fixed heat calculation

[M. D.]

Lets' see , I've got 4250 watts of fixed heat divided by 240v = 17.7 amps x 1.25 for continuous load = 22.13 amps ,. Lets call it 23 amps . 12\2 nmb is good for 25 amps @ 60 deg. there will be no insulation my question is can I use a 20amp breaker on this circuit? it should never see more than 18. amps ? the wire is good for 25 .
Also I live in Massachusetts where we have this:

334.80 Ampacity. Type NM, NMC, and NMS cable shall have conductors rated at 90?C
(194?F).
Where installed in thermal insulation, the ampacity of conductors shall be that of 60?C
(140?F) conductors. The ampacity of Types NM, NMC, and NMS cable installed in cable tray
shall be determined in accordance with 392.11.

Again there will be no thermal insulation.
I will most likely run 2 circuits.
So I guess the question is. Can the over-current device be based on 18 amps(100%) while the conductor ampacity gets based on the full load plus 125% Sitting here,. without the good book,.. I'd say no . but I'm not sure why it would ever be a problem?

 

[jbelectric777]

MD, Your right on with your calcs and 1.25 CLF which most guys forget, but you'll need two 15 or 20 amp circuits or one 30 amp circuit. The #12 copper is rated for 25 amps but the oblisk at the bottom states that in no case shall the #12 NMB be protected at more than 20 amps for #12 and 15 amps for #14 , you can however do the whole circuit in #10 protecting it with a 30....... The 25 you see is used for de-rating the conductor for temperature and number of un-grounded (hot) conductors in a raceway or conduit.

 

[G._S._Ohm]

the oblisk at the bottom states that in no case shall the #12 NMB be protected at more than 20 amps

Please pardon my ignorance of the NEC.

Does the NEC explain where this limit comes from, the physics behind it or incidents in the past that make this a good idea?

If not, are they obligated to explain the origin of this rule to anyone?
I guess I'm asking who the NFPA answers to.

There was a rule change requiring GFCIs in car washes even though there has never been an incident in a car wash so I was wondering about the basis for some of the rules.

 

[G._S._Ohm]

Never mind. I wiki'ed it.

 

[mikeames]

Can you post the wiki link? I didnt know wki had NEC references.

Nevermind I see the reference.....I think. http://en.wikipedia.org/wiki/National_Electrical_Code#Actual_vs_maximum_current_rating

 

[G._S._Ohm]

No, this one
http://en.wikipedia.org/wiki/National_Fire_Protection_Association

I believe they are answerable to no one,
I know they have considerable power,
and I think it's likely they have a conflict of interest.

"I wish I didn't know now what I didn't know then."

 

[Dennis Alwon]

M.D. since this load is rated continuous and it is over 16 amps wouldn't you need a breaker rated for 100%. I see no reason you couldn't do that.

 

[tryinghard]

If the heater and breaker are listed to operate at 100% does exception 1 of 210.19(A)(1) apply?

 

[Smart $]

If the heater and breaker are listed to operate at 100% does exception 1 of 210.19(A)(1) apply?

I don't believe the heater is considered as part of the assembly... but I'd assume it to be rated for 100% operation, i.e. run continuously for 3 or more hours.

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.

 

[kwired]

M.D. since this load is rated continuous and it is over 16 amps wouldn't you need a breaker rated for 100%. I see no reason you couldn't do that.

If you have a breaker rated for 100% continuous loading you no longer need to multiply load by 125% to determine conductor ampacity. He then will be under 20 amps with minimum conductor ampacity with a 12AWG.

Good luck finding a breaker to use in a typical 120/240 single phase loadcenter that is rated for 100% continuous loading. Even if you do find one, it will likely cost less to run either the 30 amp circuit or multiple 15 or 20 amp circuits.

 

[Dennis Alwon]

If you have a breaker rated for 100% continuous loading you no longer need to multiply load by 125% to determine conductor ampacity. He then will be under 20 amps with minimum conductor ampacity with a 12AWG.

Good luck finding a breaker to use in a typical 120/240 single phase loadcenter that is rated for 100% continuous loading. Even if you do find one, it will likely cost less to run either the 30 amp circuit or multiple 15 or 20 amp circuits.

I was stating since his load is over 16 amps (80% of 20) the breaker needs to be rated 100%. I think we are saying the same thing.

Now let's go back to what M.D. is using-- he is using NM. In 2011 this point is moot as the ampacity of #12 @ 60C is now 20 amps not 25 amps.

 

[tryinghard]

If you have a breaker rated for 100% continuous loading you no longer need to multiply load by 125% to determine conductor ampacity...

kwired, can you cite the code that supports this please.

 

[Twoskinsoneman]

If you have a breaker rated for 100% continuous loading you no longer need to multiply load by 125% to determine conductor ampacity.

I don't believe this is the case. The conductors must still have the ampacity at 125% the continuous load

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Where
a branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.

 

[Dennis Alwon]

I don't believe this is the case. The conductors must still have the ampacity at 125% the continuous load

I think he is referring to article 210.19(A)(1) exception

 

[gotmud]

In my opinion he can't use 12nm because it's only rated for 20 amps unless it falls under the exceptions of 240.4D and according to chart 240.4 E or G, a heating load is not there....so no using it at 25amps....

 

[Smart $]

I was stating since his load is over 16 amps (80% of 20) the breaker needs to be rated 100%. I think we are saying the same thing.

Now let's go back to what M.D. is using-- he is using NM. In 2011 this point is moot as the ampacity of #12 @ 60C is now 20 amps not 25 amps.

Based on his OP, the 60?C "allowable" ampacity requirement has been omitted as an amendment in MA... thus allowing a maximum ampacity under the 75? table value of 25A.

 

[Dennis Alwon]

Based on his OP, the 60?C "allowable" ampacity requirement has been omitted as an amendment in MA... thus allowing a maximum ampacity under the 75? table value of 25A.

He also said that 12/2 is good for 25 amps at 60C. Unless MA has amended the table of 2011 then it is rated 20 amps. I agree at 75C 25 amps is good but I am confused by his statement.

 

[M. D.]

He also said that 12/2 is good for 25 amps at 60C. Unless MA has amended the table of 2011 then it is rated 20 amps. I agree at 75C 25 amps is good but I am confused by his statement.

My statement was based on ignorance I was unaware of the change....

 

[kwired]

kwired, can you cite the code that supports this please.

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

210.20(A) and exception

215.2(A)(1) and exceptions

215.3 and exception 1

230.42(A)

422.10(A)

424.82

430.22(A)

430.24 and exception 1

552.46(B)(3)

625.21

Notice there is no 310 sections there. It has nothing to do with anything in 310.

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

 

[kwired]

I don't believe this is the case. The conductors must still have the ampacity at 125% the continuous load

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Where
a branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.

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.