Wall heater

What size conductors?

424.4 Branch Circuits.
424.4(A) Branch-Circuit Requirements.
An individual branch circuit shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which the branch circuit is rated. Branch circuits supplying two or more outlets for fixed electric space-heating equipment shall be rated not over 30 amperes. In other than a dwelling unit, fixed infrared heating equipment shall be permitted to be supplied from branch circuits rated not
over 50 amperes.
424.4(B) Branch-Circuit Sizing.
The branch-circuit conductors for fixed electric space-heating equipment and any associated motors shall be sized not smaller than 125 percent of the load.
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Unless there is a rule that allows you to truncate the digits after the decimal point the conductors are too small given the 125% requirement.
 
Seems to me that if the heater is supplied by a 75C wiring method, and the heater and breaker terminations are rated 75C, then the ampacity of #12 Cu is 25A. 240.4(D) still requires protecting it at 20A, but that doesn't change the ampacity. So in this scenario, 424.4(B) is satisfied. And as 424 doesn't seem to require a 125% factor on the OCPD, a 20A OCPD is compliant.

Of course, if the heater terminations are only rated 60C, then #12 Cu is too small.

hhsting said:
Would it be okay if conductors are changes to #10 awg but breaker remains 20A?
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Certainly, #10 Cu has an ampacity of at least 30A, so it complies with 424.4(B).

Cheers, Wayne
 
hhsting said:
Would it be okay if conductors are changes to #10 awg but breaker remains 20A?
Click to expand...
What do you think?

424.4(A) Branch-Circuit Requirements.
An individual branch circuit shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which the branch circuit is rated
Click to expand...
 
wwhitney said:
Seems to me that if the heater is supplied by a 75C wiring method, and the heater and breaker terminations are rated 75C, then the ampacity of #12 Cu is 25A.
Click to expand...
I think that you're on to something. The conductor size would not be reduced to 20 amps under article 240 since that mentions only the OCPD size.
 

Article 424 Fixed Electric Space-Heating Equipment​

424.4 Branch Circuits.
(A) Branch-Circuit Requirements.
An individual branch circuit shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which the branch circuit is rated.
(B) Branch-Circuit Conductor Sizing.
The branch-circuit conductor(s) ampacity shall not be less than 125 percent of the load of the fixed electric space-heating equipment and any associated motor(s).

The FLA of the heater OP asking about at 16.7 is good on the 20A breaker, there is no real starting loads to accommodate for that would need a larger breaker. But the wire must be sized to 125% of the FLA thus as mentioned the #12 at 25A is more than enough per the above referenced codes unless using NM then the 20.875A is over the rating of the #12 NM.

But in reality how long would the load have to be on for the extra continuous load of .875A to heat up the conductor to a critical point. See those small forced air heaters connected to #12 NM all the time. When sized correctly for the space never seen them operating more than 20 or 30 minutes continuously.

Typically see these wall heaters having a pigtail wire connection not a terminal, They are using high temperature wire around 250oC so a terminal derating at the heater is technically a non issue, and most breakers are now listed to 75oC terminals so worse case for #12 THHN or MC is limited to 25A, so limitation for temperature/amperage is just the branch wiring method.
 
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