Another possible useless thought from the mind of Dnk....

This one involves thermal expansion of conductors. Away I go..

Does a conductor expand latterly with heat?

My thought goes like this. When things heat up, they expand, and then contract, when they cool.
When using pvc as an example, we need to use expansion couplings, due to this effect. The pvc "grows" in length. With an I-beam of steel,we see the same effect, so we see expansion joints on bridges, to compensate for this.

So, Copper wire must do the same, but to what degree? Lets say we are backstabbing receptacles. The wire, dependant on load, would basicaly, expand into the stab, and then back out when the load is removed. Would this be accurate? Or does the slack left in the wire, compensate for this?

Is this movement actually calculateable?(wording)

Any takers?


Dnk...

Editted for sp
looking for spell checker

[This message has been edited by Dnkldorf (edited 02-03-2006).]

[This message has been edited by Dnkldorf (edited 02-03-2006).]

In my opinion, it is the heating and cooling that leads to the premature failures of the terminations in cheaper backstab receptacles.

BTW, try http://www.iespell.com/ for spell checking, it wrks greet for me.

Hey Dnk - here we are once again, heh?

You are exactly right. Copper, like most anything else, expands 3 dimensionally when it heats up. And as you know, any current through a resistance produces some small amount of conductor heating (minimal as it may be). And the expansion is quantifiable.

Each substance has a known (measured) coefficient of expansion, a measure of the degree of expansion relative to a set change in temperature. It is interesting to note that the coefficient of expansion of aluminum is greater than that of copper, which leads to one of the 2 serious problems with aluminum wiring (as noted by Ron): the constant and greater degree of cycles of expansion and contraction lead to loose connections, contributing to hi resistance connections. (The other problem with alu wiring is the tendency of the outside layer of aluminum to oxidize, also contributing to a hi resistance connection).

Somewhere I have a table of coefficients of expansion of several types of materials. If I can find it I'll scan it post it.

Radar


[This message has been edited by Radar (edited 02-03-2006).]

Nice explanation, Radar.

From Reference Data for Radio Engineers 7th ed.:

Aluminum: 22.9 ppm / degree C
Copper: 16.5 ppm / C

This means that an awg-12 copper wire heated from 20C to 75C expands in diameter by, oh, 74 millionths of an inch. And a 100-foot length gets 3/32" longer.

Edit following Dnk's comment below...

Sorry, not 3/32" -- it's 3/32 foot = 1.09"

PVC is something like 61 ppm/C, which works out to 4" expansion. Yep, there's still some difference between wire length and conduit length to compensate for.


[This message has been edited by John Crighton (edited 02-03-2006).]

3/32" that's it? Hmmm...

IF pvc expands at roughly 4.5" per 100' at that temp. difference, and the wires only expand at 3/32", wouldn't the wires get damaged some how by the force of the pvc expanding?.

Table 352.44..


Dnk...

Dnk, see my correction above. Also, from Carlon's Expansion Joints for PVC Rigid Nonmetallic Conduit:


I guess the point is that the distance between end points of a straight conduit run doesn't change, or it changes roughly in the same way that the wire length does, so the wires aren't stressed. Since the TCOE for PVC is so high compared to other building materials, the purpose of expansion joints is more to compensate for the NMC expansion than to accomodate structural movement.

I deal with it in a couple of ways.
1.) Socketed ICs on R.O.W. mounted equipment. You get alot of crunching sounds when you press on the chips. The leads and socket materials have different coefficients.

2.) Programming underground storage tank monitoring systems. #2 diesel fuel is .00045. The number is needed for temperature compensated volumes.

I know that this is a little off-topic, but we also use gizmos that differentiate between fluids with 2 thermal conductivity coefficients. These RTD (Resistive Temperature Devices) let us know if the probes are suspended in hydrocarbons or water so that we might get our filters drained before the elements swell up and block flow.
Joe