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spark88

Wire Management of PV on Tiled Roof

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Hey all,

As I have some time to burn with the lockdown, I'm busy setting up my PV panels on our tiled roof.

At the moment I'm busy with all the brackets. So far so good ... all pretty straight forward stuff.

I'm planning ahead, and I'm trying to work out the best way to handle all the strings that need to come down from the panels.

Around the whole roof we have a gutter and all the gear is located 2 stories below in the garage, so I'm kinda limited to take things into the roof.

 

So was thinking about the following setup:

Mount a junction box to the roof brackets somehow.

From the junction box run some flexible conduit to the MC4 connectors on the panels to give me the most protection on the panel side

From the junction box use flexible conduit, hooking up to the main conduit that rests in the gutter. It will run in the gutter until the point where I need to bring it down to the MPPT's

 

Any other thoughts ? Pics would be great too.

Thanks

Mark

 

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On 2020/04/25 at 11:28 AM, FixAMess said:

As per the stds, all DC cables must run in steel conduit, not PVC

Are you sure this is a regulation in South Africa? I spoke to my solar installer and according to him PVC conduit is acceptable inside the roofspace.

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Hi,

yes and no. See below, its a SANS DRAFT.

DRAFT: SANS 10142-1-2:20XX
Edition 1
33
The connection to the array frame shall be in line with the manufacturer's instructions, and shall be designed and constructed to minimize the effects of corrosion. This shall include selection of materials and components that minimize the effect of galvanic corrosion. Plated or un-plated copper tubular cable lugs shall not be used for connection onto aluminium rails.
The cable shall link to each electro-mechanically interconnected array block (typically a row of modules on a pair of aluminium mounting rails) such that all parts of the solar PV array frame are connected to earth.
c) Conduits
Where conduit is used for DC cables, it shall be metal conduit. Conduit shall be bonded at all junctions to ensure electrical continuity of the conduit. The metal conduit shall be bonded to earth at a suitable location.

SANS-10142-1-2-Rev-R-01102018 Solar.pdf

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2 minutes ago, FixAMess said:

yes and no. See below, its a SANS DRAFT.

Thanks. That seems a bit draconian, given that 220V AC in PVC is apparently just fine. My plan is to run two separate conduits, one with + cables and a separate one for the - cables for my four strings. All the + and - cables will be shorted together in the combiner box anyway (after the fuses/isolators), so this should be quite safe. There will be 4 strings, so 4 cables in each conduit, 10A @ 200V DC.

My application has been approved by CoCT, but I haven't actually started installing - the lockdown has put a stop to that. The big question on my mind now is that if this draft standard becomes final while I'm busy installing, would I have to comply with it? PVC conduit is so much easier to work with (and cheaper).

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I think the metal conduit requirement has to do with EMC rather than anything else. The MPPTs switch at 40kHz or thereabouts, which can cause radio interference. If you put the wiring inside metal conduit and earth it as properly as the SANS document says, it ends up as a kind of screening against the electromagnetic radiation.

Other than EMC, I also don't see any reason why PVC isn't sufficient.

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On 2020/03/30 at 10:21 PM, spark88 said:

It will run in the gutter

I'm not terribly confident that this (actually very common!) trick is allowed. I would much rather make some kind of a plan to push the gutter away slighty from the wall, and then push/squash a bit of flexible conduit between the wall and the gutter so the wiring can pass behind it.

About wiring in a gutter... moved into a new house. Found the most interesting wiring job. One of the points on the Quad-LNB (satellite dish) went into the attic using an air hole. There is a geyser (water heater) in the attic, and the drip tray has a thick pipe running outside into a gutter. So the coax able from the LNB is then fed through the drip tray drain pipe into the gutter outside, across to the edge of the building where it meets the flatlets in the back, then upwards over the side into some trunking... down the wall on the outside, and then drilled straight through into the next room. I ripped that cable out already... guests can watch Showmax and Netflix for all I care 🙂

 

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35 minutes ago, PierreJ said:

Thanks. That seems a bit draconian, given that 220V AC in PVC is apparently just fine. My plan is to run two separate conduits, one with + cables and a separate one for the - cables for my four strings. All the + and - cables will be shorted together in the combiner box anyway (after the fuses/isolators), so this should be quite safe. There will be 4 strings, so 4 cables in each conduit, 10A @ 200V DC.

My application has been approved by CoCT, but I haven't actually started installing - the lockdown has put a stop to that. The big question on my mind now is that if this draft standard becomes final while I'm busy installing, would I have to comply with it? PVC conduit is so much easier to work with (and cheaper).

