ampacities of branch ckts?

[thetacon]

I am trying to understand the ampacities of 15 and 20 amp ckts in 'romex'. I have had more than one boss tell me that i could not put more than 16 amps on a 20 amp circuit . the logic being 20x .80% derating for continuous duty equals 16 amps.

When i calculate it i use art 334.8 which allows me to derate using the 90 degree column of table; 310.16 unless it end up being higher than the 60 degree column which i must than use it for derating purposes.
table 310.16 puts 12 awg at 25 amps in the 60 degree column .
25 x .80% = 20 amps.

i know that 240.4(d) requires a 20 amp breaker for 12 awg. and as far as i know you do not derate breakers

so is there a step that i am missing to get down to 16 amps or have i had some mistaken bosses, and the allowable ampacity is 20 amps

thanks

 

[stickboy1375]

You are allowed to use 100% of a breaker... It is a design issue not code issue...

 

[infinity]

The 80% is for a continuous load. That doesn't mean that a multi outlet circuit can't be loaded to a full 20 amps. Look at 210.19(A)(1) and (2).

 

[celtic]

Is an outlet in a dwelling unit ever considered "continuous"?

 

[Jomaul]

look at table 210.24. Your Bosses may be refering to Table 210.21 (B) (2), which shows the recpt being derated by 80%. But that is for the load on the recpt. and not the the ckrt. suppling the recpt.

 

[haskindm]

This is a common oversimpification of the NEC. Many people say that no circuit may be loaded to more than 80% of its rated capacity. This is NOT in the NEC. A CONTINUOUS LOAD may not exceed 80% of the circuits capacity (ampacity). If the load is not expected to last 3-hour or more, it is not continuous and the circuit may be loaded 100%. Also keep in mind the restrictions of 210.23 and 210.24 which prohibits the rating of any ONE cord & plug connected piece of equipment shall not exceed 80% of the branch circuit ampere rating. The NEC is not simple and any attempt to make it simple often leads to confusion.

 

[thetacon]

thanks all of the above links clear up the confusion. for now any way....

 

[LarryFine]

thanks all of the above links clear up the confusion. for now any way....

Links? :-?

 

[sparky59]

so...a hard wired continous load of 20 amps would be ok on a 20 amp breaker and #12 wire?

 

[haskindm]

No, a continuous load must be calculated at 125%. A 20-amp continuous load would be calcuated as 25-amps, which would require a 25-amp breaker and #10 romex (NM cable).

 

[Sass5150]

1) No. Continuos load of 16 amps on a 20 amp. circuit.
2)There is nothing in a dwelling unit that would be classified as a continuos load. I debated that with an engineer/instructor. Take for example a base board heater. I know and you know it will be on all night long but it is "Thermatically controlled" and therefore not considered continuos.Same theory for other stuff you can think of.

 

[Dennis Alwon]

so...a hard wired continous load of 20 amps would be ok on a 20 amp breaker and #12 wire?

The 80% is not a derating factor of the 90C column of #12 but rather an 80% reduction of the circuit breaker. 20 amp breaker means 16 amp load.

 

[steve66]

1) No. Continuos load of 16 amps on a 20 amp. circuit.
2)There is nothing in a dwelling unit that would be classified as a continuos load. I debated that with an engineer/instructor. Take for example a base board heater. I know and you know it will be on all night long but it is "Thermatically controlled" and therefore not considered continuos.Same theory for other stuff you can think of.

A baseboard heater would be a continuous load, thermostat or not because it can be expected to run all night. 424.3(B).

Steve

 

[steve66]

so...a hard wired continous load of 20 amps would be ok on a 20 amp breaker and #12 wire?

No, but a hard wired non-continuous load of 20 amps would be OK.

Steve

 

[haskindm]

A thermostatically controlled baseboard heater is NOT a continuous load because it will NOT usually run all night. The thermostat will cycle the unit on and off. HOWEVER 424.3(B) requires that the conductors and overcurrent devices for electric space heating be sized at 125% of the total load. So same difference.

 

[Sass5150]

I had to mention baseboard heaters didn't I...Thanks for the code article Steve as well as the input from the others.
...Bob

 

[russ__20000]

No, a continuous load must be calculated at 125%. A 20-amp continuous load would be calcuated as 25-amps, which would require a 25-amp breaker and #10 romex (NM cable).

What if that ckt was to a motor..Wouldn't 12AWG, NM cable be ok,per 240.4(d)(g)? and the breaker could be as high as 50AMP?

 

[Dennis Alwon]

What if that ckt was to a motor..Wouldn't 12AWG, NM cable be ok,per 240.4(d)(g)? and the breaker could be as high as 50AMP?

Sure but I think they are talking about the general rule not the exceptions to the rule. A/C would be the same as well as others mentioned in Table 240.4(G)..

 

[russ__20000]

Sure but I think they are talking about the general rule not the exceptions to the rule. A/C would be the same as well as others mentioned in Table 240.4(G)..

Thanks...cause the more i read the more i don't feel like i know anything

 

[Dennis Alwon]

Thanks...cause the more i read the more i don't feel like i know anything

Here -- stare at this for awhile. It will make you feel better

By the way this is not an animation but an optical illusion​