Should I Pre-Wire Panels Before Putting on Roof?

[AgroVenturesPeru]

This is international, and US codes don't apply, but that said, all the youtube videos from the USA say that you can't have your PV cables touching the roof. Why not? Is the roof going to heat up to the point to fry them? How would you keep them from touching the roof? It sounds like a tall order in my situation.

I've done an internet search and looked at youtube videos, but haven't found anything that addresses my particular situation.

I am going to put sixteen PV modules on the roof of the house we're building. 4s4p. I've got some roof mount structures to put them on. It's a corrugated metal roof (see photos), and local builders recommend placing boards on this kind of roof instead of walking directly on the roof. So, already it sounds like the install is going to be time consuming, having to constantly move boards around when needing to move from point A to point B on the roof.

The PV modules, like most, have their positive and negative MC4 connection on the underside. I have plenty of MC4 connectors in a separate bag, and am also equipped with a 100 meter roll of PV red cable and a 100 meter roll of PV black cable. I'm going to have to cut & strip the cables and crimp on the MC4 connectors myself, which I have the tools to do all that. However, I'm just not sure I should work on all that up on the roof. Also the VOC rating is 49V, so it would be a bit dangerous to connect wires to a live panel. Although the MC4 connectors are meant to protect you from touching bare wire, right?

Do you recommend doing a general calculation of the length of each piece of cable ahead of time; and then cut, strip and crimp a male or female MC4 before getting on the roof? If I were to do it this way, I'd have to add a little slack to each length of cable and triple check that my calculation is correct god forbid I don't cut a long enough section of wire.

I've yet to see a single video of someone unrolling a spool of cable on the roof and connecting it to the undersides of the panels and then wiring the home run. Most tutorials online show optimizers and pre-wired Mine's an off-grid system, and the spacing is rather unconventional.

 

[snoobler]

I would do whatever it takes to avoid fabricating connectors up on the roof. I would not be concerned about the high voltage aspects of the work due to the nature of the MC4 connectors.

If described, I've missed it. I hope that you are somehow mounting only to the beams supporting the roof and not the sheet metal.

 

[AgroVenturesPeru]

I would do whatever it takes to avoid fabricating connectors up on the roof. I would not be concerned about the high voltage aspects of the work due to the nature of the MC4 connectors.

If described, I've missed it. I hope that you are somehow mounting only to the beams supporting the roof and not the sheet metal.

Yes, I'm mounting to the five beams that you can see in my first photo. This does requires perforating the sheet metal with the screw from the structure's L feet.

Out of the whole project this is the part that causes me to hesitate the most. I'm not sure how to proceed.

So, your recommendation is to measure out each length of wire that I will need and prepare the wires with their corresponding male or female MC4 before I head up to the roof? Maybe I'll list each wire out on paper ahead of time with its length and weather one end should be male or female, and if it should be red wire or black wire? Then I could label each section of wire, and that way I can just request "wire 1" or "wire 23" from my helper.

This sounds difficult. I hope I don't mess this up and use a black wire where I should've planned for a red wire or something like that.

If there's a little bit too much slack, I can always loop the extra slack and secure it to the panel support rail with a ziptie. So that's not much of a problem. The problem would be if I didn't make the section of cable long enough.

 

[HateGrid]

Normally you clip the wires to the frame using spring clips so you can take up any slack by moving the clips. You can use stainless cable ties as well. Plastic cables ties will become brittle and fail after a few years. One reason for not having wires dangling on the roof is that wind will move the wire around and eventually wear through the insulation.


Your roof is a gentle slope, so that makes it easier. The span on the purlins looks like 900mm which is will within spec for what looks like a trapazoidal roof (T-rib). Walk on the ribs and you should be fine.

As @snoobler mentioned, you will want the L-feet to be drilled into your purlins. They looks like RHS purlins - probably a couple of mm thick, so metal self-drilling roofing screws will hold very well.

This sounds difficult. I hope I don't mess this up and use a black wire where I should've planned for a red wire or something like that.

