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AfterGlow
09-16-2010, 08:51 PM
Hello fellow artisans. I'm excited to be a new member of this forum. I've been a silent observer for a few years. Thought I'd chime in from time to time.

I've found several conflicting opinions on a question I have. I'd appreciate your knowledge...
With all else being equal, what is a better conductor of low voltage that we use? A 200 foot length of 10 gauge solid copper wire or 10 gauge stranded copper cable? Again, with all other factors being equal.

Thanks!
Jeff

Terradek
09-17-2010, 10:42 AM
Jeff,
Solid wire has less resistance and therefore greater conductivity at any gauge. That being said, solid wire is rarely used in Low voltage lighting. Primarily due to the difficulty of working with the more rigid nature of solid wire. Below is information regarding 12gauge wire that will help to give you some point of reference.

12awg solid wire @ 68F resistance is 1.587 (Ohms/1000')

12awg 7 strand wire @ 68F is 1.628 (Ohms/1000')

12awg 65 strand wire @ 68F is 1.650 (Ohms/1000')

*Based on UL1581, Table 30.10
Source: Paige Electric.

Notice as the number of strands increases the resistance also increases. This increase in resistance causes greater voltage drop. So why use stranded wire? Primarily because it is easier to work with...though the resistance is greater it is not significant enough to make a big difference, especially with today's multi-tap transformers.

The primary reason that solid wire is more efficient is that it provides a larger surface (larger diameter) area for electricity to travel along. This is called the Skin effect. Typically we use 65 strand wire in Low voltage.

steveparrott
09-17-2010, 10:47 AM
Hello fellow artisans. I'm excited to be a new member of this forum. I've been a silent observer for a few years. Thought I'd chime in from time to time.

I've found several conflicting opinions on a question I have. I'd appreciate your knowledge...
With all else being equal, what is a better conductor of low voltage that we use? A 200 foot length of 10 gauge solid copper wire or 10 gauge stranded copper cable? Again, with all other factors being equal.

Thanks!
Jeff

From a practical standpoint, stranded is much better because it is more flexible, much easier to make long runs. That's why it's universally used - all landscape lighting wire commercially available is stranded.

From a voltage loss standpoint - intially, solid has less resistance. Some argue that stranded is better because current travels along the surface of the strands, implying that more surface area equals less resistance. But this is only a significant factor when high-frequency currents are used (e.g. 20,000 Hz vs. 60 Hz.)

In fact, the increased surface area of stranded cable is a problem when cables start to corrode (oxidize) through wicking. Copper oxides have much greater resistance than bare copper; more strands, more oxides, more resitance than oxidized solid wire. That's why we use tin-coated stranded wire - oxidized tin maintains good conductivity compared to oxidized copper.

From a code standpoint, there is no requirement that states that wire must be stranded - only that it's rated for underground burial and UV-resitant (SPT-type).