ECN Forum Forums Photo Discussion Forums Photos for General Discussion Multiple feeds, One Service

John,
First of all, I don't think stranded wires can be considered as "parallel" conductors, (or 'multiple conductors under one lug, for that matter) as they are in constant contact with each other through their entire length. Were they to be individually insulated, you might be on to something.

As to your first point, I suspect that this effect is ignored in your usual electronics class (when doing resistor calculations) simply because it is considered insignificantly small, both in terms of the currents involved and the physical spacing. Those simple calculations also ignore the effects caused by the current being AC, and treat it all as DC.

But- just as those electronics do get warm- improperly paralleled conductors do get quite warm at their termination.

Now, I hve never deliberately created this sort of situation. NEC 310.4 tells us that:
"The parallelled conductors ....shall comply with all of the following:
(1) Be the same length
(2) Have the same conductor material
(3) Be the same in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner"


If we could assume that "things would somehow balance out" by dividing the load between the conductors proportionally, then we are left without an explanation for either the heating that occurs, or 310.4.
Maybe there's a better explanation out there, but I've done my best :-)

Reno, thank you for that explanation. I really do appreciate understanding why it is wrong. Thank you very much.

Ron

Reno, thanks for the reminder about 310.4. I think the most frustrating thing about studying the NEC is that there are so many rules for which no rationale is stated, even in the handbook.

I suspect the restrictions on parallel conductors have a lot to do with termination methods. Trying to connect two wildly different-sized wires into one terminal could lead to some "creative" and dangerous solutions.

Fault tolerance was probably also a concern. In your hypothetical 2/0 and #2 example, the loss of continuity in the 2/0 would leave all the current flowing in the #2, which would reduce it to glowing slag. At least if the conductors are equally sized, the overload is limited to 2:1.

I believe you when you speak from experience that unequal-sized parallel wiring produces heat at the junction, but there's got to be some other mechanism at work -- possibly higher resistance in the joint itself because of unequal pressure on the conductors.

Thanks for the nice reply, John

Even if a man started in this field the same time as Tom Edison, there are sure to be things he hasn't dealt with very often, if at all. Those things he may not be very "expert" at.

I remember being taught way back about the correct way to make a parallel feed. I have, over the years, even run a few. But- and this underscores my limitations- I have never, ever seen a parallel feed done wrong before this one! So I really can't say, from personal experience, just how horrible it is to make this error.

Now, the PoCo trains its' folks extremely well, and they never, ever improvise or cut corners. (All it takes is a stay somewhere outside the USA to find out -the hard way- just how reliable our grid really is!) So, when I went to them with this pic, I had the entire engineering department saying "Huh?" and scratching their heads. It would seem that this sort of thing is beyond their experience as well.

My grasp of theory might be overly simple, and incomplete- but then who really "understands" those electrons anyway? (I want to study probability theory, I can go to the local casino!).
As I read the code, this sort of thing is a very big "no-no," and that's enough for me to know!

Anybody has a better explanation, please share it with us. I mean that.

A possible reason for requiring the same type and size wire for paralleling might be that a wire that has twice the cross sectional area of another wire will have half the resistance of the smaller wire. And will take 2/3 of the current being demanded by the load. Problem is that the heavier wire has less surface area per amp of current to get rid of the heat due to wire resistance. SO the heavier wire will get hotter. You've seen this in the ampacity tables of various gauges of wire.

Thus the code requirement that paralleled wires be of the same size, etc.

As for the POCO service, I wouldn't be suprised that the linemen didn't have heavy enough cable for that customer on the truck, and there being pressure from the boss to get the customer on line, just paralleled 2 cables to get it done. Management can get that way...

[This message has been edited by wa2ise (edited 07-25-2005).]

I know that this is an old thread, but I think that wa2ise has hit it on the head. The ampacity of a conductor is set by the combination of heat produced by the conductor, and heat which escapes through the insulation. Larger conductors have less ampacity per unit of copper cross section because for a given cross section there is less circumference through which the heat could escape.

If you want to see just how big a difference, compare the total ampacity of a single #2/0 conductor with 16 #14 conductors in parallel (separated for good heat dissipation). This is a approximately the same total amount of copper. If you could actually run the conductors at their thermal ampacity limit, you would get more than twice the current through the parallel #14s. (Note: this would not work or be safe for other reasons, and would violate a bunch of codes...just focusing on the thermal ampacity of an extreme example.)

Which leads to a thought about how to save some copper: build rope core cables where you use some sort of plastic filler material covered with copper and then covered with insulation. If you made something that had the external surface area of a #2/0 conductor, but the copper content of a #2 conductor, then my guess is that you would see a thermal ampacity perhaps 20% better than a #2 conductor, I'm guessing the equivalent of increasing the wire size by 1.5 gages.

-Jon