Check the std, the +ve and -ve are supposed to run in the same conduit, there is a reason but I can't look  through the std now (I'm in a meeting..)

Also , the steel conduit must also be grounded to earth (real earth, like in the garden), its all in the stds.

 

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South African Standard SANS 60364-7-712:2018 (which is based on the international standard IEC 60364-7-712:2017) - titled "Low voltage electrical installations - Part 7-712: Requirements for special installations or locations — Solar photovoltaic(PV) power supply systems" does not have any requirement for metal conduit.

Section 712.522.101 states that "All non-metallic cable management systems exposed to sunlight shall be of a UV resistant type"

I think that SANS 60364-7-712:2018 is likely to be the "applicable regulation" on this issue, but I am not 100% certain. 

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Hi,

you are correct, the standard I quoted is still in draft format, so probably does not apply at this point in time, but it may be worthwhile taking what is stipulated there into account if you are doing a brand new installation.

 

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On 2020/04/29 at 7:21 PM, FixAMess said:

the standard I quoted is still in draft format, so probably does not apply at this point in time, but it may be worthwhile taking what is stipulated there into account if you are doing a brand new installation.

I've read through the draft document, and the rules seem kind of strange:

In section 5.3.5.2 it states "Where conduit is used for DC cables, it shall be metal conduit."

Then in section 6.2.6 it states "Where a system features longer DC cables (e.g. >50m), consideration shall be given to the use of screened / armoured cables, or to installing the cable in earthed, metal conduit / trunking."

And again in section 6.2.8 there is the wording "Where a system features DC cables longer than 50m, the cables shall be installed in earthed metal conduit or using armoured cable."

So what I understand from this is that PVC conduit is never allowed. If you put the cables in conduit it has to be metal, and above 50m runs using such conduit is mandatory. Why the hate of PVC? Surely in runs under 50m PVC is better than nothing?

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On 2020/04/29 at 3:54 PM, FixAMess said:

Check the std, the +ve and -ve are supposed to run in the same conduit, there is a reason but I can't look  through the std now

The exact wording in section 6.2.6 is: "Positive and negative cables shall be run alongside each other, to prevent the formation of induction loops;"

If I put the two metal conduits next to each other, touching, will that pass muster? It just seems safest to me to put a physical barrier between the +ve and -ve cables.

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17 minutes ago, PierreJ said:

So what I understand from this is that PVC conduit is never allowed.

It seems that you are allowed to use PVC trunking if it is short enough (and due to other rules, if it is indoors).

5.3.5.2 says that IF conduit is used, it must be metal (so your reading is correct).

6.2.8 says that if the cable is longer than 50m, you must use conduit (not something else).

15 minutes ago, PierreJ said:

If I put the two metal conduits next to each other, touching, will that pass muster? It just seems safest to me to put a physical barrier between the +ve and -ve cables.

If you've seen PV cable, you'll see it is already double insulated and good up to 1000V. I don't think an additional barrier is necessary. That is also why you fuse this stuff. Additionally, you have to think of the conduit as the core of a transformer. With both wires inside the same core, any inductive action (there will be some, due to the high frequency switching of the MPPT) cancels out. But placing two transformer cores side to side does not have the same effect. I don't think this is a good idea.

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9 minutes ago, plonkster said:

If you've seen PV cable, you'll see it is already double insulated and good up to 1000V.

I haven't actually. I have ordered 6mm^2 cable online (which is massive overkill for the 10A it will be carrying), but I've never held it in my hands. Perhaps once I see it I'll be reassured.

9 minutes ago, plonkster said:

I don't think an additional barrier is necessary.

I'm leaning towards not using conduit at all now. I'm slightly worried about rodents gnawing through the cables, but if I look around in my ceiling I see plenty of tasty AC cables that have been there for at least a decade that haven't been touched.

Thanks for the advice.

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9 minutes ago, PierreJ said:

I haven't actually.

Aaah good. Because if you did, what I wrote would have been a tad condescending. Which of course was not intended, sometimes the outgoing filter just isn't quite there... 🙂

PV cable has an outer black layer (or sometimes a different colour, but black usually), and once you strip this away there is a second tougher and thicker white layer. If you use a wire-stripper to strip it (some people just use a sharp knife like a carpaet knife) it is quite common for the black layer to pull off easily, and then the white layer takes a bit more work. It reminds me somewhat of the inner insulation in a TV coax cable, though not quite as plasticky.

I figured a picture is worth more than all my words above. So I went and took one for you. This is 4mm^2 cable, somewhat badly stripped, but you get the idea.

pv-cable.jpg.06c887a3265422aaccc8068c01c958e6.jpg

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