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Joined: Feb 2002
Posts: 840
C
Member
Bob,

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


Peter
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Joined: Jan 2003
Posts: 4,391
I
Moderator
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]

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Apr 2001
Posts: 518
J
Member
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.

Joined: Jan 2003
Posts: 4,391
I
Moderator
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).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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