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

[FionaZuppa]

Most conductors can handle inrush with no issue.

A bare copper wire about thick as human hair, held in liquid nitrogen under pressure, can handle thousands of continuous amps, just takes a very long time for the liquid to vaporize and copper heats and melts.

Using 125% rule does not make much sense if the main idea is that the OCPD should not be any bigger than the rated ampacity of the wire. A motor that has inrush of 28A but runs full load at say just 16A can perhaps still pose an issue, like you run #12 wire and use a 25A ocpd, what happens when the load goes south and begins to run 21A after startup? OCPD does not care, but the #12 does (using NEC rules, etc).

 

[kwired]

190510-1258 EDT

kwired:

Read the Chromalox reference.

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

Start at page 15 for a general discussion, then jump to page 38 thru 40 and in particular the comment on example 4.3 on page 40.

.

What exactly is "running current"? Doesn't amount of current depend on temperature which changes the resistance? If anything it should have a rating at a specific temperature. If you are intending to use it at a low extreme, you maybe need to make adjustments or use some control method that will limit current during "warm up"?

I know, you are probably going to tell me to read the reference you linked to.:ashamed1:

 

[gar]

190510-2344 EDT

kwired:

The heating element is made up of carbon and other material. The resistance of this composite heating material (a mixture) has a very large positive temperature coefficient. At startup at some cold point the resistance is low, high current, as the material heats up resistance increases and current goes down. Long thermal time constant makes this a long time event.

By contrast nichrome or most other metallic resistance materials don't change much with temperature. The resistance of a typical space heater changes about 10% from room temperature to an orange glow.

Yes, read the material I referenced. Also go buy some of this type of heat tape and run experiments.

.

 

[kwired]

Read some of it, didn't have any completely new revelations. Conditions of use still will determine how much current will flow. If you plan to have any extreme cold starts, you need to factor that into design, as well as how long it may take to warm up the object you are trying to regulate temperature of. Appears as though 50F is the design temp at which ratings are based on.

If you are using the cable for freeze protection you probably don't have cold starts, if you do it is too late and freeze damage you are trying to prevent already has occurred.

If you have a high temp maintenance application, chances are you don't have extreme cold startup. Media probably already much warmer than room temperature at startup, you are just trying to maintain that temp say in a pipeline or something but if you do start such a process in extreme cold then beware what current will be like when starting.

 

[electron flow]

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?

GFPE is required by code to protect this load. Most manufacturers require GFPE in the installation manual. Arcing and fire can occur with conventional breakers or fuses. I have replaced heat tape that has started on fire. Unless this fuse is protecting conductors that feed a controller with GFPE you should be using a GFPE breaker.

 

[gar]

190511-0538 EDT

The primary and fundamental purpose of a fuse or circuit breaker is to protect the wiring on the output side of the protective device. The protective device may also protect a single load on said circuit, but in general that load is not what determines the protector size. If the protector is used to protect the load, then that protector can not be made larger than the criteria defined by the wire.

As I said before the power dissipation per unit length of the wire is a function of the current in the wire squared. While temperature rise in the wire is proportional to power dissipation. A 1.414 times increase in current will double the power dissipation in the wire. In this post 25/20 = 1.25, 1.25 squared = 1.56. So temperature rise would go up by 50%.

There is a long history that a copper wire with ordinary insulation in an ordinary ambient temperature can tolerate 20 A continuously with good insulation life. Thus, considered safe.

This does not mean you can pack a large number of wires together is a bundle and have all run at the full rating compared to a single or pair of wires together. It is fundamentally a heat, thermal resistance, and insulation problem.

Also there are transient vs steady state considerations. The criteria is to protect the wire.

The protective device does not protect what precedes the protector other than by limiting of current into the protector.

.

 

[jeremy.zinkofsky]

A bare copper wire about thick as human hair, held in liquid nitrogen under pressure, can handle thousands of continuous amps, just takes a very long time for the liquid to vaporize and copper heats and melts.

Using 125% rule does not make much sense if the main idea is that the OCPD should not be any bigger than the rated ampacity of the wire. A motor that has inrush of 28A but runs full load at say just 16A can perhaps still pose an issue, like you run #12 wire and use a 25A ocpd, what happens when the load goes south and begins to run 21A after startup? OCPD does not care, but the #12 does (using NEC rules, etc).

If the OP is trying to understand the physics of electrical design, then what you are saying is helpful. But if you are an electrician, the NEC exists to idiot proof your work. I would love to be creative with my designs and use my engineering prowess but am constrained by general rules governed by relevant building codes. That's what we're talking about here, a general rule applied to all installations. There will inevitably be situations where the code doesn't exactly make sense.

That being said, motor loads are required to have (2) modes of protection. The oversized OCPD can handle inrush but will safeguard against fault current. The secondary OCPD is sized for overcurrent and thus protects the wire.

 

[jeremy.zinkofsky]

Its done all the time for motors and welders in the NEC. Heat loads in the CEC. Most fixed loads in BS7671.


I was strictly speaking about linear loads. The NEC requires different approaches for different load conditions as you stated above.



"Its the job of the installer to make sure the circuit can handle the added load."


I disagree. The engineer of record is mandated to ensure that an electrical system is safe, not the installer. We have a building code and inspectors to ensure that the installer does the job right when an engineer is not engaged.

 

[jeremy.zinkofsky]

190511-0538 EDT

The primary and fundamental purpose of a fuse or circuit breaker is to protect the wiring on the output side of the protective device. The protective device may also protect a single load on said circuit, but in general that load is not what determines the protector size. If the protector is used to protect the load, then that protector can not be made larger than the criteria defined by the wire.

As I said before the power dissipation per unit length of the wire is a function of the current in the wire squared. While temperature rise in the wire is proportional to power dissipation. A 1.414 times increase in current will double the power dissipation in the wire. In this post 25/20 = 1.25, 1.25 squared = 1.56. So temperature rise would go up by 50%.

There is a long history that a copper wire with ordinary insulation in an ordinary ambient temperature can tolerate 20 A continuously with good insulation life. Thus, considered safe.

This does not mean you can pack a large number of wires together is a bundle and have all run at the full rating compared to a single or pair of wires together. It is fundamentally a heat, thermal resistance, and insulation problem.

Also there are transient vs steady state considerations. The criteria is to protect the wire.

The protective device does not protect what precedes the protector other than by limiting of current into the protector.

.

Amen

 

[jaggedben]

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.

Heaters are used in some places to keep parts of track switches from freezing. That arguably has a role in the operation of rolling stock. I'd say we don't know if this covered by the NEC based on the info given.