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

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mbrooke said:
The Canadian Electrical Code which is modeled after the NEC.


OP is asking a good question. In theory it is doable without any danger.
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And depending on the "cold start" temperature of the heat trace, the starting current for a heat trace circuit that would normally have a load of less than 20 amps may have a cold start temperature that exceeds 20 amps. However, under the rules in the NEC, the 12 AWG for this circuit cannot have an OCPD rating that exceeds 20 amps.
 
don_resqcapt19 said:
And depending on the "cold start" temperature of the heat trace, the starting current for a heat trace circuit that would normally have a load of less than 20 amps may have a cold start temperature that exceeds 20 amps. However, under the rules in the NEC, the 12 AWG for this circuit cannot have an OCPD rating that exceeds 20 amps.
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How long does the current last though?
 
gar said:
190509-2427 EDT

So far davepfr71 has not described the reason for his question.

A potential useful reference is
https://www.chromalox.com/-/media/files/training-manuals/en-us/tm-pj324-heat-trace.pdf
Starting current for self regulating tape can be 2 to 3 times running current for several minutes.

.
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As an analogous example, consider the amount of
gasoline required to accelerate from 0 MPH to 60 MPH and maintain speed at 60MPH.
The amount of fuel and energy required to accelerate is much greater than the amount
of fuel needed to simply maintain speed
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good one....

~RJ~
 
ActionDave said:
That was just a guess, but I know there is inrush on heat tape.
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Is on an incandescent lamp also, as soon as it begins heating it is changing resistance though. If we rated incandescent lamps on cold resistance we would need to run much larger circuits to them. I think it is fair to at least rate the VA of this cable per expected resistance after it has passed that short warm up period, maybe have to consider some minimum temp rating to account for how much surge current may occur in extreme cases.

mbrooke said:
If its for a few minutes, I would not be worried.
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I'd think majority of the resistance change on a cold start happens within a few seconds at the most.
 
190510-1120 EDT

kwired:

Read the Chromalox reference I provided. Suppose the starting current was 3 times steady state, then power is 9 times steady state, and this is about 9 times for the temperature rise. Further the duration can run several minutes.

These are probably worst case, and vary over the startup time, but not insignificant.

.
 
davepjr71 said:
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.
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The main purpose of the OCPD is to protect the conductors not the load. You cannot have an OCPD rated for more than what the conductors can handle. Inrush is the exception only because it is transient and only lasts for a few cycles. Most conductors can handle inrush with no issue. Sustained low overcurrent is the main driver of conductor insulation break down.

For example, if you have a 16A linear load fed with #12 and protected by a 25A OCPD (assuming 60 degree terminals). Theoretically the sustained current seen by the conductors will never be more than 16A right? What if someone adds more load over the years? What if the load fails and creates an overcurrent condition?

In short, you want the OCPD rating to be equal or less than what the conductors can handle. Otherwise the conductors will burn up and the breaker will never trip. Yes, it is a generalization but i think it is a good and safe strategy to adhere to, even if you don't like to blindly following orders.
 
jeremy.zinkofsky said:
The main purpose of the OCPD is to protect the conductors not the load. You cannot have an OCPD rated for more than what the conductors can handle.
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Its done all the time for motors and welders in the NEC. Heat loads in the CEC. Most fixed loads in BS7671.



Inrush is the exception only because it is transient and only lasts for a few cycles. Most conductors can handle inrush with no issue. Sustained low overcurrent is the main driver of conductor insulation break down.

For example, if you have a 16A linear load fed with #12 and protected by a 25A OCPD (assuming 60 degree terminals). Theoretically the sustained current seen by the conductors will never be more than 16A right? What if someone adds more load over the years? What if the load fails and creates an overcurrent condition?
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Its the job of the installer to make sure the circuit can handle the added load.


In short, you want the OCPD rating to be equal or less than what the conductors can handle. Otherwise the conductors will burn up and the breaker will never trip. Yes, it is a generalization but i think it is a good and safe strategy to adhere to, even if you don't like to blindly following orders.
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Depends on the load.
 
gar said:
190510-1120 EDT

kwired:

Read the Chromalox reference I provided. Suppose the starting current was 3 times steady state, then power is 9 times steady state, and this is about 9 times for the temperature rise. Further the duration can run several minutes.

These are probably worst case, and vary over the startup time, but not insignificant.

.
Click to expand...
I don't know what is all out there in the world for heavy duty heat trace, but the lighter heat trace I have commonly encountered I wouldn't think would handle that kind of current for several minutes. I also don't think it would take several minutes to warm it enough for resistance to start to noticeably increase. If designed for maximum thermal transfer ability and you had it installed firmly against a very cold pipe - maybe. They have to have electrical insulation to prevent undesired current flow and that usually reduces thermal conductivity to some extent, so they are going to retain some heat instead of transferring it really easily.
 
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