Which wire to use?

[bdosborn]

FWIW, I'm on the fence over solid versus stranded. I was a rabid stranded fan until I started looking at the wiring in old trailers. ALL of them had solid core wires for the 120V system and some of them were from the 60s. I also noticed that the wood shops I looked at during job walks had solid wire (saws, drills, lathes, etc) and that's a high vibration location. I used MC cable in the trailer for the 120V system and its all solid wire (couldn't get stranded MC at HD). No failures after ~15 years.That being said the van is all stranded wire because it's so much easier to pull and terminate than solid. I've come to the conclusion that solid wire is probably okay for a trailer and how well you terminate the wire is a more important reliability factor than solid vs stranded.

Bruce
P.S. in all my years as a consultant I never had a solid vs stranded discussion with clients, code officials or contractors. It wasn't ever an issue I encountered in the critical facilities I worked on.

[tony.latham]

P.S. in all my years as a consultant I never had a solid vs stranded discussion with clients, code officials or contractors. It wasn't ever an issue I encountered in the critical facilities I worked on.

And that speaks volumes.

Tony

[featherliteCT1]

My understanding is that stranded wire should/must be used with low voltage DC current (e.g., 12v) where surface contact is critical to avoid fires. The surface contact issue is not as critical with high voltage AC current (e.g., 120v).

Also, the insulation temperature rating is critical.

For example, my Victron charger manual says:

Screenshot 2023-09-25 at 8.35.59 PM.jpg (310.9 KiB) Viewed 443 times

[Tom&Shelly]

FWIW, I'm on the fence over solid versus stranded. I was a rabid stranded fan until I started looking at the wiring in old trailers. ALL of them had solid core wires for the 120V system and some of them were from the 60s.

OK, but maybe they were just getting away with it, in terms of number of failures. Wonder how many failures there were in those trailers? After all, there were six successful Apollo flights using the same cryo tanks until Apollo 13's failed. Extreme example, of course, but we are considering a somewhat unlikely event. How big of a percentage of failure is acceptable? For trailer manufacturers, from what I've heard, it's incredibly high.

I also noticed that the wood shops I looked at during job walks had solid wire (saws, drills, lathes, etc) and that's a high vibration location.

Right to the equipment? That surprises me as the shops I've been in had stranded wire from the walls to the equipment. Probably industry standard, though I never asked. Probably solid wires down the walls to the termination, but again, I never paid that close attention.

P.S. in all my years as a consultant I never had a solid vs stranded discussion with clients, code officials or contractors. It wasn't ever an issue I encountered in the critical facilities I worked on.

Well, it is specified in the code, isn't it? NEC or industrial "equivalent"? (eg NFPA 79)

Certainly the wiring for spacecraft was spelled out in the n'th degree, and it was always stranded.

Tom

[bdosborn]

My understanding is that stranded wire should/must be used with low voltage DC current (e.g., 12v) where surface contact is critical to avoid fires. The surface contact issue is not as critical with high voltage AC current (e.g., 120v).

Also, the insulation temperature rating is critical.

For example, my Victron charger manual says:

Screenshot 2023-09-25 at 8.35.59 PM.jpg

A detail that no one ever discusses is that it's the lugs on the device that dictates the type of wire that can be used. The lugs use the wire as a heat sink to cool the device so using the wrong type of wire can cause overheating. In the US, all circuit breakers/switchgear/etc. have 75C rated lugs (including MV CBs) so you're required to use 75C rated insulation when doing wire ampacity calculations. The lugs are also rated for specific stranding of wires. The fact that Victron is requiring the fine wire isn't so much its standard practice, it's because that's what the lugs they use require. Finally, the manufacturer's installation requirements always trump general code requirements so if Victron recommends it you should use it.
Since I know that all the 120V lugs that are sold in the US are rated for both solid and stranded wire I should amend my statement that solid core wire is probably fine for 120V devices in a trailer. And what a PITA it would be to use solid wire on 12V devices, I guess I missed that in the original post.
Bruce
P.S. There have been complaints regarding the push-in connectors that Victron is using on the Multiplus II inverters. I've seen a few pictures of charred lugs on the 120V output side. We wouldn't allow push-in lugs in our specs as it was generally agreed in our office that they sucked.

