7500W Heater Wire Sizing

[kwired]

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.

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?

 

[mbrooke]

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?

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:

https://library.industrialsolutions.abb.com/publibrary/checkout/DES-065?TNR=Time%20Current%20Curves%7CDES-065%7CPDF&filename=DES-065.pdf

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.

 

[mbrooke]

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?

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

 

[romex jockey]

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


View attachment 2554475

I can recall a number of FPE 'it went boom' incidents, emphasis on the boom part....~RJ~

 

[mbrooke]

I can recall a number of FPE 'it went boom' incidents, emphasis on the boom part....~RJ~

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.

 

[romex jockey]

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.

AIC was in da house!

~RJ~

 

[mbrooke]

AIC was in da house!

~RJ~

If those breakers could even interrupt what they claimed.

 

[xformer]

Guess the electrician could have ran #8 for the circuit and changed out the Ocpd as required.

 

[flashlight]

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.

 

[sw_ross]

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.

 

[don_resqcapt19]

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.