Wire recommendation from panel

[Teocalli]

I would like to spec in 6/2 or similar from the panel to my CC (why not cable is cheap), the length of the run is going to be maybe 20' ? Might wind up soldering and shrink wrap connection to the existing 12/2 MC4 connections instead of a breakout to a box.

I have been looking at UF-B, THHN UV wire but wondering if someone can point me to a proper cable that will hold up well on the roof of the RV ?

cheers,

Dave

[SunEagle]

I would like to spec in 6/2 or similar from the panel to my CC (why not cable is cheap), the length of the run is going to be maybe 20' ? Might wind up soldering and shrink wrap connection to the existing 12/2 MC4 connections instead of a breakout to a box.

I have been looking at UF-B, THHN UV wire but wondering if someone can point me to a proper cable that will hold up well on the roof of the RV ?

cheers,

Dave

By code you really need to use a cable designed for outside and pv dc voltage applications. They usually have the insulation designation of USE- 2/ RHW-2 which are both UL 4703 listed.

Also any time you solder wire you end increasing the chance of a failure. I would suggest using a small junction box with a UL listed terminal block to land the cable from the panel to whatever you run the longer distance.

[Teocalli]

I wound up ordering UF-B wire 6/3, it has solid conductors, unless I am mistaken stranded is specified in solar simply for ease of use. I don't see how soldering increases the chance of failure especially when I can mechanically strengthen the connection with a loop or some strain relief.

[SunEagle]

I wound up ordering UF-B wire 6/3, it has solid conductors, unless I am mistaken stranded is specified in solar simply for ease of use. I don't see how soldering increases the chance of failure especially when I can mechanically strengthen the connection with a loop or some strain relief.

Solder can actually weaken the wire since it has thermal properties which can make it soften when a lot of current runs through it or become brittle over time and break.

So to be safe it is recommended to use a hydraulic crimping machine to attach a connector or terminal on the end of the wire. That will get you a low resistance connection which is much better in the long run.

[Sunking]

Do not use any solder. Since this is a RV, there are no NEC code requirements. RV's fall under SAE requirements and use different cable insulations, and amperage limits. For RV's and Boats you want to use Marine Grade Wire like 6/2 Duplex Safety wire. It is resistant to oil, grease, fuel, abrasion, Sun Light, and high temperatures. Make sure it is UL1462 and BC5-W2 listed.

[Sunking]

I wound up ordering UF-B wire 6/3, it has solid conductors, unless I am mistaken stranded is specified in solar simply for ease of use. I don't see how soldering increases the chance of failure especially when I can mechanically strengthen the connection with a loop or some strain relief.

Again DO NOT USE SOLDER. It does not have the mechanical strength. or the low resistance of irreversible compression. DO NOT USE SOLID CONDUCTORS, use a Fine Stranded and compression terminals like I listed above. NEC Aircraft, Marine and SAE do not allow soldered connections because they are a Fire hazard and high failure point.

[ncs55]

SunEagle and Sunking are absolutely correct. The reason for using stranded in all DC applications including but not limited to solar, is also because the electrons flow on the outside of each strand in your cable. All cars, boats and RV's use stranded for the DC side. A solid cable will fail at a solder joint eventually due to heat and resistance. You should use a combiner box with fuse provisions for each circuit if you really want to be safe, especially if it is a low voltage charge controller that requires high current. You can use PV wire for the roof, and it is designed for that application. PV_600VLT.pdf

[Sunking]

The reason for using stranded in all DC applications including but not limited to solar, is also because the electrons flow on the outside of each strand in your cable.

Well thanks for the Cudos, but that is not the reason to use stranded wire. AC current flows on the outside, not DC. RV's and Marine use stranded wire to:

1. Resist fatigue failure from vibration and movement.
2. Easier to work with in tight areas and with sharp bend radius.
3. Easier to terminate with lower connection resistance. Also relates to more secure connection like Ring Terminals.

Solid wire is used in smaller gauges because it is less expensive, smaller, and where it will be ran in Raceways secured with no movement or vibration.

Almost forgot another advantage of fine stranded wire is for high frequency application. One would be DC or AC motor power using high frequency PWM speed control. The fine strands off a much larger surface area vs a solid of B Stranded wire.

[ncs55]

Well thanks for the Cudos, but that is not the reason to use stranded wire. AC current flows on the outside, not DC. RV's and Marine use stranded wire to:

1. Resist fatigue failure from vibration and movement.
2. Easier to work with in tight areas and with sharp bend radius.
3. Easier to terminate with lower connection resistance. Also relates to more secure connection like Ring Terminals.

Solid wire is used in smaller gauges because it is less expensive, smaller, and where it will be ran in Raceways secured with no movement or vibration.

Almost forgot another advantage of fine stranded wire is for high frequency application. One would be DC or AC motor power using high frequency PWM speed control. The fine strands off a much larger surface area vs a solid of B Stranded wire.

