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Mixing Line Voltage and Thermostat Wires in a AC Disconnect Switch.

What's the consensus? Seems to be a violation of the code?

[Linked Image from]
Where is the ground wire to the unit

I hate it when guys ramset boxes to a wall
It was not installed!

and ... Posted: Jul 31, 2004 8:43 AM Post subject: OK!


Dennis: OK, I think you done real good! and below see also the discussion on the question first posted in the link above:

The low voltage thermostat wires are not permitted to be run in the same cable or conduit because they do not have sufficient construction specifications to permit them to be installed with electric light, power conductors and any failure of the cable insulation due to a fault could lead to hazardous voltages being imposed on the thermostat circuit conductors.

The use of the same insulation for circuits up to 600 volts for both AC and DC is acceptable when run in the same cable or conduit, but not for this application.

It is better to see that thermostat wiring run in a separate cable or conduit, or wrapped around the cable or conduit that will supply the unit.
Agreed it's not a good practice, but if the wiring is all 600v rated, is it a violation? What Code reference, so I can tell the HVAC guys next time I see this done.
300.3(C) Conductors of Different Systems.

(1) 600 Volts, Nominal, or Less.

Conductors of circuits rated 600 volts, nominal, or less, ac circuits, and dc circuits shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway.

All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway.

Exception: For solar photovoltaic systems in accordance with 690.4(B).

FPN:See 725.55(A) for Class 2 and Class 3 circuit conductors.
725.55 does not permit under any circumstance for Class 2 conductors to be installed in RACEWAYS with any power conductors, even associated conductors are not permitted in the same raceway.
Enclosures and other items yes, under specific restrictions, but not raceways.

where is the T-stat wire gaining access to the electrical system here anyways??? It looks like its spliced into the #12TW's on the left? This is a strange installation from what I'm used to... [Linked Image from]

OK, the picture is not too clear to me. We have the heavy black and light blue conductor pairs that look like line and load. We have an orange and a blue spliced to the aux contacts on the breaker. And we have some sort of a coil of unidentified white wire that doesn't really seem to start or end anywhere. What's with the white - what is it? Where does it originate and terminate?
The blue wire on the very left of the photo comin gout of the yellow wire nut goes up and then back down ans seems to end in a spade connector that is not attached to anything. The smaller blue wire from the same yellow worenut goes almost directly to the right and ends up in the coil of white, green, red, and yellow wire. What is going on here?
No you can not run class 1 and 2 in the same raceway but you can reclassify class 2 or 3 circuits to class 1 then run them in the same conduit with other class 1 conductors [Linked Image] 725.52(A)Exception No. 2

I doubt highly that is the case of the wiring in Joe's Picture.


If you "reclassify" the low voltage conductors to Class 1, then don't the reclassified low voltage conductors have to be installed per Class 1 wiring methods?

In that case, you would not be permitted to run class 2 thermostat cable, but have to use NM, MC, AC, EMT, etc. Correct?

It seems to me that in most cases, reclassifying won't save any time or money, but might actually add labor and cost.

Peter you are absolutely correct, most times this will not save anything.

The times I have been able to take advantage of this have been for small air conditioning split systems.

Both Mitsubishi and Sanyo make units that the feed goes from the panel to the outdoor condenser, then you bring line voltage from outside to run the indoor section.

Along with this line volt feed between the units you also need to bring two 5 VDC control wires.

I have used 12/4 MC or NM between the two units to handle both voltages. [Linked Image]

Only recently have I begun doing A/C work. Heck, I even took an A/C course.

NEC clearly prohibits running the control wires together with the power leads. BUT...

The practice here seems to be:
If residential, run the control wires with the a/c line set. At the condenser, the control wires loop through the air and enter the condenser through a factory-provided bushing. Run this way, the wires are exposed to the much higher ambient of the line set.
If commercial, the control wires are run together with the power wires- even for 480 v. units.

As I see it, there are two issues here: insulation and induction.
Others have already addressed the insulation issue.
Most a/c systems seem to require the control wires to close a contactor, or otherwise deliver a few amps, so induction is not a problem. This may change as fancier electronic (plc-type) controls are used. PLC inputs simply read a voltage, so a voltage induced by the control wires proximity to power wires could cause control problems.

It doesn't help that the control (or t-stat) wires are often run by the HVAC guys.
The picture is also complicated in that some designs don't have control wires to the condenser at all.

Many times, the control wires and power wires come from differing directions, so they are run together only through the flex from the disconnect. This mitigates any risk.

Whatever the code may say, I've yet to see a condenser with two pieces of flex running to it.
I'm at a loss, and would like to learn if "code" contradicts "practice" elsewhere.
John come on out to the jobs we work you will see two different raceways coming out of the roof for RTUs. [Linked Image] Unless allowed otherwise.

I was always under the impression the reason the code wanted these conductors separated was to prevent a fault of the class one conductors imposing their voltage on the class 2 or 3 conductors.

As an example I repaired a factory wired display case that had cat 5 point of sale wiring in the same raceway as the 120 power.

The 120 power faulted and put 120 onto the cat 5 wire, this took out the data switch and the cash register.

IMO this is why they will allow you to reclassify class 2 and 3 to class 1 if you then treat the entire control circuit as class 1.

Imaging a 480 fault energizing 18/2 thermostat wire run through the building as class 2 or 3.

IMO the NEC is not concerned with mis-operation caused by induction, that is an inconvenience not a danger.

JMO, Bob [Linked Image]

[This message has been edited by iwire (edited 08-22-2004).]
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