25A fuse on a #12 for a dedicated single known load of under 20A.

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davepjr71

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I am trying to find out if I can use a 25A fuse to protect a #12 conductor for a single heat tape load. The load on the circuit is under 20A. I understand the code states that a 20A fuse is required for a #12. However, I know there are some exceptions like for AC systems. Is there anything that may apply in this situation?
 
Is that fuse the primary ocpd?
I not aware of a NEC allowance for such.
How close to 20A is the actual load?
 
FionaZuppa said:
Is that fuse the primary ocpd?
I not aware of a NEC allowance for such.
How close to 20A is the actual load?
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Yes it is the primary. There is a main breaker (50A) in a panel with a #6 conductor that is then split up on a terminal strip to feed the different heating cables with each cable having it's own fuse. The fuse is located within 20' of the heating cable. This is not located inside a building and is located on a rail transit system. Thank you.
 
I don't know of any rule that would allow what you are describing. But I need to ask: why would you want to? Why not use a 20 amp fuse, and eliminate all questions?
 
charlie b said:
I don't know of any rule that would allow what you are describing. But I need to ask: why would you want to? Why not use a 20 amp fuse, and eliminate all questions?
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Because I'm not the type of person that always just rolls with what I'm told blindly. I fully understand why the ratings were implemented for Facilities. However, even the code has variations for conductor sizes and protective circuit ratings out side of your normal outlet/lighting loads. The code would never change if we always went with what is always done or said.

My area of electrical engineering is outside of the norm and there are always questions popping up that are gray due to the installation.
 
It seems pretty clear. 240.4(D)(5) limits the OCP to 20 amps unless otherwise allowed. HVAC gets its break in that it is listed in 240.4(G) but there does not appear to be any such except for heat tape applications.
 
augie47 said:
It seems pretty clear. 240.4(D)(5) limits the OCP to 20 amps unless otherwise allowed. HVAC gets its break in that it is listed in 240.4(G) but there does not appear to be any such except for heat tape applications.
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And it's the "otherwise allowed" that makes it not so clear. I've always followed the 20A rule. I have a unique installation and wondering if unique installations have been dealt with by anyone on here.

The #12 will be "SO" Cord if that makes any difference?
 
Flexible cord likely would have an ampacity of 25 amps per 400.5 but taking 240.5(B) into account as well as the limitations in 400.8m {'14 Code} it's difficult to visualize a situation where that could be applied on a branch circuit.
 
I think the fundamental question has already been asked, will 20A fuse work?

Will a 20A fuse work ok? If yes, then install a 20A fuse. If not, then why is it an issue? Do heat tapes have any inrush amps to worry about?
 
growler said:
Would this even by covered by the NEC ? 90.2 (B) (3)
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I think that it is covered, and I think that article does not apply. This circuit does not appear to have a role in the "operation of rolling stock" and is not related to signaling or communications.

 
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190509-0737 EDT

davepjr71:

The NEC is a rule based system. To an extent it is a system designed to have the smallest number of rules to cover the greatest number of different conditions. However, the NEC has grown to be huge. The rules are based on theory, assumptions, experiments (testing), judgements, and arguments between experts in the field.

You have a load that is under 20 A, it is probably close to a linear resistance, and will have resistance that is temperature sensitive, and probably a positive coefficient. What is wrong with using a 20 A fuse? If you think there are short surges, then a slow blow fuse. The general design of a fuse or breaker is that it will carry rated current continuously at normal ambient temperature conditions. If in an overheated area it may not. If in a cold environment it will carry more. The NEC may in effect change the rating of the protective device by setting a criteria of maximum load being some percentage of the protector's rating. An added safety factor.

In reality a copper wire in free space (air at standard temperature and pressure) with no insulation will have a higher current capability than the same wire with plastic insulation tightly packed with other current carrying conductors in a PVC tube. This is all a thermal and materials problem.

You need to make it clear what your real question is. More specifics.

.
 
190509-0854 EDT

davepjr71:

To add to my previous post consider the approximation ---

The temperature rise of a resistance is approximately proportional to the power dissipation in the resistor.

If you go from a 20 A load to a 25 A load, then this is a current ratio of 25/20 = 1.25, and a power ratio of 1.25^2 = 1.56. Thus, temperature rise is about a factor of 1.6 or a 60% increase.

If an insulation has a long life, 50 to 100 years, at its normal rating, then operating at a considerable increase in that temperature can greatly shorten its life to possibly minutes or hours.

Insulation failure occurs at some absolute temperature. Thermoplastics melt at some temperature and soften before this. Failure is not based on temperature rise, but on temperature rise from some absolute temperature. So ambient temperature and thermal resistance to that ambient determines the absolute temperature of the insulation.

We need to hear why you want a 25 A fuse instead of a 20 A.

.
 
davepjr71 said:
I am trying to find out if I can use a 25A fuse to protect a #12 conductor for a single heat tape load. The load on the circuit is under 20A. I understand the code states that a 20A fuse is required for a #12. However, I know there are some exceptions like for AC systems. Is there anything that may apply in this situation?
Click to expand...
If the load is under 20 amps there is no need for 25 amp breaker. Keep in mind minimum ampacity will be 125% of continuous load, so if the load is continuous 18 amps, your conductor size will be based on 22.5 amps.

Most the items that are permitted to be higher OCPD have starting currents that may need higher settings, they often still have additional overload protection that still protects the conductor from overloading.
 
charlie b said:
I don't know of any rule that would allow what you are describing. But I need to ask: why would you want to? Why not use a 20 amp fuse, and eliminate all questions?
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Take a look at 62-114 (6)


(6) Where a service, feeder, or branch circuit is used solely for the supply of energy to heating equipment, the
load, as determined using Rule 62-116, shall not exceed

(a) 100% of the rating or setting of the overcurrent devices protecting the service conductors, feeder conductors, or branch circuit conductors when the fused switch or circuit breaker is marked for
continuous operation at 100% of the ampere rating of its overcurrent devices; or

(b) 80% of the rating or setting of the overcurrent devices protecting the service conductors, feeder
conductors, or branch circuit conductors when the fused switch or circuit breaker is marked for
continuous operation at 80% of the ampere rating of its overcurrent devices.

(7) Service, feeder, or branch conductors supplying only fixed resistance heating loads shall be permitted to
have an ampacity less than the rating or setting of the circuit overcurrent protection, provided that their
ampacity is

(a) not less than the load; and
(b) at least 80% of the rating or setting of the circuit overcurrent protection.

(8) Notwithstanding Subrule (7)(b), where 125% of the allowable ampacity of a conductor does not
correspond to a standard rating of the overcurrent device, the next higher standard rating shall be
permitted.
Click to expand...
 
Knuckle Dragger said:
At a quick glance some people could get confused looking at table 310-16.
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Maybe so, but what OP is suggesting involves more than just the table.

First is determining minimum ampacity needed, for a continuous load, that means 125% of actual load. Then we have the small conductor rule that comes along even though OP's 12 AWG is good for more than 20 amps in the table. Then there is overcurrent protection rules. On top of that there is still situations that change these general rules at times.
 
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