I'm thinking the current requirements of modern thermostats/furnaces
is pretty low.
Rates 24 at .57A for power transmission. That would seem to be plenty.
Most volt ohm meters have a current scale, if the OP wishes to check.
I suppose you could wire the 4 pairs in cat 6 in parallel which would
give the same capacity, that would seem unlikely to be needed, but I
know little about thermostats.
It all depends upon what the thermostat is controlling. Lots of equipment
use tiny solid state relays and ice cube relays that don't draw much, but
some systems use zone valves of various types and quantities, which draw
more. 18 gauge is probably a safe bet for almost everything, which is
probably why HVAC systems typically call for it. There is also the color
coding as gfretwell mentions. It's just a lot cleaner and easier to use
It's good to use what's designed for the usage. That way, the next
people who work on the system will be less confused. "Hey, bub...
isn't that phone wire? What's that doing inside a furnace?" "Dunno.
Lets rip it out."
On this, at least, we agree. Avoid doing things in non-standard ways,
but if/when you have to, leave notes for the next guy. Either yourself
in five years when you won't remember what the heck you did, or some
poor tradesman in 20 years after your heirs start fixing the place up to
sell it off.
I've also found to avoid the absence of not using double negatives to
contradict the refutation that previously rescinded the cancellation
which had previously but not at present been issued to disregard the
information which had been revoked.
Yeah, I'm funning with you. Once in a while I see a double negative
like "avoiding non". Just gets me to thinking that it's often easier
to write in positive voice. Sometimes I've no clue why the standard is
this or that way. And some times I do things in the usual way. But,
Thermostat wire is normally 18 gauge, Cat 6 is 22 or 24. (The smaller
the number, the larger the conductor.) So you might want to pair up 2
conductors of the Cat 6 for 1 "wire" of the thermostat. Especially if
it is a fairly long run from T-stat to furnace.
24 gauge should have a resistance of 25 ohms per 1000 meet. Considering
standard loads and voltages for a thermostat loop with #24 you could end
up with a 10 -15% voltage drop over all. That might not be enough to
effect a simple circuit but it would be better to pair up the wires.
And I believe that is likely a violation of the NEC. They have
requirements for low voltage wiring too. And I would believe that
the prohibition against paralleling conductors would apply there as
well. Perhaps someone knows for sure.
Overall, given the low cost of thermostat wire, using it for a
thermostat is a bad idea for all the reasons already given.
At the risk of getting slapped around, i always use sprinkler wire, it
is 18ga and 1/2 the price of thermostat wire, usually 5 conductor or
more (handy for heat pumps and remote sensors), and direct bury-able
for what that is worth.
Cat 6 is an expensive wire to use, but if you have it, you could use it
depending on what you are operating. If you only need 2 wires for a simple
heat only thermostat or possibly an air conditioning thermostat you could
double or triple up the wires. If you are running a heating/cooling system
with fan controls you need a 5 wire cable. Cat 6 may not be able to handle
the load because the fan and system switching on the thermostat operate
relays and may cause a voltage drop causing the relays to not stay closed or
open whichever state they should be in.
Failing to provide enough information about what you are using it for, makes
the accuracy of the answer questionable.
And I think that would likely be a violation of the NEC, which only
allows doing that under a few special conditons.
Thermostat wire is inexpensive. Why anybody would want to screw
around, is beyond me. Put the Cat 6 on Ebay and buy some thermostat
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