Speaker Wire / Low Voltage Lighting

[Karl H]

Got a call to trouble-shoot some track lighting that wasn't working in a
large design facility today. 12v Alfa tracks, all fed with what appears to be
#10 alum clad copper or tinned copper stranded, with a transparent outer
sheathing. It looks like and I believe it to be 10AWG speaker wire. There
are no numbers,manufacture,type letters, or UL on the wires at all. Due to
brittle or burned condition of the outer sheathing, it is definitely not suited
for the high current low volt system it is supplying. I've tried looking it
up but, I've yet to find a type letter or an ampacity rating of the insulation
commonly used on speaker wire. Does anyone know what type of insulation
speaker wire uses.

The wires will be replaced with a suitable LV cable. I'm just curious what the rating is for speaker wire. Sorry no pics.

BTW there are 90 tracks wired in this manner. Physics exposes Ignorance!

:grin:

 

[iMuse97]

Suppose maybe they (original installers) thought these were LV tracks so they could use LV speaker wire? Gave it some more thought and said to themselves, "yeah, they use them (these speaker wires) to feed 100w speakers so it ought to be good?" What a joke.

Thanks for the post.

 

[zbang]

Does anyone know what type of insulation
speaker wire uses.

Yes, since most of my speaker cable is SO-type . Generic speaker cable is going to be either cheap thermoplastic or cheap PVC. OTOH, I've been known to use CAT5 for small stuff.

 

[electricmanscott]

What is the load thses tracks are drawing? If the insualtion is that bad perhaps the wire is undersized.

 

[LarryFine]

#10 is #10, and should have similar ampacities. It sounds like you are seeing tinned copper. The insulation may be a different story. It needs to be rated for in-wall use if so used, and for the temperatures it sees.

This is from another, similar discussion: http://forums.mikeholt.com/showpost.php?p=1071557&postcount=30

 

[iwire]

The wires will be replaced with a suitable LV cable. I'm just curious what the rating is for speaker wire.

Most likely you will need to use a Chapter 3 cable and I am willing to bet the cables are undersized resulting in a lot of voltage drop.

 

[Karl H]

What is the load thses tracks are drawing? If the insualtion is that bad perhaps the wire is undersized.

250 watts. It's the definitly the insulation. Speaker wire cannot be used
for a 300 wiring method. I'm just curious as to the ampacity rating of common
speaker wire. I guess in the Audio feild, insulation rating isn't a factor as
I have not of yet seen a table or anything in the form of a spec for current
tolerence. I have seen C degree ratings though.

 

[Karl H]

#10 is #10, and should have similar ampacities. It sounds like you are seeing tinned copper. The insulation may be a different story. It needs to be rated for in-wall use if so used, and for the temperatures it sees.

This is from another, similar discussion: http://forums.mikeholt.com/showpost.php?p=1071557&postcount=30

I've come to the same conclusion, it has to be tinned copper. Yeah, speaker
wire is good for 12,24,120 volts, etc. But, what current can the insulation
handle, is the REAL question.

 

[Karl H]

Most likely you will need to use a Chapter 3 cable and I am willing to bet the cables are undersized resulting in a lot of voltage drop.

I don't think they are undersized, #10 @ 40'. It's not the correct insulation
or wiring method for the load or app. 250 watt load.

 

[electricmanscott]

I don't think they are undersized, #10 @ 40'. It's not the correct insulation
or wiring method for the load or app. 250 watt load.

The wire size may be ok for the load it's carrying but there has to be some serious voltage drop there.
This is a guide from tech lighting for sizing transformer loads to keep voltage drop in check. This is for a 300 watt transformer at 12 volts.

LOW-VOLTAGE WIRE SIZE TABLE
Use to determine the correct THHN wire size for a recommended 3 percent drop
in voltage at the specified wattage. The length shown is the length of wire from
the transformer to the power feed.
12 Volt 5 ft. 6-15 ft. 16-20 ft. 21-40 ft. 41-60 ft. 61-90 ft.
300 Watt #10 GA #6 GA #4 GA #1 GA 1/0 3/0

 

[iMuse97]

LOW-VOLTAGE WIRE SIZE TABLE
Use to determine the correct THHN wire size for a recommended 3 percent drop
in voltage at the specified wattage. The length shown is the length of wire from
the transformer to the power feed.
12 Volt 5 ft. 6-15 ft. 16-20 ft. 21-40 ft. 41-60 ft. 61-90 ft.
300 Watt #10 GA #6 GA #4 GA #1 GA 1/0 3/0

Yeah! Let's run some 2" RMC and 3/0 for those LV track lights!!!:grin:

What engineer thought that one up? The obvious conclusion: Run speaker wire.

 

[LarryFine]

Well, 250w at 12 v is 20.8 amps. A #10 of any kind of wire should be able to handle that without getting hot enough to damage the insulation, and voltage drop would only reduce the current.

My conclusion is that the terminations and/or proximity of lamp heat is what's causing insulation damage. Is the damage all along the wire, or only near the track feed?

 

[Karl H]

The wire size may be ok for the load it's carrying but there has to be some serious voltage drop there.
This is a guide from tech lighting for sizing transformer loads to keep voltage drop in check. This is for a 300 watt transformer at 12 volts.

