9.8kW DIY Grid-tie - Question about Metal Clad cable in attic and general feedback on my design?

[JoelE]

I've done my best to make this install as cheap and easy for myself as possible, while still attempting to meet all the NEC 2020 code requirements as far as I've read them.

I wanted to solicit your feedback on my current design (note: there should be a junction box between array and disconnect for the roof penetration pass through of DC).



On the roof:

• 2 strings of 11 panels each - Canadian Solar BiHiKu CS3W-445MB-AG 445W - 10AWG PV wire connecting the 2 strings to the Soladeck
• Tigo TS4-A-F at each panel for RSD
• Unirac Solarmount system with Flashloc attachments (so I don't have to mess with getting under shingles at all except for the junction box)
• Soladeck or EZ Solar XL junction box to pass through the 2 strings through the roof

In the attic:

• 10/4 w/ ground Metal Clad cable, runs from roof junction box all the way to outside under the patio - Red and black will be used for string 1 and 2 hot side.

Outside (under patio):

• 10/4 w ground MC runs down from attic to DC disconnect switch
• 10/4 w ground MC to Growatt MIN 7600 TL-XH-US Inverter, connecting both strings
• Inverter to AC Disconnect with 8 AWG copper in conduit.
• AC Disconnect to existing sub-panel with 8AWG copper in conduit
• Existing Sub-panel runs to Service Panel with 6AWG copper

Outside (At meter):

• 6x6 placard at the Meter with a sitemap to the location of the disconnect. Meter is approx 10ft from the disconnect switch.

Few things to note:
1. I have a split bus main service panel, and I'm relying on the exception in NEC 2020 that allows for this. No plans to upgrade the MSP.
2. I have an existing 50a branch originally permitted for a hot tub install, which I plan to derate to 40a for my inverter which has a max output of 32a. The hot tub is decommissioned but the sub-panel is in a perfect spot for me to mount my inverter near and backfeed into. The 40a backfeed breaker will sit on the 100a busbar, protected by a 70a breaker on the main (200a) busbar. [(70 + 40) = 110a. 110 < (100*1.2)]

Questions:
1. Any reason I can't use this particular MC and run it all the way from soladeck to the DC disconnect, and can you recommend a conduit connector for it for attaching to the soladeck?
2. For the PV wire MC4 connections at the strings, can I run the cable with MC4 connector on one side, and directly into the soladeck on the other side? For example, if I buy an 1 extension cable of sufficient length with MC4 already crimped on each side, can I cut it in half and save myself the need to buy a crimping tool or any MC4 connectors? So I'd have on string one the negative side running back to the jbox. On string 2, I'd have the positive side running back to the jbox?
3. If you think my plan to use the split bus panel is not to code, can you please give a reference supporting that? Or if you think I should do a supply side tap instead, keep in mind I'm getting this hybrid inverter with a plan to expand into a battery system in the future. And I prefer to not have to shut down all the power to my home and make things more complicated for myself unless I have to.

 

[Madco]

You will need to check the ampacity of the #10.

You'll have 4 CC (current carrying) conductors in the raceway, so that is one adjustment. Check NEC Table 310.15(B)(1).
Should be a factor of 0.8.

Then, you'll have elevated temperature in the attic, that's another adjustment. Check NEC Table 310.14(C)(1). This one depends on the wire type in the MC- could vary from 0.41 to 0.76.

The MC should be OK (with the above factors) as long as it complies with NEC 240.118(10).

 

[Madco]

The temperature derates assume a 130F attic, BTW.

 

[hwy17]

690.31(D) Direct-Current Circuits on or in Buildings.

Where inside buildings, PV system dc circuits that exceed 30 volts or 8 amperes shall be contained in metal raceways, in Type MC metal-clad cable that complies with 250.118(10), or in metal enclosures.

www.electricallicenserenewal.com

 

[zanydroid]

Red and black will be used for string 1 and 2 hot side.

This wording indicates a misunderstanding… PV conductors these days are generally ungrounded. So both sides are hot. That said you can relabel the whites to ungrounded conductors since they are in a cable assembly. You will need to do a derating calculation but for 4-6 #10 in a bundle it’s usually fine.

