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 Advanced Electricity and Magnetism Advanced Electricity and Magnetism Physics Help Forum Aug 26th 2016, 05:17 AM   #1
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Current Carrying Capacity

I am trying to solve for the current carrying capacity of a wire in a glass-to-metal seal. I have been trying to use the neher-mcgrath formula to do so but have been unsuccessful. This may or may not be the appropriate formula for what I am trying to do. My end goal is to have a working understanding of a formula that will enable me to make a chart similar to the one titled "Current Capacities of Standard Kemlon Glass Seals (52 alloy) in Amperes with 50 Degree [Fahrenheit] Temperature Rise" on this page:

Kemtite High Pressure Connectors Technical Information

The main difference is I want to be able to make a chart in excel that will let me fill in the required properties and excel will do the math to give me current carrying capacity for that setup.

What I have done so far:
I have found the neher-mcgrath formula for current carrying capacity (ampacity).
This formula reads I = square root((T sub c - (T sub a - delta T))/(R sub dc * (1+Y sub c) * R sub ca)) Sorry if that is hard to understand, here is a link to where I found it:

Understanding the Neher-McGrath Calculation and the Ampacity of Conductors

Where I = ampacity, T sub c = conductor temperature, T sub a = ambient temperature, delta T = conductor temperature due to dielectric loss, R sub dc = conductor dc resistance, Y sub c = loss increment due to conductor skin, R sub ca = thermal resistance between conductor and ambient temperature

As I mentioned at the beginning of this post, the wire would be within a glass to metal seal. There is no conductor skin (unless air counts?) and I read that Y sub c was negligible for wire sizes smaller than AWG 2. I read that if the voltage will be less than 2000V delta T is negligible. That said I used the shortened version labeled "Equation 1" which omitted those values. For conductor temp I was trying to match the chart given as an example to verify I was performing the calculations correctly, so I used 35 degrees Celsius. I used 25 degrees Celsius as my ambient temperature. The conductor (52 alloy) dc resistance is .00000429 ohm-mm which I multiplied by the length of my pin then dived that by the cross sectional area to get .001517277 ohms. The thermal resistance I was using (which is most likely in error) was that of the 52 alloy is .0132 watt/mm Celsius which I divided by the length to get 481.060606 watts Celsius. When i run the calculation for a .030 dimater pin as shown in the chart i get 3.7 ampacity which is close to their 3.8. At this point i was feeling pretty good but then as I tried the other pin diameters and temperatures listed in the chart the numbers I got were way off mark. Any help in understanding where I went wrong would be greatly appreciated.

I will attach the excel file I was working with as well.
Attached Files Ampacity.xls (23.0 KB, 7 views)  Tags capacity, carrying, current Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post Walter Lewin Electricity and Magnetism 1 Feb 8th 2011 09:08 PM greencheeseca Advanced Electricity and Magnetism 3 Mar 15th 2010 04:59 PM tanaki Thermodynamics and Fluid Mechanics 1 Dec 10th 2009 12:40 AM ssadi Electricity and Magnetism 3 Aug 14th 2009 03:49 PM helloying Thermodynamics and Fluid Mechanics 2 Apr 29th 2009 03:25 PM