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I wanted to ask you a quick wiring question - I need to run a wire to my new thermostat from the furnace - I have a spare telephone cable in the wall that has 4 22 gauge wires. I'll be running about 30ft and require 24v to power the system (drawing about 0.5 amps) The 4 wires are are independent insulated as well as being in cased in a blue phone cable.

My question is - would you know if 22 gauge wire will suffice? If not would using all 4 wires act as a higher gauge wire? I am trying to get around running a new wire.

update:
Thank you gents. Turns out as I opened the wall to retrieve the 22 gauge wire found a 18 gauge that was already fished to my furnace room. Better to be lucky than right. Thank you again

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  • \$\begingroup\$ According to some ampacity charts it should be good for around 0.9 amps but something to consider is what happens if your thermostat shorts (i.e. you should probably add a fuse at the source end) and any possible building/insurance code problems. From that point of view it might be a better question for DIY.SE (home improvement stack) but rather than cross-post if you agree you can do a flag→other on the post and ask for it to be migrated to DIY.SE. \$\endgroup\$ – PeterJ Jan 24 '15 at 14:25
  • \$\begingroup\$ I, too would be concerned about using telephone wire for a thermostat because of the insurance ramifications. \$\endgroup\$ – mkeith Jan 25 '15 at 7:49
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The conservative NEC set the maximum current density of about 700A/cm^2 for normal house electrical installation with common gauges copper wire. With AWG22 even at 1A the current density it is half of this limit (about 300A/cm^2).

However the PVC insulation of the telephone cable is terrible, because their purpose is other than current feed line use. But even the AWG22 bare wire, temperature rise is only 10 degr Celcius if carry 1A.

The voltage drop of a 30 ft long wire AWG22 carrying 1A at 24 DC voltage is about 1V or 4%, with a maximum allowable drop of 5%.

All above valid for straight cable suspended in air at a normal environment

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