Just draw it out, maybe put a few panels up to get a hang of it, and then fabricate the rest on the ground. You shouldn't be able to easily mess up the positive and negative wiring since the males are typically positive and females are negative.

Just give yourself plenty of time. The first couple of panels will take 75% of time time and the rest will take the remaining 25% of the time.

 

[AgroVenturesPeru]

Normally you clip the wires to the frame using spring clips so you can take up any slack by moving the clips. You can use stainless cable ties as well. Plastic cables ties will become brittle and fail after a few years. One reason for not having wires dangling on the roof is that wind will move the wire around and eventually wear through the insulation.
View attachment 47900

Your roof is a gentle slope, so that makes it easier. The span on the purlins looks like 900mm which is will within spec for what looks like a trapazoidal roof (T-rib). Walk on the ribs and you should be fine.

As @snoobler mentioned, you will want the L-feet to be drilled into your purlins. They looks like RHS purlins - probably a couple of mm thick, so metal self-drilling roofing screws will hold very well.


Just draw it out, maybe put a few panels up to get a hang of it, and then fabricate the rest on the ground. You shouldn't be able to easily mess up the positive and negative wiring since the males are typically positive and females are negative.

Just give yourself plenty of time. The first couple of panels will take 75% of time time and the rest will take the remaining 25% of the time.


View attachment 47901

Thanks for the reply. I feel reassured.
Yes, we are using self-drilling roofing screws. And yes the purlins have a wall thickness of 2mm. No, not 900mm. The purlins are spaced at about 1100mm (I just measured this yesterday). The handyman here now is familiar with working on this type of roof, and he suggests walking on wooden boards in the valleys of the sheet metal, because otherwise you can compromise the integrity of the existing screwpoints where the roofing was attached to the purlins, and this can cause leaks in the roof.

I think you're right. Probably the most important thing is to not be in a hurry.

 

[pollenface]

View attachment 47901

Gosh, save it for the bedroom....

 

[AgroVenturesPeru]

Gosh, save it for the bedroom....

LOL

 

[AgroVenturesPeru]

Probably a dumb question. When I have the panels side by side, is it OK to just link the negative (female) MC4 connector directly to the positive (male) MC4 connector of the next panel? (I'm talking about the cables and connectors that you can see in my photo that are already incorporated in the PV module.)

Or do I need to add a separate stretch of wire for some reason in between the two?

 

[MarkSolar]

Probably a dumb question. When I have the panels side by side, is it OK to just link the negative (female) MC4 connector directly to the positive (male) MC4 connector of the next panel? (I'm talking about the cables and connectors that you can see in my photo that are already incorporated in the PV module.)

Or do I need to add a separate stretch of wire for some reason in between the two?

You can connect them directly to each other, no extra wire needed. But be careful, with your Voc and string configuration, you won't be able to connect them during the day when the sun is on the panels. You'll get a big arc that will probably melt the connectors. Do it late in evening or early morning, check the connectors with a meter to make sure there isn't more than a volt or two being produced.

 

[HateGrid]

You can connect them directly to each other, no extra wire needed. But be careful, with your Voc and string configuration, you won't be able to connect them during the day when the sun is on the panels. You'll get a big arc that will probably melt the connectors. Do it late in evening or early morning, check the connectors with a meter to make sure there isn't more than a volt or two being produced.

You will only get an arc if there is a load. You should have the disconnect switches off until you are ready to energize the array.

The typical approach is to leave an accessible PV panel connection disconnected until all of the wiring is complete and then you can do the final connection to complete the circuit. You do not want to be crimping connections on live wires. The panels should be isolated (+/- are separate from the grounded frame), so in theory you cannot be shocked if you touch an exposed wire, but I wouldn't risk it.

Probably a dumb question. When I have the panels side by side, is it OK to just link the negative (female) MC4 connector directly to the positive (male) MC4 connector of the next panel? (I'm talking about the cables and connectors that you can see in my photo that are already incorporated in the PV module.)

Definitely use the leads on the panel. The fewer the connections, the better. You will want to cut off the plastic cable ties and use stainless clips or stainless straps to hold the wire. You do not want to coil wires together since that can cause additional heat to build up.

