(Up to) 35 Amp Load on 10 Gauge Wire?

Question

I'm looking to connect an electric pottery kiln to an existing subpanel that is serviced by a 10/4 copper line and want to understand if the wire can acceptably tolerate the load.

The wire (installed by previous owner) has the following markings

• "CSA Type ST"
• "105 degrees C"

When in operation the kiln is rated by the manufacturer to draw a steady-state 27 amps (240 V), and that it should be wired on a 35 amp breaker circuit.

Checking my handy ampacity chart notes that #10 can do 30 amp @ 60 C, 35 @ 75 C and 40 @ 90 C. Does the 105 C on the wire mean that I can in theory go beyond 40 amps and still be within safe operating limits? What does it actually mean for a wire to be running at 75 C, 90 C? So long as it's behind walls / in conduits / you're not grabbing it with your hands, does that elevated temp pose any risk?

(I'd have put an #8 wire to this panel if I were installing new, but this line is buried going to a outside workshop so trying to avoid digging that up if at all possible)

Thank you!

Answer 1

ST is cordage. It's only good for a cord.

ST means "Service Grade" and "Thermoplastic".

What this means is, it's allowed for a flexible cord going from the appliance to the plug (or if hardwired, the first place attached to the wall).

Appliance cords follow a different set of rules, defined by CSA. They are allowed a somewhat higher ampacity specifically because they are always "out in the open" and have more access to cooling air. Don't coil them!

Cordage is NOT allowed to be used as a substitute for the permanent wiring that is part of a building or between buildings. Sometimes people use cordage instead of Romex, or staple/nail it to walls, or run it inside conduit. None of that is allowed.

Building wiring must follow CEC (similar to NEC)

That means S family cordage won't be used at all from first wall attachent back to main panel. You need NM, NMD90, Teck, or other type of cables made specifically for building wiring. It must be installed properly according to the rules for that cable type.

However it happens all the time where a nitwit uses cordage for wiring, or NM for cordage. This is a code violation and unsafe, and needs to be fixed.

For a 35A circuit the right stuff is 8 AWG copper or possibly 8 AWG or larger aluminum if it is authorized. If you need to replace the underground run, you might consider a large section such as #2 aluminum going to a subpanel - most professionals view aluminum as perfectly safe in large sizes, and the price is excellent.

2-2-2-4 aluminum is probably cheaper than that 10/4 cordage, actually. Because of that, it's really not worth trying to scrimp on feeder ampacity - someone thinks they're penny-pinching by using #8 copper instead of #6, is actually wasting money since #2AL would be a considerable savings even on the #8, and give twice the ampacity too!

Note that with cordage, the / number includes ground, so it is +1 more than the equivalent building wiring.

Answer 2

The temperature rating is for the wire types listed below the temperature rating on the chart. Find the printed on the wire label that tells you the type of insulation and then find that rating on the chart. That then telly you the ampacity of that exact cable/wire. Never exceed that ampacity, ever! Fire Hazard.