fixed heat calculation

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It would seem that both the overcurrent device and the wire ampacity capability have to be 125% so 23 amps +- .. in Mass I do believe we are allowed to use nm-b @ 75c as long as it is not run in insulation. so even though the ampacity meets the conductor requirement I don't seem to be able to use a 25amp breaker to meet the overcurrent protection requirement .. does that make any sense??
 
M. D. said:
It would seem that both the overcurrent device and the wire ampacity capability have to be 125% so 23 amps +- .. in Mass I do believe we are allowed to use nm-b @ 75c as long as it is not run in insulation. so even though the ampacity meets the conductor requirement I don't seem to be able to use a 25amp breaker to meet the overcurrent protection requirement .. does that make any sense??
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Sort of makes sense except I think you could have a 25 amp breaker in this case just not a 20 amp breaker. 23 amps already has the 125% in it so a 25 amp ocpd should suffice.
 
Dennis Alwon said:
Sort of makes sense except I think you could have a 25 amp breaker in this case just not a 20 amp breaker. 23 amps already has the 125% in it so a 25 amp ocpd should suffice.
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If MA hasn't amended 240.4(D)(5), he's still stuck with not more than a 20A breaker on a 12/2 nmb circuit.

That said, an 18A continuous load on a 20A 100%-rated assembly using 12/2 nmb copper (not run through insulation) is compliant... in MA (AFAICT, with emphasis squared :cool: )
 
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Smart $ said:
If MA hasn't amended 240.4(D)(5), he's still stuck with not more than a 20A breaker on a 12/2 nmb circuit.

That said, an 18A continuous load on a 20A 100%-rated assembly using 12/2 nmb copper (not run through insulation) is compliant... in MA (AFAICT, with emphasis squared :cool: )
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He did say that MA can use the 75C col. for NM where there is no insulation.
 
Smart $ said:
If MA hasn't amended 240.4(D)(5), he's still stuck with not more than a 20A breaker on a 12/2 nmb circuit.

That said, an 18A continuous load on a 20A 100%-rated assembly using 12/2 nmb copper (not run through insulation) is compliant... in MA (AFAICT, with emphasis squared :cool: )
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And they have not so no 25amp breaker for me and no 100% rated one either.

what is AFAICT ???
 
Dennis Alwon said:
He did say that MA can use the 75C col. for NM where there is no insulation.
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It,s not so much that we can use 75c it is the next weak link ,.. it says that they shall have a temp rating of 90c but then we have 110.14 limitations to deal with so for all intents and purposes it becomes 75c when not installed in insulation . I guess the Ma. code comes in handy for some applications when using NM-b ,. just not this one.
 
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Forgive me for not having the code book by my side while enjoying this discussion, so I may be very wrong in what I am about to say, but it seems to me that the branch circuit is required to be calculated at 125% for the continuous load, however, the OCPD does not as it is serving to protect against over current draw, and short circuits. The heater should never draw more than the its rating. The concern is the constant draw on the conductor, thus requiring it to be increased in size. Again, I do not have the 'bible' at my side so I am open to being corrected.
 
Mr 3phase said:
Forgive me for not having the code book by my side while enjoying this discussion, so I may be very wrong in what I am about to say, but it seems to me that the branch circuit is required to be calculated at 125% for the continuous load, however, the OCPD does not as it is serving to protect against over current draw, and short circuits.....
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it is because it is considered a continuous load this is from an EC&M article written by Mike Holt in 2008

If you are sizing branch-circuit conductors for fixed electric space-heating equipment, you must consider these to be a continuous load [424.3 (B)]. The branch-circuit conductors and overcurrent devices for fixed electric space-heating equipment must have an ampacity not less than 125% of the total heating load [210.19(A)(1) and 210.20(A)].
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240.4 (D) will require a 12AWG copper to be protected by no more than a 20 amp OCD. Fixed resistance heating is not one of the exceptions to this rule.

210.20 will require branch circuit overcurrent device to be not less than 125% of continuous load.