I wouldn’t think MC is cheaper, I think it is faster though than conduit. If you use MC or FMC please be mindful of the protection requirements (guard strips and running boards) that they require if you don’t use EMT or better.

Did you consider TIGO with monitoring? I know it costs like $500 more but knowledge is power, ?

I used the same Unirac flash feet. I used EZ Solar XL instead of soladeck. I know people that like the Dinkle DIN mount terminals for DC connections.

Not sure where your busbar question was. NEC has progressively become more lenient on mid-fed buses.

 

[zanydroid]

For MC into the back of the Soladeck/EZ solar, find an FMC fitting for conduit of the same diameter as that multi conductor cable. Buy a couple 3/8, 1/2, and 3/4 and see how they spark joy. My bet is on 1/2 being the right one.

 

[zanydroid]

Your bus calculations look good. I believe the latest NEC allows 120% rule in center fed MSP, so 1.2*200-200 gives you the 40 you need. The only failure modes I see is if you misidentify the MSP you have or misread the specs therein.

Line side tap opens up POCO can of worms. We can’t answer those for you unless someone that sees your post accidentally happens to be in your POCO and attempted it.

I don’t see solar string ampacity calculations, with the 1.56 and CCC and temp derates.

 

[JoelE]

This wording indicates a misunderstanding… PV conductors these days are generally ungrounded. So both sides are hot. That said you can relabel the whites to ungrounded conductors since they are in a cable assembly. You will need to do a derating calculation but for 4-6 #10 in a bundle it’s usually fine.

I wouldn’t think MC is cheaper, I think it is faster though than conduit. If you use MC or FMC please be mindful of the protection requirements (guard strips and running boards) that they require if you don’t use EMT or better.

Did you consider TIGO with monitoring? I know it costs like $500 more but knowledge is power, ?

I used the same Unirac flash feet. I used EZ Solar XL instead of soladeck. I know people that like the Dinkle DIN mount terminals for DC connections.

Not sure where your busbar question was. NEC has progressively become more lenient on mid-fed buses.

I can get a roll of 50ft of this for $110, and that will be enough for what I need. Also - I've never worked with EMT or flex, I would have to do more learning re: pulling wire, etc. I'm not familiar with the protection requirements... Can you point me to where I can find that? Can I just secure it under the rafters, at least where it sits under the panels?

 

[JoelE]

Your bus calculations look good. I believe the latest NEC allows 120% rule in center fed MSP, so 1.2*200-200 gives you the 40 you need. The only failure modes I see is if you misidentify the MSP you have or misread the specs therein.

Line side tap opens up POCO can of worms. We can’t answer those for you unless someone that sees your post accidentally happens to be in your POCO and attempted it.

I don’t see solar string ampacity calculations, with the 1.56 and CCC and temp derates.

The thing that worries me is that it is a split-bus panel without a single main breaker - GE TX2420 made in either 1977 or 1978 I think. I'm not worried that it has 'worn out' or anything, I just worry that the inspector won't like it. NEC 2020 says that the overcurrent protection device for the MSP is the "up to 6" breakers - And in my case I do have 6.

 

[zanydroid]

I can get a roll of 50ft of this for $110, and that will be enough for what I need. Also - I've never worked with EMT or flex, I would have to do more learning re: pulling wire, etc. I'm not familiar with the protection requirements... Can you point me to where I can find that? Can I just secure it under the rafters, at least where it sits under the panels?

MC is the better choice for sure, and that seems like a good price. Took me forever to do my EMT/FMC installation.

This is in 690.31(D)

(1) Flexible Wiring Methods. Where flexible metal conduit (FMC) smaller than metric designator 21 (trade size ¾ ) or Type MC cable smaller than 25 mm (1 in.) in diameter containing PV power circuit conductors is installed across ceilings or floor joists, the raceway or cable shall be protected by substantial guard strips that are at least as high as the raceway or cable. Where run exposed, other than within 1.8 m (6 ft) of their connection to equipment, these wiring methods shall closely follow the building surface or be protected from physical damage by an approved means.

