Stranded vs. Solid Conductors - Pros & Cons

To All:

We have a customer asking about the pros and cons of stranded versus solid copper conductors in our building wire. Of course, the first thing that popped into my mind was the flexibility factor, that stranded was much more flexible than solid, so that would be a pro for stranded should someone be looking for flexibility in their use of the product.

Well, now the customer wants more than just flexibility as a selling point (they are pushing the stranded). I cannot think of anything else. The amp load is the same, the circular mil area is roughly the same, and the DC-Resistance values are roughly the same as well.

Does anybody have any thoughts on this?

Thanks,

Blake Slayton
Quality Manager
United Copper Industries

Hi Blake,

Building Wire covers a lot of products as you well know. Many are stranded because they are designed to be pulled into ducts, electrical conduit or raceways in commercial/ industrial buildings under very specific rules.

On the other hand, circuit size copper building wire for residential and multi-residential buildings, NM-B, UF-B and Type-MC, has little axial strength and thus are not made from stranded wire until the 8 AWG conductor size is reached.

A Rule of Thumb

A solid wire gets more rigid and harder to handle as the gage size decreases and the conductor diameter increases. Stranded conductors solve this issue but at a cost. (The rule here is that a stranded conductor has an almost identical (but not less than) cross-sectional area of a solid conductor of the same gage size. This of course is by the sum of the areas of the individual strands making up that stranded conductor.

Disadvantages

The disadvantages are as per the following example and discussion:

Let’s consider a solid 12 gage conductor and an equivalent seven wire strand.

The solid wire diameter is 0.080808 inches and the area is 6529.93 circular mils

The area of each of the 7 wire strands for the 12 gage is 932.85 circular mils and the diameter of each strand is 0.03055 inches. (Slightly moved up from 0.03054 inches to make sure the cross sectional area is OK at 6533.12 circular mils.)

Now, the best possible diameter of the stranded conductor is 0.0917 inches and this is 10.9 mils larger in diameter than the solid gage conductor. It also means that the insulation and jacket usages increase. Likewise the cost of the conductor increases because of the extra drawing and stranding processes. Moreover if you compact the conductor to bring it closer to the original solid conductor diameter, you would lose your quite small flexibility advantage.

There is another far more serious issue.

Suppose an electrician manhandles these small gage size, fairly low tensile strength cables during installation in a residence.

If it was a solid conductor and it broke, the electrician would probably find it by loss of circuit integrity.

Suppose however it was a stranded conductor and one strand broke. You would have a higher resistance at one spot. In service a hot spot would be created and a fire could result. How would you to address that issue? By putting in axial strength members to protect the stranded copper conductors? I think you would you would be legally responsible for that fire if the forensics pointed to the stranded conductor cable as the fire initiator?

I hope this gives you some insight.

Sincerely,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Peter,

Thanks for the help. I have passed this on to my customer.


Blake