7500W Heater Wire Sizing

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mbrooke said:
Agree. But 31 amps is pushing it, 125% would be a better option. And meet Code in the CEC.

Though yes, breaker won't trip in reality at 31 amps :)

However modern breaker trip curves are getting closer and closer to their handle rating.
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Have they changed at all (particularly trip curve) in like 30-50 years for typical miniature beakers that plug into load centers, mostly talking QO, CH, BR, QP, or THQL lines? Trip curve for FPE or Zinsco is a straight horizontal line isn't it? :)

Also isn't the generally published trip curve for a 40C ambient at the breaker location, but would change with a different ambient?
 
kwired said:
Have they changed at all (particularly trip curve) in like 30-50 years for typical miniature beakers that plug into load centers, mostly talking QO, CH, BR, QP, or THQL lines? Trip curve for FPE or Zinsco is a straight horizontal line isn't it? :)

Also isn't the generally published trip curve for a 40C ambient at the breaker location, but would change with a different ambient?
Click to expand...


Both the thermal and magnetic trip components have changed dramatically in the last 30-50 years and are still changing each day.

The latest batch of THQB bolt on breakers, 5-7 magnetic pickup with a steeper thermal curve starting closer to the handle rating:


Correct 40*C ambient, but many manufacturers are now including higher and lower temps in their trip curve data.

IIRC- breakers are ambient compensated so a change in temperature doesn't produce as much of a shift as it would otherwise.
 
kwired said:
Have they changed at all (particularly trip curve) in like 30-50 years for typical miniature beakers that plug into load centers, mostly talking QO, CH, BR, QP, or THQL lines? Trip curve for FPE or Zinsco is a straight horizontal line isn't it? :)

Also isn't the generally published trip curve for a 40C ambient at the breaker location, but would change with a different ambient?
Click to expand...


Here is FPE's trip curve. They wish it started around 105-125%...


1606968468358.png
 
romex jockey said:
I can recall a number of FPE 'it went boom' incidents, emphasis on the boom part....~RJ~
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Well, you had busbars that melted down shorting out to the can, fake 100 amp breakers (they just used the 125 amp mech for a number of breakers), time current curves that often started well over 135%, no mag trip and jamming of 2 pole breakers.
 
mbrooke said:
Well, you had busbars that melted down shorting out to the can, fake 100 amp breakers (they just used the 125 amp mech for a number of breakers), time current curves that often started well over 135%, no mag trip and jamming of 2 pole breakers.
Click to expand...
AIC was in da house!
:eek:
~RJ~
 
I just had similar situation with 4000W heater to be installed on existing #12 wire. Would have worked just fine, I'm sure,
but had to be upgraded to #10.
 
How did the size (7200 watts) get decided?
Obviously there was a lack of communication somewhere.

Definitely gives me ideas about stating in a bid for “garage heater circuit” what size circuit is included and what size heater should be installed.

My standard thought for a garage heater is 5kW. If it wasn’t in writing I would normally run #10’s.
Lesson learned.
 
A 30 amp breaker is required to carry 30 amps forever in a 40°C ambient...it is unlikely that the breaker is in a 40°C ambient so the trip curve shifts to the right. For one brand of breaker, it shifts ~25% when the breaker is in a 50°F ambient. I would doubt that the breaker would ever trip at 31 amps, even where that is a continuous load.
 
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