LOW-VOLTAGE WIRE SIZE TABLE
Use to determine the correct THHN wire size for a recommended 3 percent drop
in voltage at the specified wattage. The length shown is the length of wire from
the transformer to the power feed.
12 Volt 5 ft. 6-15 ft. 16-20 ft. 21-40 ft. 41-60 ft. 61-90 ft.
300 Watt #10 GA #6 GA #4 GA #1 GA 1/0 3/0

Am I not clear in my post? This is IMO "Speaker" wire. What "Type Letter"
is speaker wire??????? What is the ampacity rating of common,typical
speaker wire? I know you mean well!:grin::grin::grin:

 

[Karl H]

Even if I reduced the VD to an acceptable %. I still can't make Speaker
wire compliant for this install. Yes, I know this is a weird one. The
un-compliant conductors will be replaced with a compliant wiring method.
But, for my own education I would love to know what Type letter or
ampacity speaker wire is rated for. I bet a 10 AWG speaker wire insulation is
not even close to being able of carrying 21 I for any length of time without
insulation failure.:grin:

 

[Karl H]

Well, 250w at 12 v is 20.8 amps. A #10 of any kind of wire should be able to handle that without getting hot enough to damage the insulation, and voltage drop would only reduce the current.

My conclusion is that the terminations and/or proximity of lamp heat is what's causing insulation damage. Is the damage all along the wire, or only near the track feed?

Good catch! The failure is at or before the terminations. The heat from the lamps isn't the factor IMO as the first lamp is 18" from the power feed
terminations and all connections within 6 inches of the transformers
secondary terminations.

 

[Karl H]

All the conductors look fine where they are exposed to "Free Air", yet all of
the burned or damaged conductors are in confined spaces IE, transformers,
fixtures, or small "cramped" spaces.

 

[iMuse97]

So sorry; you are, and have been, clear.

It seems I, and possibly the others, are so entranced with the idea that someone would use "speaker wire" and all the other things that go with your story, that we've missed the point completely.
However, I cannot think of anything that would allow me to definitively say what the ampacity of speaker wire is, because I do not know the specifications of such wire.

I would assume a couple things and go from there:
1. the insulation on the wire in question is essentially similar to lamp cord insulation.
2. the ampacity would be the same as lamp cord, therefore.
If the foregoing are true, then the ampacity of a AWG #10 speaker wire would be 30A. And I would, with Larry, be wondering why the wire failed. Voltage drop does by definition produce heat, but not enough
to destroy the wire, I wouldn't think.
Your other question has to do with insulation on speaker wire: Thermoset or Thermoplastic would be a guess, and would be correct if it were the same insulation as common lamp cord. From T400.4

Now, if the ampacity is OK, and by inference the insulation is OK, then why did this thing fail prematurely?
And would another type of installation not fail?
This baffles me as it seems it has baffled you.
Would the continuous loading be an issue? or other sources of heat?

Something is very unusual about this whole scenario.
Please keep us updated.

 

[iMuse97]

All the conductors look fine where they are exposed to "Free Air", yet all of
the burned or damaged conductors are in confined spaces IE, transformers,
fixtures, or small "cramped" spaces.

Is there is some sort of resistance/impedance that is built into "speaker wire?" This is just weird. I'm wondering if the fine stranding and twist or lack of twist in speaker wire causes some problem. I'm with you on the idea that this wire couldn't possibly be able to sustain a 20A load. I'm googling speaker wire specs right now, but not getting anything substantive.

 

[iMuse97]

Here's some info on speaker wire that suggests that not all of it is just ordinary wire:

"Although some expensive wires can have low resistance, several have a high capacitance of 100 to 300 picofarads (pf) per foot. These can introduce a significant amount of capacitive load, particularly for longer lengths. For a 50 foot length, this adds up to 5000 to 15,000 pf, which is the same as .005 to .015 microfarads (mfd)."

"In comparison, ordinary wire can have low resistance but also very low capacitance. Twelve gauge Romex has a capacity of only 15pf/foot. Fifty feet has a total capacitance of only 750 pf, which is the same as 000750 microfarads (mfd). Twelve gauge line cord has a capacitance of only 18 pf/foot and 50 feet has a total capacitance of only 900 pf, which is the same as .000900 microfarads (mfd)."

"Cobra Cable was sold by Polk Audio many years ago but has long since been discontinued. The idea was to reduce the cable inductance but in doing so, increased cable capacitance. Cobra wire had many separate strands that were individually insulated with a thin material. These all ran in parallel and comprised one conductor. The insulation was colored black for one group. The other conductor was a similar group of strands but the insulation was colored red. The strands of each color were interwoven with each other and formed a braided but cross-coupled two-conductor cable. Of course, the strands being very close to each other and with thin insulation, resulted in an unusually high cable capacitance."

This info. is located here: http://www.roger-russell.com/wire/wire.htm#resistancehigh

"

All inexpensive wires are not the same, However. This wire at the right was sold as speaker wire several years ago by such places as Home Depot and Lowes. It sold for 33 cents/foot. It had transparent insulation and was 12-gauge. It was much less expensive than the brand names. It did not have any coding to identify one of the wires for proper phasing. I had some of this wire for about 6 months and noticed it was turning color. Now it has turned a very pronounced green on the surface of the copper wire, indicating a chemical interaction with the insulation and the copper. A new piece of wire is at the right for comparison. Although the wire may not corrode any further, it doesn't inspire confidence, particularly if the insulation comes close to the connecting terminals.

Perhaps the transparent insulation was an attempt to mimic the more expensive speaker wires. Without researching the chemical properties of the insulation or the need to code one of the wires, it was not well thought out for use as speaker wire."

 

[iwire]

I don't think they are undersized, #10 @ 40'. It's not the correct insulation
or wiring method for the load or app. 250 watt load.

They are undersized.

20.8 amps on 10 awg running at 12 volts 40' long will have high voltage drop.

I come up with at least 14% drop, you would need 4 AWG just to get under 4%.

Move the transfomers closer to the load.