ECN Forum Forums Photo Discussion Forums Violation Photo Forum Ampacity of a #2 Phillips?

Submitted via Joe Tedesco:

133.2 Ohms for a 1/4" dia steel screwdriver shaft placed between 4" apart tabs.

R=p l/A
R=Resistance
p= specific resistance for material - steel is 100
l=Length
A=Area in Kcmils

R=100 (.333/.250)

Of course, I had to really dig around to find this one...never had to use that formula before. I'm probably wrong, but it killed some time, and fired up the brain cells... good post!

Can't belive someone actually tried to answer the question!

Nice try but there must be something wrong with that calculation. Probably a decimal point somewhere. I think the resistance of steel is approximately ten times that of copper. This (I think) means that you should move ten sizes AWG to get the equivalent copper screwdriwer. Remeber to use only a listed electricians screwdriver for this. Come to think of it, I haven't seen any of those lately...

If the diameter is 1/4" the area is roughly 30 mm2 (note my systematic use of units) or approx 2 AWG. Thus, the ampacity is similar to that of a 12 AWG copper conductor.

Somebody, please correct this!

DougW,

Right calculation, wrong units.

When the area is expressed in Kcmils, and the length is expressed in _1000 feet_, K for copper is something like 10 or 12 (depends on temperature and the reference that you are looking at). I'll trust you that the K for steel is 100; for iron it is about 50, but for different steels it can be all over the place.

Area in circular mils is the diameter in mils _squared_.

So we get:
area = 250^2 = 62.5Kcmil
length= 4 inches /12000 inches = 3.3333*10^-4 Kfeet

resistance = 100 * 3.3333*10^-4 / 62.5 = 0.5 milliohm

-Jon

OK,... thought the distance in feet was supposed to go into the calculation.

I really had to look around to find something even close to caluclate R for steel...

Went to http://www.electrician.com/articles/ampacity.htm
and eventually wound up at http://www.ibiblio.org/obp/electricCircuits/Ref/REF_3.html
For the "Coefficients of Specific Resistance", since the only ref I had at work was the NEC. I couldn't find a formula to convert in2 to cmils. Peeved me, lemme tell ya...

Hey, it was late, I was bored... good brain workout anyway.

update - Just got home & got out my American Electrician's Handbook. Let's see...1/4" = 62,500 Cmils... steel R=81.179/cmil ft....

[This message has been edited by DougW (edited 04-16-2004).]

A tremendously useful reference for these sorts of things:
http://www.unc.edu/~rowlett/units/
http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html

you should also be aware of the 'google calculator' http://www.google.com/help/features.html#calculator

-Jon

Wouldn't that be measured in Stanleys? Or was it in Kleins? I think 4 Stanleys = 1 Klein, or something like that.