Ampacity of non standard materials

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mic

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I am trying (unsuccesfully) to calculate the ampacity of a kovar (iron, nickle, cobalt alloy) stud.

My client uses it to carry current to an electrode but I am concerned it has been undersized (gut feeling based solely on the resistivity numbers; 0.49 micro-ohm*m for kovar vs. 0.017 for copper, almost 30:1).

The calculation in the NEC (310.15 (C)) assumes that the conductor is rated for conducting current and requires a temp rating for the cunductor (TD).

The stud is fully exposed so can I assume Delta TD=0 or do I have to use air as my insulation?

This is a DC system with no ac cable nearby so can I assume that YC=0?

RCA would just be the thermal resistance between kovar and air.

I still don't have TC so I am stuck there.:confused:

If anyone has ideas I would appreciate the help

Chris
 
Wouldn't it be easier to just run a wire?

NEC ampacity's in 310 are based on a maximum conductor temperature (which you understand) - so if the stud isn't getting hot, I don't think 310.15 is what you want to be looking at.

I would suggest you look at 110.5 or 110.8. Although these might not apply depending exactly on what the electrode is for, or what your application is.

Steve
 
mic said:
I am trying ... to calculate the ampacity of a kovar ... stud.
Click to expand...
What are the dimensions of this piece? Are we talking about a bolt or a rod? What's the diameter? An inch long or a foot? What's the current?
 
mic -
I've never heard of using Neher-McGrath for calculating ampacity of an open-air conductor. I have only seen it used for duct banks - so I am no help there.

The only place I've seen kovar used is sealed in ceramic\glass insulators. As I understand, kovar is used because the expansion characteristics are close to the ceramic.

So, you appear to be looking at a bare conductor, part of process equipment. This is not premises wiring. I'm not even sure if the term "ampacity" applies.

The only issue I see is temperature. If tensile strength is not an issue, you may only be limited by the support insulators - if they are ceramic, I don't see 1000F as too much.

Q: Are you trying to estimate the temperature the kovar stud while in operation (carrying process current), because you can't see it while in operation? Otherwise you would just use an IR gun or a thermocouple?

If so, isn't this just an ME heat transfer problem? You know the resistance, temp coef, current, power dissipated, geometry, ambient. This one should be cookbook.

Perhaps we could use more detail - could be I don't understand yet

cf
 
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No Easy Answer

No Easy Answer

This stud in the air is not a conductor so Neher-McGrath (upon which NEC tables are based) does not apply. You might be able to approximate using the IEEE formula for bare conductors. However, these formulae consider the safe operating temperature of the conductor. What is the safe operating temperature of the stud? What kind of heat sink is it attached to.

Empirically, use an IR camera to find out how hot the stud and attached wires are operating. If any exceed their ratings, you need to fix the problem. If the stud heats and the heat conducts to the wire and the wire is now how, you have a problem. If the stud operates at 100C but the wire is at 90C and is so rated, there may be no problem.
 
The NEC ampacities are based on protecting the conductor insulation from overheating. The stud can probably take much higher temperatures than typical building wire.
 
The stud is about 3/16 diameter and about 2.5" long with 25-30 A load. It is part of an electrode for chemical processing equipment (hence the material). The reason I was trying to calculate the ampacity is because it passes through a polypropylene barrier so heat levels are critical. I guess I should have been looking for the thermodynamic equations instead of ampacity.

I agree with beanland, empirical data is probably the easiest and the best way to verify the integrity and safety of the system.

Thanks to everyone for the help (especially chris kennedy ;)).

Chris
 
mic said:
The stud is about 3/16 diameter and about 2.5" long with 25-30 A load. It is part of an electrode for chemical processing equipment (hence the material). The reason I was trying to calculate the ampacity is because it passes through a polypropylene barrier so heat levels are critical. I guess I should have been looking for the thermodynamic equations instead of ampacity.

I agree with beanland, empirical data is probably the easiest and the best way to verify the integrity and safety of the system.

Chris
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I'm inclined to agree with you.
I have a spreadsheet that I use for calculating copper busbar current ratings.
Using your diameter, if the stud was copper it could carry about 100A in free air for a 50degC rise. Taking the resistivity of the kovar at around 30 times that of copper suggests that a 3/16 inch diameter stud would be significantly under rated for 25-30A.
However, the stud isn't very long. The cable at one side and the electrode at the other probably act as heatsinks and help in keeping the temperature down and maybe somebody has taken that into account. Maybe there are other installations where operating experience has shown it to be satisfactory. But maybes won't show that adequate care has been taken to assess risk and show that all reasonably practicable measures have been taken to mitigate it.

In the absence of proven data to the contrary, I'd advise the client that I was not happy with the installation. I certainly wouldn't pass it.

For what it's worth, I'd be uncomfortable with a stud diameter of anything less than about 15mm. (Approx 0.6 inches, or 5/8)
Rough and ready but based on the maximum working temperature of polypropylene.
 
mic said:
I am trying (unsuccesfully) to calculate the ampacity of a kovar (iron, nickle, cobalt alloy) stud.

My client uses it to carry current to an electrode but I am concerned it has been undersized (gut feeling based solely on the resistivity numbers; 0.49 micro-ohm*m for kovar vs. 0.017 for copper, almost 30:1).

The calculation in the NEC (310.15 (C)) assumes that the conductor is rated for conducting current and requires a temp rating for the cunductor (TD).

The stud is fully exposed so can I assume Delta TD=0 or do I have to use air as my insulation?

This is a DC system with no ac cable nearby so can I assume that YC=0?

RCA would just be the thermal resistance between kovar and air.

I still don't have TC so I am stuck there.:confused:

If anyone has ideas I would appreciate the help

Chris
Click to expand...

Where did you get the other technical information you have on this substance? Can you not get the ampacity from the same place you got the resistivity (ask the manufacturer...)?
 
drbond24,

Resistivity (or conductivity) is a commonly quoted characteristic of metals and their alloys. Ampacity is generally only given on wire and is conditional on the insulation material.

The manufacturers are typically metal bashers that haven't got a clue about anything electrical (its all smoke and mirrors :wink::wink:).


Thanks to everyone for the input.

I have asked the client to monitor the temperature and if it stays below 75 C it's all is good.

c
 
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