# 25 amps on #10 THHN



## Almost Retired (Sep 14, 2021)

Spark Master said:


> #10 THHN, and then Romex for the final device run. continuous load for a 240v 6000watt heater. 25 amps all day.
> 30amp circuit at 80% is 24 amps max for continuous load.
> 
> Would you do it with 25 amps? 1 amp over the 80% 24amp rating .....


when you say all day, are you talking truly continuous ?
if so ... i would go up to 8 at least and a 40A breaker
the heat from the elements will heat up the wire without any current draw considered
the element connections will be the most trouble prone spot in the circuit

do you have 75*C terminations and breaker ? that will matter as well
if you have stud and bolt connections on the heater, i would get some mechanical/set screw lugs and bolt them on

im guessing this is for a hot tub or similar, with continuous circulation
on the other hand if this a storage tank WH with dual elements and two thermostats
just like a normal resi kind
it is likely that only one element runs at a time


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## Spark Master (Jul 3, 2012)

6000 watt infrared heater. It will run 8 hours in my garage if I'm out there all day.


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## Almost Retired (Sep 14, 2021)

Spark Master said:


> 6000 watt infrared heater. It will run 8 hours in my garage if I'm out there all day.


well i dont know how i got water in there ..........
but still #8 on a 40


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## samgregger (Jan 23, 2013)

Why would you want to waste a portion of you power heating up the THWN vs actually heating up the heater? I agree #8


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## samgregger (Jan 23, 2013)

Also 6Kw running all day long? Must be nice to be rich. Buy a used salamander heater off Craigslist and it will heat the entire garage for a fraction of the price.


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## u2slow (Jan 2, 2014)

What's the voltage-drop at the heater on load? Are you actually pulling 25A? 

In my own shop, I'd see if a 30A holds, if not, try a 35A. If it's a short run getting full voltage, then #8 is a short length to upgrade to.


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## SWDweller (Dec 9, 2020)

Why the race to the minimum allowed? Do you take this attitude with everything you do?


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## Spark Master (Jul 3, 2012)

#10 THHN is rated for 40 amps. It's the NEC that limits it to 30amps.


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## emtnut (Mar 1, 2015)

Spark Master said:


> #10 THHN is rated for 40 amps. It's the NEC that limits it to 30amps.


No it isn't


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## Spark Master (Jul 3, 2012)

emtnut said:


> No it isn't


90 degree #10 THHN is rated for 40 amps. I use the Cerro ampacity chart.


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## Almost Retired (Sep 14, 2021)

Spark Master said:


> #10 THHN is rated for 40 amps. It's the NEC that limits it to 30amps.


let me finish that statement for you ...
#10 is rated for 40amps *@90*C*
the breaker and termination temp ratings limit it to less
the NEC justs lists all ratings
the point being that the wire will heat up at the higher temp ratings and transfer that heat to the terminations
this can cause the breaker to trip prematurely (it has a temp sensing trip mechanism in it ... bimetallic strip)

as i mentioned earlier .... what are your terminations rated for ???
if 75, not 90, then you cant use 40amps

on the other hand its your heater ,, treat it like it belongs to you


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## backstay (Feb 3, 2011)

samgregger said:


> Also 6Kw running all day long? Must be nice to be rich. Buy a used salamander heater off Craigslist and it will heat the entire garage for a fraction of the price.


Just under $6 for 8 hours @.12 per kW.


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## Spark Master (Jul 3, 2012)

You guys like propane combustion sucking the O2 out of your air. I do not. I'll pay the $10 per day.


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## Almost Retired (Sep 14, 2021)

mine is 17 cents per these days


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## emtnut (Mar 1, 2015)

Spark Master said:


> 90 degree #10 THHN is rated for 40 amps. I use the Cerro ampacity chart.


Are you an Electrician ?

SMH


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## Kevin (Feb 14, 2017)

emtnut said:


> Are you an Electrician ?
> 
> SMH


A real electician wouldn't ask if too small of wire is AOK to use...


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## kb1jb1 (Nov 11, 2017)

Check my math and code usage.
6000 watts X 125% is 7500 watts
7500 watts ÷ 240 volts is 31.25 amps. The next size breaker is 40 amps and #10 is limited to 30 amps in most applications.


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## kb1jb1 (Nov 11, 2017)

6000 watts ÷ 240 volts = 25 amps
25 amps X 125% = 31.25 amps.
#8 to be code compliant


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## paulengr (Oct 8, 2017)

NEC uses calculations done by Neher-McGrath. There is NO “fudge factor”.

Most terminals are rated for 75 C so if you use the 90 C rating the terminals will overheat. You can never get terminal resistance down to equal cable resistance (Google alpha spots contact resistance). So the choice is between making larger lugs that cool better (90 C) o using larger cables (75 C rating). Cerrowire gives 40 A at 90 C, 35 A at 75 C. When we say “continuous” this means we test for 3 hours. After 3 hours temperatures can be higher.

Also breakers are affected by heat from other breakers in the panel and as stated we use “next size up” so we have to account for this somewhere. The exact “fudge factor” may not be 125% but it has been in place for 50 years without failures. I’ve seen lots of very marginal situations where industrial plant operators adjust things to run at name plate maximums. Those NEC numbers are spot on. If anything at all goes wrong the insulation gets brittle and falls off.


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## Spark Master (Jul 3, 2012)

emtnut said:


> Are you an Electrician ? SMH


Is electrician a new pronoun ???


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## kb1jb1 (Nov 11, 2017)

paulengr said:


> NEC uses calculations done by Neher-McGrath. There is NO “fudge factor”.
> 
> Most terminals are rated for 75 C so if you use the 90 C rating the terminals will overheat. You can never get terminal resistance down to equal cable resistance (Google alpha spots contact resistance). So the choice is between making larger lugs that cool better (90 C) o using larger cables (75 C rating). Cerrowire gives 40 A at 90 C, 35 A at 75 C. When we say “continuous” this means we test for 3 hours. After 3 hours temperatures can be higher.
> 
> Also breakers are affected by heat from other breakers in the panel and as stated we use “next size up” so we have to account for this somewhere. The exact “fudge factor” may not be 125% but it has been in place for 50 years without failures. I’ve seen lots of very marginal situations where industrial plant operators adjust things to run at name plate maximums. Those NEC numbers are spot on. If anything at all goes wrong the insulation gets brittle and falls off.


Part of your post mentioned temperatures can get higher if more than 3 hours. That was a point I was trying to make on a thread about the EV wall connectors. Everything is monitored after the wall thing but the only thing before the wall connector unit is a circuit breaker. How much extra heat is generated for drawing 48 amps for 8 hours continuously? Maybe rating cables 125% is not enough and those small terminals should be made Bigger.


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## joe-nwt (Mar 28, 2019)

paulengr said:


> Most terminals are _tested_ for 75 C so if you use the 90 C rating the terminals _might_ overheat.


FIFY.

But yes, #8 on a 40A.


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## Veteran Sparky (Apr 21, 2021)

samgregger said:


> Also 6Kw running all day long? Must be nice to be rich. Buy a used salamander heater off Craigslist and it will heat the entire garage for a fraction of the price.


Infrared heats up objects...i.e. human, vs. heating up the entire garage.


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## samgregger (Jan 23, 2013)

Veteran Sparky said:


> Infrared heats up objects...i.e. human, vs. heating up the entire garage.


Yes I know, what are you going to do - wire it up to a motion tracking robot arm so it is always pointing at you when you move about your garage?


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