You would also have to find and follow the general MC rules, the ones above are PV specific. For NM if you run it "perpendicular" to rafters you need running boards. Even if MC does not require it, I would personally do it, because MC isn't really stronger than NM. If you use good self-drilling screws like GRK cabinet screws (I would think the #8 1.5" ones would be great) they'll go through the rafters and 1x4 boards like butter.

 

[JoelE]

For MC into the back of the Soladeck/EZ solar, find an FMC fitting for conduit of the same diameter as that multi conductor cable. Buy a couple 3/8, 1/2, and 3/4 and see how they spark joy. My bet is on 1/2 being the right one.

I'll make sure to get the right size, I just wasn't sure if I need a water tight fitting or something with the box being on the roof?

 

[zanydroid]

The thing that worries me is that it is a split-bus panel without a single main breaker - GE TX2420 made in either 1977 or 1978 I think. I'm not worried that it has 'worn out' or anything, I just worry that the inspector won't like it. NEC 2020 says that the overcurrent protection device for the MSP is the "up to 6" breakers - And in my case I do have 6.

Oh that is different, that is hot bus with split. Can you post a picture? The bottleneck for me is that I have to convert your written description to a picture, and some days that is too much mental labor...

 

[zanydroid]

I'll make sure to get the right size, I just wasn't sure if I need a water tight fitting or something with the box being on the roof?

EZ solar instructions says you need to use a watertight fitting. You won't find one for NM or MC because those are not wet listed.

My take: (1) it doesn't exist, which should be a good stopping point for thinking. (2) if water leaks there, you got bigger issues than not using a wet listed fitting. (3) if you use MC it'll tolerate the water a lot more than NM (even if it's not listed for it), so you're better than code minimum.

I did not use watertight fittings because I did not always use wet-rated cable/conduit on the bottom side of the EZ solar.

 

[zanydroid]

I'll make sure to get the right size, I just wasn't sure if I need a water tight fitting or something with the box being on the roof?

Another thing with FMC fittings is that they do have a pretty wide range of ergonomics... I'd recommend buying like two or three kinds and playing with it.

My standard thing on my install this year (the only one I've done and probably the last I'll ever do) is:
- close conduit nipple held in place by lock nuts. If you use longer ones you'll have a bitch of a time getting the EZ solar to go in, might rip shingles.
- screw on a female Rigid coupler to the bottom
- screw on FMC or EMT fitting on the bottom, or directly attach a box

Compared with doing FMC or EMT directly onto the bottom - the advantage is that you get a lot more room to adjust.

But I recommend buying all the parts and playing with them in hand until it makes sense.

 

[JoelE]

MC is the better choice for sure, and that seems like a good price. Took me forever to do my EMT/FMC installation.

This is in 690.31(D)

(1) Flexible Wiring Methods. Where flexible metal conduit (FMC) smaller than metric designator 21 (trade size ¾ ) or Type MC cable smaller than 25 mm (1 in.) in diameter containing PV power circuit conductors is installed across ceilings or floor joists, the raceway or cable shall be protected by substantial guard strips that are at least as high as the raceway or cable. Where run exposed, other than within 1.8 m (6 ft) of their connection to equipment, these wiring methods shall closely follow the building surface or be protected from physical damage by an approved means.

You would also have to find and follow the general MC rules, the ones above are PV specific. For NM if you run it "perpendicular" to rafters you need running boards. Even if MC does not require it, I would personally do it, because MC isn't really stronger than NM. If you use good self-drilling screws like GRK cabinet screws (I would think the #8 1.5" ones would be great) they'll go through the rafters and 1x4 boards like butter.

You will need to check the ampacity of the #10.

You'll have 4 CC (current carrying) conductors in the raceway, so that is one adjustment. Check NEC Table 310.15(B)(1).
Should be a factor of 0.8.

Then, you'll have elevated temperature in the attic, that's another adjustment. Check NEC Table 310.14(C)(1). This one depends on the wire type in the MC- could vary from 0.41 to 0.76.

The MC should be OK (with the above factors) as long as it complies with NEC 240.118(10).

As for ampacity of the #10 - If this table is right I think I should be OK?

