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I've seen this question about receptacles, but what about thermostats? My house in Washington state has really old mechanical thermostats that don't have their rating specified anywhere, but judging by 12 AWG wire they are 20 amps (all wiring in the circuit is 12 AWG, too, and the breaker is 20 amps). The thermostats I want to replace them with are rated 15 amps. There are total of 3 heaters (in 3 rooms, 1 thermostat per heater) in the circuit, each draw less than 10 amps and total draw should be around 14 amps. Can I use 15 amp thermostat in circuit with 20 amp breaker?

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You cannot compare this question to the allowance of a 15A receptacle on a 20A circuit. This is apples to oranges.

You can have as many 15A switches, thermostats, controls, etc, on a 20A circuit, provided the load each item is controlling is within its own rating.

So you can have a 15A thermostat, controlling a 10A load, on a 20A circuit.

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The short answer is yes. You can have multiple stats on one circuit. The code requires more than 1 you have 3 so this is up to code. This requires 12awg wiring that you have so you should be code compliant. The OP lives in Washington state. Oregon and Washington have similar limitations on residential circuits. The maximum breaker size on a general purpose branch circuit is 20A. 422.11.E. Single non motor appliance. (2) not exceed 20 amp for loads less than 13.3 amp. (3) Not exceed 150% of rated current. There is an exception for lighting circuits but I have not found it yet. This is why I said more than 1 was OK.

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    the OP did not ask about whether or not 3 thermostats was acceptable. He asked if it was acceptable to install a device rated at 15 amps on a circuit protected by a 20 amp breaker. Can you explain why this is OK? Who cares about wire size if the device itself heats up and catches on fire due to a failure. – Jimmy Fix-it Aug 24 '16 at 4:07
  • Jimmy code allows a 20 amp breaker with more than 1 15 amp device on the circuit. Since his state and mine have recripicial licensees, and code allows for a 20 amp breaker with 12awg AND more than 1 outlet or device it is legal and code compliant in both our states and many more.he did state 3 rooms with thermostats in ea room – Ed Beal Aug 24 '16 at 5:15
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    @EdBeal, if you are talking about 210.23, that part of the code is talking about receptacle rating where two or more outlets and/or receptacles share a circuit, not simply outlet rating. ............ Besides, a thermostat is a control, not an outlet. The heater would be considered the outlet. – Speedy Petey Aug 24 '16 at 11:39
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    @JimmyFix-it,the thermostat is a control. It's no different than being allowed to have a 15A switch on a 20A circuit, as long as the load it is controlling is within it's rating. – Speedy Petey Aug 24 '16 at 11:41
  • Can you cite the code section you're talking about? – Tester101 Aug 24 '16 at 12:35

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