There seems to be a lot of confusion on this subject and Technicians are taught differently than say a college student. I was taught that Electrons travel in one direction and current travels in the opposite direction. Electrons flow on the surface of the wire and current inside the wire. Every electrician I have ever met or asked this question is very careful about damaging the outside of the wire when stripping the insulation off, and when I ask them why. They tell me because that damage to the outside of the wire will restrict the flow of electrons, causing resistance and heat. I have searched the web and asked this question many times to many people including master electricians and University teachers and it always boils down to the two present theories taught. which are conflicting.

[foo1bar]

Every electrician I have ever met or asked this question is very careful about damaging the outside of the wire when stripping the insulation off, and when I ask them why. They tell me because that damage to the outside of the wire will restrict the flow of electrons, causing resistance and heat. I have searched the web and asked this question many times to many people including master electricians and University teachers and it always boils down to the two present theories taught. which are conflicting.

What is conflicting?

Electricians are careful to avoid damaging the wire.
Damaging the wire does result in a higher resistance in that area than in the rest of the wire.

It's basically the same as changing to a smaller diameter wire and then back.
Wire sizes are often chosen to be as small as possible while having a reasonable safety margin. Damaging the wire uses up some (or possibly all) of that safety margin.

A tiny amount of damage to the wire won't be an issue. May be indistinguishable from the normal variation in wire diameter for a given wire size. But it can be difficult to tell whether visible damage is small (ex. a tiny fraction of the wire diameter) or large and likely to be where things fail in a few years as things heat and cool repeatedly.

And not damaging the wire is also something that is a show of their workmanship. They have lots of practice in stripping wire, so doing it cleanly becomes much easier.

[ncs55]

What is conflicting?.

What is conflicting, are that there are two theories currently taught about electron flow and or current flow, which is what I was trying to explain was taught to me.
There is nothing conflicting about wire stripping or the potential thereof. That was just an example of what I have been told by electricians in the past, more often than not.

[foo1bar]

What is conflicting, are that there are two theories currently taught about electron flow and or current flow, which is what I was trying to explain was taught to me.

There's no conflict between current flow and electron movement.

It's just that electrons were picked to be "negative" and protons to be "positive" - therefore when electrons move in one direction the current is in the opposite direction.

Had there been more knowledge that electrons were the things that moved, maybe they would have been picked to be "positive" - but that wasn't known until a long time after the convention for "positive" and "negative" charge was well established.

http://xkcd.com/567/

[Teocalli]

Stranded is easier to work with and for DC applications there is really no difference between both types , what's important is the voltage drop over my 20' and that is why I am using 6/2 instead of 12/2 from the panel, I did follow Sunking's advice and went with marine 6/2 just to be safe.

Thanks for all the advice everyone but remember it's just a "freakin' cable on top of an RV not a commercial project with 440v 3 phase powering a hospital
Gotta love when a bunch of engineers (myself included) get too far down in the weeds.

cheers,

Dave

[Sunking]

What is conflicting, are that there are two theories currently taught about electron flow and or current flow, which is what I was trying to explain was taught to me.
There is nothing conflicting about wire stripping or the potential thereof. That was just an example of what I have been told by electricians in the past, more often than not.

There is nothing conflicting, nor theory. Electron flow is current. Makes no difference which way you say it flows, reference Electron or Conventional Flow. Only thing that changes between Electron or Conventional is the integers or either positive or negative.

As for what part of a wire current uses is well known. DC currents uses all the cross-sectional area, and AC Currents use the SKIN EFFECT. The higher the frequency to more pronounced the effect is.

As stated both electricians, technicians, engineers, and scientist use great care skinning a wire as to not nick or cut a strand as it affects mechanical strength. If it results in cutting a strand reduces current capacity and mechanical strength. Look in any high frequency application or Lightning protection and signal paths tend to be wide with a lot of surface area due to SKIN EFFECT. Power Frequencies 300 Hz and less) have very little Skin Effect.

Now there are special circumstances in Motors and Controls where High Stand Count in wires is important. Most DC Motors, and variable speed AC motors use a form of PWM that operates at High Frequencies of several Khz between 10 and 20 Khz.

But for the most part stranded wire is used sparingly because it is much more expensive, and larger than its solid counter part. Therefore it is reserved for larger sizes because anything above 4 AWG is impossible to work with unless your knuckles drag the ground when walking upright, in tight spaces, Lot of bends and turns, and vibration/movement is encountered.

I can only suggest is if you perceive conflicting information is you have been misinformed by people who do not know what they are talking about. Do a simple search Stranded vs Solid Wire, and Skin Effect. There is no conflicting information or theory. It is physical facts .

Hope that helps Norman.

[inetdog]

They tell me because that damage to the outside of the wire will restrict the flow of electrons, causing resistance and heat.

In the case of small nicks which do not significantly reduce the cross section, that is garbage.

Particularly with solid wire a nick forms a stress concentration which makes it much more likely that the wire will break at that point under bending and vibration.