Thermostat Terminal Identification

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The National Electrical Manufacturers Association is at present attempting to standardize thermostat markings in order to aid the installer in wiring and servicing. Thermostat terminals have been given standard identification letters that specify the function of the terminal. These identification letters are also matched, in most cases, with the color-coding of the wire. A partial list of equivalent terminal markings is given in Table 4-1.

Thermostat installation literature will generally contain at least one internal view and/or wiring diagram in which the various terminals are identified by a specific letter.

table 4 1 Thermostat Terminal Identification

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  1. Hi, my name is Barb and I’ve been up allll night trying to figure out how to remove the unmarked wiring in my very old ITT General Controls thermostat, which all seem to be soldered onto the two mercury bulby thingys After HOURS of research, finding this site has restored some hope in my belief that i CAN remove the old ITT, and replace it with my new Honeywell RTH221b Thank you so much for th Table 4-1 Common Terminal I.D. Im going to snip off the wires at their soldering points, and just go for it, using your letter/color chart …If my trailer blows up, i won’t blame you..if it doesn’t, and my installation is successful, i will send you some of my yummy choc/toffee cookies! Wish me luck!

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