 

[Substrate]

This is international, and US codes don't apply, but that said, all the youtube videos from the USA say that you can't have your PV cables touching the roof. Why not? Is the roof going to heat up to the point to fry them? How would you keep them from touching the roof? It sounds like a tall order in my situation.

Although your situation with well-elevated panels is different, just in case DIY lurkers are wondering.

1) Yes, panels shouldn't be simply placed on a roof with no elevating structure of many inches to provide air cooling. There are regulations for your region, but this isn't about that.

2) The more common problem, is poor connections like MC4's just laying on a roof surface.

Thing is, MC4's are *supposed* to be standard right? Even if they are meticulously made by the DIY'er, the actual tolerances between the connections can be high, especially if using an off-brand, or merely mixing brands!

An example of this problem is this seemingly intelligent guy going into detail about how ultimately his MC4 connectors were at fault. Although he's an rv-boondocker, having an MC4 connector go into nuclear-meltdown on top of his roof shows the problem - finding it near the 8 minute mark:

He sussed out his situation very intelligently. Nice vid.

So while he's got a metal roof, imagine if those out-of-tolerance MC4 connectors were laying on an old shingle-roof!

This is why when DIY'ing, I always advise the use not just of multimeters to check things out, but supplement your install inspection with inexpensive IR handheld thermometers, and do a baseline check and record of values with a load on it.

Had this guy used a cheap IR gun with a laser beam and sharks.. Wait - just a simple inexpensive IR handheld thermometer you can get from a home improvement store, he might have been able to spot this early after install, and not risked burning his RV down.

No drama here, it's just cheap and easy to do a final IR-gun temp check for added piece of mind and safety.

 

[AgroVenturesPeru]

You will only get an arc if there is a load. You should have the disconnect switches off until you are ready to energize the array.

Where are the disconnect switches located? Is that something on the PV modules?

 

[AgroVenturesPeru]

What is the proper handling of the solar panels? How fragile are they? Is it like handling a glass window?

How can we pass them up to the roof to each other?

 

[AgroVenturesPeru]

You will want to cut off the plastic cable ties and use stainless clips or stainless straps to hold the wire. You do not want to coil wires together since that can cause additional heat to build up.

How about these? They're nylon zip ties. I can't imagine these would break in my lifetime, especially on the underside of a panel out of the sunshine.

 

[HateGrid]

No nylon ties unless they are truly UV stable and designed for solar. Just get some stainless steel cables ties. They come in silver and black and if you can't find them locally, just order them from Aliexpress. Same for the clips.

The panels are just like single-glazed (paned) glass windows and typically in the 15 to 25kg range. You should be able to pass them up or just hire a lift of some type (boom lift, scissor lift, wifey lift). The last one is expensive and has a limited operating range.

Where are the disconnect switches located? Is that something on the PV modules?

No, these are specific switches that are used to disconnect the panels from the inverter for maintenance. Some codes require them to be outside and accessible for the fire department. If you can have exposed cables, something like this works:



Have a good read through the forum and take a look at Will's videos on Youtube and check out the DIY Solar Blueprints section which links to https://www.mobile-solarpower.com/.

Please educate yourself before you go playing with the wiring and come up with a good plan including how you are going to work on it without it being live. DC power can be very dangerous if you do something wrong. The main differences between the AC wires in your house and the DC solar wiring is:

1. DC solar wiring normally runs at maximum load whenever the sun is bright, your household wiring almost never runs at full rated power for longer than a few minutes
2. High voltage DC (say anything over 120V) will sustain an arc. Once the arc starts, it will require a large air gap before it stops.
3. Because of the arc issue, you have to very carefully read the voltage ratings for circuit breakers, switches, and fuses and make sure they are rated for the maximum voltage that you can encounter.

That said, solar panels themselves are current limiting. You can never get more current out of them than the short-circuit current, so you can design your wiring sizing, etc around that and you don't have to worry about fault currents.

Once you get to the battery side then you have the fun of extremely high fault currents (e.g. a short-circuit). The short circuit current can easily reach 20,000 A which is the limit for most mid-grade switches, fuses, and circuit breakers. If a fuse is rated for too low of a voltage, then it may arc continuously after blowing. If it is rated for too low of a fault current then it could explode or catch fire.