Load in the OP was 17.7 amps X 1.25 = 22.13.

The overcurrent device must be larger than 22.13 so next size is 25 amps.

The smallest conductor allowed to be protected by 25 amps according to 240.4(D) is a 10 AWG - it does not matter if it is THHN/THWN or NM cable it can not be smaller than 10. It could possibly need to be derated to require larger than 10 but in no case can it be smaller than 10.
 
M. D. said:
And they have not so no 25amp breaker for me and no 100% rated one either.

what is AFAICT ???
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100% rated is out due to economics... perhaps other limitations... but otherwise would be compliant within an assembly.

AFAICT = as far as I can tell
 
kwired said:
Did you read the exceptions to this section? They apply directly to what you are disagreeing with and are there for 100 percent rated devices only.

The exceptions do not apply most of the time but understanding why they are there makes the whole picture more clear.


Add: Exception 2 is for grounded conductors not connected to an overcurrent device.

The overcurrent device is the reason for 125% not the conductor.
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yeah I see that now. Thanks
 
Smart $ said:
?Typically the "assembly" is the 100%-rated breaker, it being installed in a listed for the purpose enclosure or a proerly sized enclosure (perhaps required with ventilation also), the conductor insulation rated 90?C or better, and the conductor ampacity determined by the 75?C column of the applicable table. Having met all these conditions, yes, the exception kicks in.
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kwired said:
I will give you all 11 that I found in 2008 NEC.

210.19(A)(1) and exceptions - many of the others have a reference back to this one?

The reason for increasing ampacity by 125% with a continuous load is not for the ability of the conductor to carry the current, it is because standard thermal type overcurrent devices depend on the first few inches of conductor as a heat sink to draw heat away from the device. A 100% rated device does not have the need to use the conductor as a heat sink. That is the reason for the wording in most of the sections mentioned or the exceptions
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So really M.D's application is not an assembly therefore the exceptions are out; correct (understanding the continuous load of course)?
 
tryinghard said:
So really M.D's application is not an assembly therefore the exceptions are out; correct (understanding the continuous load of course)?
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Typical thermal magnetic circuit breakers are not 100% rated. And the majority of the time you do not need to even think about it just size the conductor to 125%.

I think the most common devices you will find that are 100% rated will be large frame breakers and likely will have electronic current sensing devices instead of thermal heating devices directly in line with the circuit. You probably will not find any in typical miniature breaker types used in 240 volt load centers.
 
kwired said:
Typical thermal magnetic circuit breakers are not 100% rated. And the majority of the time you do not need to even think about it just size the conductor to 125%.

I think the most common devices you will find that are 100% rated will be large frame breakers and likely will have electronic current sensing devices instead of thermal heating devices directly in line with the circuit. You probably will not find any in typical miniature breaker types used in 240 volt load centers.
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kwire, I understand all this it's what qualifies as an assembly that I was questioning; a building wiring circuit is just not an assembly. :)
 
tryinghard said:
kwire, I understand all this it's what qualifies as an assembly that I was questioning; a building wiring circuit is just not an assembly. :)
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The assembly consists of the 100%-rated breaker, an enclosure which meets the breaker listing for 100%-rated operation, and minimum 90?C-rated circuit conductors with ampacity rating capped at 75?C. The load(s) are not part of the assembly AFAIK.
 
Eric R 1973 said:
Did no one think to look at Art 424
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I did when I wrote post #30. The only thing I found that applies to this thread is 424.3(B) which states that fixed electric space heating shall be considered continuous load. I never specifically mentioned that section though.
 
kwired said:
I did when I wrote post #30. The only thing I found that applies to this thread is 424.3(B) which states that fixed electric space heating shall be considered continuous load. I never specifically mentioned that section though.
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The reason i ask is Table 240.3 lists Fixed electric space heating equipment with Art 424 so doesn't that mean that Art 240.4D doesn't apply here since fixed electric space heating is listed as specific equipment. Someone cited this article in reference to #12 awg conductors and not exceeding 20a O.C.P was the reason I asked.
 
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