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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!

GRB
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    Is this up in Canada? – ThreePhaseEel May 23 '22 at 03:43
  • Since most of it's buried, at leas that portion should not be a fire hazard. Though I don't know your local codes, I might consider putting heavier gauge wire on the above-ground portion. Check local codes. – DrMoishe Pippik May 23 '22 at 04:20
  • Looking at the NEC you can not use 90c table for anything but derating. As far as the 75c table it can be used if all parts are 75c rated however a kiln is a fixed heating device so you can only use 80% of the 75c table. – Ed Beal May 23 '22 at 04:38
  • @EdBeal It looks to me like he's already derated. 27A steady draw on a 35A circuit. 27/0.8=33.75 which is less than the 75c wire rating. – DoxyLover May 23 '22 at 04:51
  • Is the wire direct buried or is it in conduit? – FreeMan May 23 '22 at 11:38
  • @doxylover I did not state the op did not derate I would not say it was acceptable based it is going to an outside sub, with that info I would not install it , this would be like running your car at red line and to me that is not safe so I just stated what the code says I used to find non code compliant all the time especially with small subs. – Ed Beal May 23 '22 at 13:21
  • @ThreePhaseEel yes I'm in Canadia. – GRB May 23 '22 at 13:29
  • @FreeMan I don't know for sure (again since this was all by prior owner) but working assumption is that it's in a conduit. Will be something I'll confirm prior to starting the work – GRB May 23 '22 at 13:31
  • @EdBeal I appreciate that perspective. I'm just wondering, what does it actually _mean_ to have the wire at 75 C? That's below combustion of wood, etc. - not that I'm looking on taking it that close, I'm just trying to contextualize how **hot** that really is (and despite this coming from a Canadian who only really knows C anyway!) – GRB May 23 '22 at 13:34
  • 75c is the maximum temperature the insulation can handle conductors heat up. most haven’t felt this other than on a overloaded extension cord that feels warm. Bad connections is probably the most common point of overloaded wires causing burnt insulation, running an electric kiln at the max off a sub panel means the feeder will probably end up overloaded. in free air there would be enough wiggle room but not conduit or earth (both heat up) this s why I only answered with a comment as there are a few questions but I never recommend using the minimum it is penny pound foolish. – Ed Beal May 23 '22 at 16:59

2 Answers2

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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.

Harper - Reinstate Monica
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  • Ah I see. So I have two issues - not just the gauge, but now the class of wire too. Thank you, I was having a hard time nailing down what ST was (other than "service"). My first instinct was always to replace - now I know for sure before the hard work begins! – GRB May 24 '22 at 18:19
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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.

Bob Roberts
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