Wiring #10 to a #12 device

[codetalker]

30A dual pole 240v, running #10 NM to a relay (in a metal box) for electric heating (15.8A), the relay RC840 is good for 22A. I'm using #10 wire because I can and for voltage drop. The relay unit has #12 (built in pigtail) for hookup. Obviously if there is a short somewhere this #12 wire becomes a concern. There is no recommended wire size for relay install. Working on the best approach for protection; use a 20A breaker on #10 wire? Inspection to follow.
Thanks

 

[rlundsrud]

You're fine with the number 10, it's not a code violation and is done all the time for voltage drop.

 

[LarryFine]

Working on the best approach for protection; use a 20A breaker on #10 wire?

That's exactly what I would do.

 

[codetalker]

That's exactly what I would do.

Since the current is fine for a 20A breaker does the relay device wire being #12 really come into the decision process in your mind, if the heaters required 20A for instance, is that #12 wire something I should look at or is it just a UL thing and none of my business.

In a over current condition, by using the #10 wire it actually increases the chance of the #12 wire burning as opposed to using #12 through-out the whole system and distributing the over-current/voltage drop condition. Hmm!
Thanks

 

[roger]

It's a pretty good bet that the insulation of the #12 is of a type rated for 25 to 30 amps, so even though you can't protect it a 30 amps (for this particular application) it's fine.

Roger

 

[WarrMann]

30A dual pole 240v, running #10 NM to a relay (in a metal box) for electric heating (15.8A)

What's the 30A dual pole for? If your question is about sizing a breaker? Im just looking for clarification on your question. It sounds like you should be using a 20A to protect that heater circuit rated at 15.8A. Which #10 is definitely able to handle

Is your main concern just that the #12 pigtails will be the first to go in the event of a short? If that's making you nervous, you could put an inline fuse holder between the different size conductors, and use a fast blow fuse. It would look strange though, and might not fit in the enclosure.

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[SceneryDriver]

30A is well within the damage curve for #12 wire. If there's a short circuit, the breaker will trip so fast the wire won't even try to get warm. Your #10 wire actually helps; the lower impedance of the circuit will lead to faster clearing times on the over current protection.

Short answer: you're fine.


SceneryDriver

 

[kwired]

Device is listed as is for 22 amps. You should not be concerned about size of that pigtail. It might have higher insulation temp rating or other design that will allow for heat dissipation that isn't necessarily accounted for in general building wiring methods.

Though below is a good point.

What's the 30A dual pole for? If your question is about sizing a breaker? Im just looking for clarification on your question. It sounds like you should be using a 20A to protect that heater circuit rated at 15.8A.

 

[GerryB]

On 240v baseboard heat with line voltage wall stats (also rated 22 amps) you cannot protect that with a 30 amp breaker. If you had three 8 foot heaters at 200 watts each plus 1.25 continuous duty you have 7500 watts or 31 amps. You need two circuits or a low voltage relay. As said if you only have 15 amps why do a 30 amp breaker? And your relay is only 22a.

 

[JFletcher]

I agree with others in that a 20 amp breaker is the maximum needed or that you should use. 15.8 amps is less than the 80% continuous load allowed on a 20 amp breaker.

 

[kwired]

OP doesn't appear to need a 30 amp circuit, but his switching device is rated up to 22 amps and could be used on a 22 amp load, which would have to be on a 30 amp circuit if a continuous load. Most line voltage thermostats and many relays intended for controlling heating are rated 22 amps from what I have noticed though the majority of them are only used on 20 amp circuits.

 

[codetalker]

Thanks for all your input. I mentioned the 30 amp breaker as a default size for the #10 wire. Sorry for the confusion.

Question: When calculating wire/breaker size and a relay/switch is put in, does the wiring need to handle the maximum rated relay current? In my case, I could use #12 wire for the heater at 15.8A or does code demand the wiring be designed for maximum relay current (22 A) which would be #10?

Thanks

 

[WarrMann]

Thanks for all your input. I mentioned the 30 amp breaker as a default size for the #10 wire. Sorry for the confusion.

Question: When calculating wire/breaker size and a relay/switch is put in, does the wiring need to handle the maximum rated relay current? In my case, I could use #12 wire for the heater at 15.8A or does code demand the wiring be designed for maximum relay current (22 A) which would be #10?

Thanks

You should you size your wire for the load. The relay will never have more current than the than the load crates going through it.

Ive done a few jobs now where we have replaced HID lighting with LED. Some of those situations have called for replacing some of the wiring. Long story short we put #10 wire on a 60 amp contactor. We put that #10 on a 20 amp breaker.

Breaker is always sized for the load too. Not the devices controlling the load

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[codetalker]

Breaker is always sized for the load too. Not the devices controlling the load

That's what I thought also, the breaker protects the wire and the other devices in the circuit are on their own per design.