30*.8= 24 and my short circuit current is 11.54a. Run in series, each string with 11 panels, it should be open circuit voltage 538v, 11.54a. I suppose I could even drop to 12awg? I don't have the specs of this particular MC, but looks like solid copper if that makes a difference.

 

[JoelE]

EZ solar instructions says you need to use a watertight fitting. You won't find one for NM or MC because those are not wet listed.

My take: (1) it doesn't exist, which should be a good stopping point for thinking. (2) if water leaks there, you got bigger issues than not using a wet listed fitting. (3) if you use MC it'll tolerate the water a lot more than NM (even if it's not listed for it), so you're better than code minimum.

I did not use watertight fittings because I did not always use wet-rated cable/conduit on the bottom side of the EZ solar.

I agree - I guess I'll just try to get one where it's not sticking out too far so I can actually slip the box on more easily.

 

[JoelE]

Oh that is different, that is hot bus with split. Can you post a picture? The bottleneck for me is that I have to convert your written description to a picture, and some days that is too much mental labor...

 

[JoelE]

MC is the better choice for sure, and that seems like a good price. Took me forever to do my EMT/FMC installation.

This is in 690.31(D)

(1) Flexible Wiring Methods. Where flexible metal conduit (FMC) smaller than metric designator 21 (trade size ¾ ) or Type MC cable smaller than 25 mm (1 in.) in diameter containing PV power circuit conductors is installed across ceilings or floor joists, the raceway or cable shall be protected by substantial guard strips that are at least as high as the raceway or cable. Where run exposed, other than within 1.8 m (6 ft) of their connection to equipment, these wiring methods shall closely follow the building surface or be protected from physical damage by an approved means.

You would also have to find and follow the general MC rules, the ones above are PV specific. For NM if you run it "perpendicular" to rafters you need running boards. Even if MC does not require it, I would personally do it, because MC isn't really stronger than NM. If you use good self-drilling screws like GRK cabinet screws (I would think the #8 1.5" ones would be great) they'll go through the rafters and 1x4 boards like butter.

So... I have to drop down from the rafters a bit with a 2x4, and secure every 6 ft. with a strap? Or do I only need to drop down in areas where there are no solar panels above it? I'm actually meeting with my AHJ in a couple weeks to go over my plan and ask them specifically, but want to be sure I have all my questions lined up for that meeting. I don't see 'guard strips' here in the pic below, and I'm not sure what that means.

 

[zanydroid]

As for ampacity of the #10 - If this table is right I think I should be OK?
View attachment 162912
30*.8= 24 and my short circuit current is 11.54a. Run in series, each string with 11 panels, it should be open circuit voltage 538v, 11.54a. I suppose I could even drop to 12awg? I don't have the specs of this particular MC, but looks like solid copper if that makes a difference.

You need to multiply ISC by 1.56

1.25 for continuous and 1.25 for meteorological conditions boosting output above STC.

Solid vs stranded makes no difference.

Do not consider aluminum for this application.

 

[zanydroid]

So... I have to drop down from the rafters a bit with a 2x4, and secure every 6 ft. with a strap? Or do I only need to drop down in areas where there are no solar panels above it? I'm actually meeting with my AHJ in a couple weeks to go over my plan and ask them specifically, but want to be sure I have all my questions lined up for that meeting. I don't see 'guard strips' here in the pic below, and I'm not sure what that means.

View attachment 162920

That picture might be of 1” FMC or bigger.

Guard strip means a length of surrounding wood that is higher than the MC or FMC to protect from impact.

Running board means a board that runs parallel to and “behind/above” the cable. Actually it’s possible a guard strip counts as a running board.

There should be code book guides with pictures showing how this simple carpentry is done.

Do you have the MC in hand already? Handling it may give you a better idea why it needs to be protected. 600V of DC is no joke.

Another possible approach is to sleeve in SCH80 but that would be quite expensive and time consuming compared to wood. Wood is easy esp if you can scale down the size. 2x1 or 2x2 sounds way easier than 2x4. 1x may or may not be thick enough to protect your MC, you will have to check the OD of the MC.