[bdosborn]

How big of a percentage of failure is acceptable? For trailer manufacturers, from what I've heard, it's incredibly high.

I would be willing to bet you a jelly doughnut that the failures are from terrible installations rather than solid wire. The general build quality for commercial RVs is terrible, isn't that why we're all building our own trailers?

Right to the equipment? That surprises me as the shops I've been in had stranded wire from the walls to the equipment. Probably industry standard, though I never asked. Probably solid wires down the walls to the termination, but again, I never paid that close attention.

I can't remember ever seeing stranded wire in the field that was smaller than a #8. Strain relief was always by flexible conduit to the device if it was high vibration.

P.S. in all my years as a consultant I never had a solid vs stranded discussion with clients, code officials or contractors. It wasn't ever an issue I encountered in the critical facilities I worked on.

Well, it is specified in the code, isn't it? NEC or industrial "equivalent"? (eg NFPA 79)

Certainly the wiring for spacecraft was spelled out in the n'th degree, and it was always stranded.

I checked the NEC 550 RV code and the only place where stranded is required is from the generator to the first point of connection.

E) Supply Conductors. The supply conductors from the engine generator to the first termination on the vehicle shall
be of the stranded type and be installed in listed flexible conduit or listed liquidtight flexible conduit. The point of first
termination shall be in one of the following:
(1) Panelboard
(2) Junction box with a blank cover
(3) Junction box with a receptacle
(4) Enclosed transfer switch
(5) Receptacle assembly listed in conjunction with the generator

However, after checking 552 Park trailers I found the following:

(B) Low-Voltage Wiring.
(1) Material. Copper conductors shall be used for low-voltage circuits.
Exception: A metal chassis or frame shall be permitted as the return path to the source of supply

Conductor Types. Conductors shall conform to the requirements for Type GXL, HDT, SGT, SGR, or Type SXL or
shall have insulation in accordance with Table 310.104(A) or the equivalent. Conductor sizes 6 AWG through 18 AWG or
SAE shall be listed. Single-wire, low-voltage conductors shall be of the stranded type.

So there you have it, per the NEC, LV wiring is required to be stranded in a Teardrop but solid is allowed for 120V.

Bruce

[featherliteCT1]

bdosborn and tom,

Thank you! You guys are giving me quite the education… much appreciated!

[Tom&Shelly]

bdosborn and tom,

Thank you! You guys are giving me quite the education… much appreciated!

You're welcome. I certainly respect Bruce's knowledge of electrical stuff and would generally go with what he says. But I just don't see an advantage to using solid wire other then a very little savings in cost, especially for low voltage applications (where, as Bruce pointed out, stranded seems to be necessary to meet code).

Wrecit, when all is said and done, of course it's your build. I would suggest going ahead and buying stranded wire of the proper gauge. It will make it easier to install and likely to cause less trouble for you down the road. Personally, I have lots of scraps of Romex from various projects and always find it handy to keep around for small household wiring projects.

Incidently, for our build, I used 14 gauge speaker wire for most of the 12 volt stuff. I already had a big spool, so I do use what I have when I can. Saw some posts from way back from someone saying you shouldn't use speaker wire, but he didn't give any reasons worth a darn. For our 120 vac wiring, I bought a 14 gauge extension cord (stranded) and cut it apart.

Tom

[bdosborn]

Thanks guys for a great discussion. Honestly, this is the only forum I would include those details without putting on my asbestos pants first.
Bruce

[Wrecit]

I actually conned wife into letting me buy some stranded for the runs to and from the switch box. Going with the Romex from battery to breaker box though. Went one gage bigger than recommended just to "over build" lol


Will say the stranded for vibration argument doesn't seem to hold water for me. Spent a large portion of my working life working in power plants and guess what kind of wire they use there. There is stranded wire but the individual strands are around 1/8 diameter and used because no one wants to try to bend solid 2 inch diameter copper wire (could you imagine lol).

The discussion/argument on both sides of this discussion may be summed up best by possibly the smartest thing Sigmund Freud ever said. .......sometimes a cigar is just a cigar.........meaning there's not really a right or wrong as long as the insulation on the wire can take the vibration. Personally I went with wire with the harder casing hoping it will fight abrasion better.

[featherliteCT1]

I actually conned wife into letting me buy some stranded