 

[AgroVenturesPeru]

Normally you clip the wires to the frame using spring clips so you can take up any slack by moving the clips. You can use stainless cable ties as well. Plastic cables ties will become brittle and fail after a few years. One reason for not having wires dangling on the roof is that wind will move the wire around and eventually wear through the insulation.

I can see a cable tie chaffing the wire insulation significantly more than a smooth roof surface like the one I have.

Why would a cable tie even need to be UV resistant if it is underneath the panel and never exposed to the sun?

I'm not trying to be smart, it's just that shopping here is a PITA, and we haven't found that stuff locally. Or even on the website of the biggest home improvement chain store, Sodimac. Importing stuff down here is even more of a PITA. It might be easier to just go up on the roof after ten years and replace the nylon zip ties should they require replacement.

This is the best I could find online. It doesn't even look like you could tighten them though.

Amarracables de Acero inoxidable – Eesa Perú

eesaperu.com

 

[HateGrid]

If you don't mind replacing them every 5 to 10 years and you do not need to pass an inspection, then the nylon cables ties should work. The stainless clips and cable ties are designed to last the lifetime of the solar array, so they are still the preference.

Since you have limited materials available and will be tempted to do just enough to get it working, please do remember to ground the frames of the panels with a separate grounding wire.

Edit: Looks like you have already asked this question here. Great!

 

[AgroVenturesPeru]

You will only get an arc if there is a load. You should have the disconnect switches off until you are ready to energize the array.

The typical approach is to leave an accessible PV panel connection disconnected until all of the wiring is complete and then you can do the final connection to complete the circuit. You do not want to be crimping connections on live wires. The panels should be isolated (+/- are separate from the grounded frame), so in theory you cannot be shocked if you touch an exposed wire, but I wouldn't risk it.


Definitely use the leads on the panel. The fewer the connections, the better. You will want to cut off the plastic cable ties and use stainless clips or stainless straps to hold the wire. You do not want to coil wires together since that can cause additional heat to build up.

I will be making the wire lengths with mc4 connectors in advance. Am I still at risk for electric shock if I hook up the pieces of cable with MC4 connectors on both ends during the middle of a sunny day? I'm not talking about connecting the panels together in series, and then adding a wire to parallel the series, where you would then have the other end of the wire outputting about 196V during the peak of the day. I just want to measure a lenght of wire, crimp both ends with MC4s, and then attach it as an intermediary between panels.

4 panels in series with 41.6Vmp. Wouldn't the other end of that wire be deadly?

For the home runs that connect to my string box, I'll probaby just wait until the sun goes down and then cut those to size, strip, and MC4 crimp so that they can attach to the MC4 inputs of the string box. But everything else, I'd like to attach during the day, if safe.

 

[HateGrid]

Wouldn't the other end of that wire be deadly?

Yes, any exposed wire should be treated as deadly once you have more than 2 panels in series. However, the conductors inside of the MC4 connector are recessed, so they are safe if you are just connecting them. Disconnecting under load is not safe as there will be a DC arc. It also helps that the panels should be isolated (frame is not connected to - or +), so you would need to touch both the + and - PV wires to get shocked unless there is a wiring fault. So while in theory you are safe, I would NEVER cut or crimp wires attached to solar panels or attach an MC4 wire that is not attached at the other end. You only need to make a fatal mistake once.

For the home runs that connect to my string box, I'll probaby just wait until the sun goes down and then cut those to size, strip, and MC4 crimp so that they can attach to the MC4 inputs of the string box. But everything else, I'd like to attach during the day, if safe.

Just attach to your isolation switch first, turn the switch off, run the wiring up to the roof, cut to length, crimp the MC4 onto it, and then plug it in to the solar panel. No working in the dark that way.

 

[AgroVenturesPeru]

Is this equivalent to an isolation switch?

How to tell if a circuit breaker is in the ON or OFF position?

Is this photo showing a circuit breaker in the On position or in the Off position?

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