questions on LV wire conductivity

Discussion in 'Landscape Lighting' started by starry night, Feb 22, 2013.

  1. starry night

    starry night LawnSite Gold Member
    Messages: 3,043

    I was reading through an old thread from two years ago about connection methods. The last post in the thread was by Tom Williams who posed some questions which were never answered. The first question is one that I have wondered about. I hope that Tom will not object to my posting these questions again as he wrote them:

    I'm an old theatre guy, not an electrical engineer
    I'd like to find out if what I've was told about Low Voltage wire is true.

    Low-Voltage wire is made of many strands of copper wire. I was told that Low-voltage current likes to run on the surface of the wire and that is the reason LV wire is made the way it is.

    Is that true?

    Second question: How does tarnish and eventually corrosion on the surface of the wire affect the wire's ability to conduct LV power?

    3rd question: what are the issues for a low-volt system if and when water finds a way to wick up the wire through capillary action?

  2. steveparrott

    steveparrott Sponsor
    Messages: 1,247

    1. All (regardless of voltage) AC current mainly flows along the outside of wire conductors - more accurately, it flows through the 'skin' of the conductor. The depth of the skin varies with conductor material, voltage, and frequency. The biggest of the variants is frequency - with very high frequency currents (e.g. electronic transformers at ~20,000 Hz) traveling through a very thin skin. The low frequency of low voltage currents (60 Hz) travels through a much thicker skin - maybe through an entire strand. If the skin is very thin then stranded cable is a better choice because of the increased surface area (vs. solid conductors) - for a thick skin (like with our low voltage current), stranding has very little impact on how the current travels through the wire. That's why resistance of a 12V 60 Hz current through a solid #10/2 is about the same as a stranded #10/2.

    The primary reason we use stranded cable for low voltage applications is because of it's increased flexibility (can bend withough damage). (Though, some do claim the skin effect is important, too - a bit of a controversy.)

    2. When copper wire corrodes (surface becomes oxidized), it does diminish in conductivity. That's why tinned wire is a good choice - tin oxides are more conductive than copper oxides. Tin is also sacrificial to the copper, meaning that the thin coating of tin will oxidize instead of the copper core.

    3. As in above answer, wicking detroys copper wire, greatly reduces it's conductivity, and makes it brittle.

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