen a wiring diagram of one mini-split in which the signaling wire is fed as an offset from one of the 240V phase conductors rather than being ground or neutral referenced. The offset signal voltage is small (10V?) but since there is no indication of whether this is current limited or to what value, the signaling wire must also be interrupted by the disconnect. Otherwise hazardous voltages will still be present in the disconnected unit.
That does not mean that the OP's system is wired that way, but it cannot be ruled out, especially if the manufacturer says that it must be switched!

I agree and I did say if the manufacturer calls for it then we must use it but it is good to know that some of the unit diagrams do use the phase conductors.

 

[kwired]

I don't believe it is part of the 240V system. Obviously it would be required to be on a switch if it were. I was of the understanding that it is was just a communication wire -- I didn't think that would have 120V on it.

I have seen a wiring diagram of one mini-split in which the signaling wire is fed as an offset from one of the 240V phase conductors rather than being ground or neutral referenced. The offset signal voltage is small (10V?) but since there is no indication of whether this is current limited or to what value, the signaling wire must also be interrupted by the disconnect. Otherwise hazardous voltages will still be present in the disconnected unit.
That does not mean that the OP's system is wired that way, but it cannot be ruled out, especially if the manufacturer says that it must be switched!

Unless I wasn't paying attention in basic electricity class, there has to be an opposing polarity somewhere in this control circuit, otherwise you will get no useful function out of a single conductor. I haven't wired many of these units but have seen some with a need for three conductors between the two units. I have never connected one that had a neutral, so that means the control circuit is either 208/240, or is picking up 120 from the equipment grounding conductor. We just put in a requirement for a grounded conductor at most lighting switches recently because of the use of the EGC as part of a control circuit with some lighting controls

 

[GoldDigger]

Unless I wasn't paying attention in basic electricity class, there has to be an opposing polarity somewhere in this control circuit, otherwise you will get no useful function out of a single conductor. I haven't wired many of these units but have seen some with a need for three conductors between the two units. I have never connected one that had a neutral, so that means the control circuit is either 208/240, or is picking up 120 from the equipment grounding conductor. We just put in a requirement for a grounded conductor at most lighting switches recently because of the use of the EGC as part of a control circuit with some lighting controls

Here is a feeble attempt at graphics:

1   240 A
       | difference = 240
2   240 B, and Control B
       | difference is 10 volt DC pulsed signal 
3   Control A

So the control voltage is only 10 volts DC, but one end of that 10 volts is one of the 240 hot lines. No neutral involved at all.

Depending on how you look at it, the control signal is either 10 volts DC or is 240 AC / 240 AC plus 10 volts DC, also describable as 10 volts DC with 240 volts AC common mode "noise".

 

[kwired]

Here is a feeble attempt at graphics:

1   240 A
       | difference = 240
2   240 B, and Control B
       | difference is 10 volt DC pulsed signal 
3   Control A

So the control voltage is only 10 volts DC, but one end of that 10 volts is one of the 240 hot lines. No neutral involved at all.

That still says to me that you will have 120 volts to ground from at least one direction if you open that conductor.

 

[GoldDigger]

That still says to me that you will have 120 volts to ground from at least one direction if you open that conductor.

Absolutely right. And you have 120 volts to ground in both units if you do not open it. Which is why you need a three pole switch.
But the signal receiver does not care anything about the presence of that 120 volts to ground in common mode on the two control wires (one of them shared with power.)
And no neutral or ground connection is needed to make both power and control work. (Although the ground is required for safety, no control current flows through the ground, unlike the case for older presence sensor light switches.)

 

[kwired]

Absolutely right. And you have 120 volts to ground in both units if you do not open it. Which is why you need a three pole switch.
But the signal receiver does not care anything about the presence of that 120 volts to ground in common mode on the two control wires (one of them shared with power.)
And no neutral or ground connection is needed to make both power and control work. (Although the ground is required for safety, no control current flows through the ground, unlike the case for older presence sensor light switches.)

Ok, I think we are on same page, I have thought all along that a 3 pole switch is the only way to disconnect the indoor unit and be assured there will be no voltage within.

 

[Dennis Alwon]

I just left a job where the hvac guy ran this cable thru the walls for the split system. I told him he needs to rip it out. They are using tray cable. Type TC-ER and that is only allowed in cable trays or raceways. It was a solid wire cable and I thought they required stranded --He said this is what the manufacturer requires and I said they must meet the NEC as well. Called the state about it and he totally agreed and he has been dealing with this issue forever. The manufacturers just don't get it.

 

[Dennis Alwon]

BTW, I think we are just going to run 14/3 nm and be done with it.

 

[suemarkp]

Washington dealt with this problem by changing our local code. See the WAC 296-46B-336 section of this: http://lni.wa.gov/TradesLicensing/Electrical/files/currents/elc1301Special.pdf

They basically allow you to treat TC cable as NM. It would be nicer if some of these companies understood the code better and just porvided an NEC compliant cable from the get go (or say use NM or UF cable).

 

[Dennis Alwon]

I am very curious as to why TC cannot be used as nm. It looks like it would be as good and perhaps better than NM. There must be some reason it must be in a raceway or a cable tray to be compliant.

 

[GoldDigger]

I am very curious as to why TC cannot be used as nm. It looks like it would be as good and perhaps better than NM. There must be some reason it must be in a raceway or a cable tray to be compliant.

Possibly just that UL requires different test conditions for the two and the TC conditions do not completely cover the NM conditions, making it necessary to spend more to get dual listing?
Among other things, NM has to be self-supporting over some vertical and horizontal span.

 

[Dennis Alwon]

Possibly just that UL requires different test conditions for the two and the TC conditions do not completely cover the NM conditions, making it necessary to spend more to get dual listing?
Among other things, NM has to be self-supporting over some vertical and horizontal span.

Not sure what self supporting means but this TC-ER cable was stiffer than NM. I am thinking that it is more expensive so they never bothered to have it tested for indoor. But you are correct that there may be some characteristics that make it not suitable-- I am just wondering what those characteristics may be. It seems strange that an entire state would allow this to be installed indoors as stated if there was a safety issue with it.

 

[ActionDave]

There is no way that anybody can convince me that TC is inferior to NM. IMO it is nothing more than bureaucratic non-sense.

I used to use it in houses all the time for swamp coolers till I found out it wasn't allowed (the only downside I have found to joining this forum).

 

[kwired]

Is possible TC cable is just fine for indoors, just nobody has ever done any evaluation for such purposes.

 

[ckelley3]

How can rubber cord be used for a permanent wiring method?

The instructions with the units I have seen say a disconnect must be provided near the unit.

But there is a 3-wire interconnect, so can it be done with a double-pole switch?

Normally a 3 pole switch, such as a Leviton MS303-DS is used at the indoor unit for a disconnect.