How to Ground Rooftop System?

[jmoles]

I've been doing a lot of reading and have realized there are different types of grounding requirements and I may be getting stuff mixed up.

Is there a "standard" method for grounding a rooftop array with microinverters (hoymiles HMS-1600-4T) that have a ground wire in the trunk cable ?

My thinking right now is this:

I have three things to ground. Microinverters, mounting hardware, and EMT conduit. These may require different wiring and different end points.

The microinverters have a ground wire in the trunk cable, this goes to a junction box, splices to a #10 thwn-2, and through the conduit, combiner box, disconnect box, then to the MSP. Can anything else get grounded through this run?

Now for the hardware, the rails, the pv frames and the other metal pieces, do they need a GEC that runs to a ground rod separate from my msp's grounding/ ground rod? If so, this should be a #6 bare copper down to the ground rod? Should this be run through the EMT conduit? Are there other methods or solutions for grounding this hardware?

I'm using EMT conduit so that needs to be grounded as well? If so how should the EMT be grounded? And I assume I'd need to use jumpers when I hit a plastic junction box, but where would the ground wire terminate?

The help is much appreciated!

 

[zanydroid]

This is what I learned and did on my HM- series install.

BTW it's an EGC in this case I believe.

You can share grounds as much as you want in all situations you listed, as long as the minimum AWG on the path is high enough for the maximum circuit ampacity among the circuits sharing it. Once you get access to a EGC you can splice branches of that to anything. I believe the most natural place for this to happen with HM micros is at the box where your trunk cable terminates. Coming up from the house you will have #10 L L G.

Going out of the box to the trunk you will have 10/2 TC-ER with ground. Add a #6 EGC going out another gland and you can start your array bonding.

So you bond the following 3-4 things
#6 exposed EGC
#10 TC-ER EGC
#10 THHN EGC (going down)
(optionally) bond #10 THHN EGC to bonding bushing attached to the EMT entering this box.

The microinverters have a ground wire in the trunk cable, this goes to a junction box, splices to a #10 thwn-2, and through the conduit, combiner box, disconnect box, then to the MSP. Can anything else get grounded through this run?

You can optionally splice grounds wherever you want, including conduit bodies (if you can fit it). You can even splice grounds on the rail with correct hardware. You are required to splice all traversing grounds at a box if there are any CCCs being spliced in that box.

EGC for a circuit needs to go in the same conduit/wire as the circuit. Except when exposed under the array.

(There is also some PV specific exemption that lets you have separation between CCC and EGC, but it's not useful in rooftop IMO and I don't know if it applies.)

Now for the hardware, the rails, the pv frames and the other metal pieces, do they need a GEC that runs to a ground rod separate from my msp's grounding/ ground rod? If so, this should be a #6 bare copper down to the ground rod? Should this be run through the EMT conduit? Are there other methods or solutions for grounding this hardware?

Sizing it at GEC size is a very old rule. So for this specifically you want to make sure to use recent forum posts / tutorials. There are still some ancient things showing up on Google results for this.

I believe it needed to be sized to your AC service size (#6 works for most? not familiar with the GEC table). For about 10 years I think EGC is sufficient, usually #10 for typical residential system sizes.

#6 is the minimum exposed EGC size for solar. You can drop it to #10 once it leaves the array and goes into conduit.

I'm using EMT conduit so that needs to be grounded as well? If so how should the EMT be grounded? And I assume I'd need to use jumpers when I hit a plastic junction box, but where would the ground wire terminate?

All metal components must be grounded. How I would do it is:

- I don't like bonding through EMT, so there is always an EGC going through it. So I just need to figure out how to bond the EMT to that EGC. I'm not even sure it's code compliant to bond through EMT on a roof (I know it's already prohibited in some situations).
- Regularly bond EMT to EGC traversing it so there is some redundancy, esp when there are fittings breaking the EMT
- EMT can be re-bonded at every box using bonding bushings (these screw onto the EMT fitting that holds the EMT to the box)
- Bonding bushings are also useful for dealing with plastic box gaps.
- Typically bonding bushings are lay-in style (IE you load it in the side rather than threading through) so you just have to strip off a little bit of insulation and slip the bare wire into the bushing.
- I believe the EGC size rule applies to the EMT bonding. EG the bonding path needs to have minimum AWG above the minimum EGC size for the largest circuit, otherwise a fault to the EMT will not clear properly. In this case #10 is fine since the OCPD on Hoymiles is 15A-30A

In short, this ends up with a bonding bushing at a lot of boxes (and they are mandatory on the plastic boxes). If you feel bonding bushing at only one end of a particular segment of conduit feels solid enough, sure that's fine.

random trivia

So microinverters with ground wire can actually be listed with racking to bond between racking and micro. The HM series probably supports this in some jurisdictions, however since I did not see instructions in the US manual for HM nor was HM listed (like Enphase M series was) for bonding this way by my racking manufacturer.

So this is more like trivia / "just in case" redundant bonding, IE every rail that you mount the HM to will have an extra bonding path through the HM (though probably not compliant for US purposes. Also the inverters get moved around while installing or servicing so it feels hacky to depend on it).

(I did verify the bonding continuity through HM frame to the rails)

 

[jmoles]

Thanks for the detailed info. This was very helpful. I was losing my mind with all the stuff I was reading on GEC which I see is now outdated with microinverter installs.

 

[zanydroid]

No problem. I actually looked that up when I noticed the designer for my new system put #6 as the ground size all the way back to the panel. While my previous one had #10. Really did not want to pull nor pay for it. Either a mistake or they didn’t get the memo.

DC can also now use EGC sizing.

 

[jmoles]

What racking hardware did you use?

Iron Ridge says only one grounding lug per subarray is required (and may not be required at all w/ enphase).

So I could basically put a ground lug near the j-box and run the #6 from the lug into the j-box. Seems like that would look pretty goofy but would save a lot of wire.

I see your point with the redundant grounding.

 

[zanydroid]

I used Unirac SM Light. It was one lug per row (but I usually had extra and if there were some suspicious places I added extra lugs. Also sometimes shit happens like messing up a bonding rail splice, time to bust a lug to jump across it).

One row = the rails to hold one row of panels (so usually two rails)
I don't see how you can bond an adjacent row without adding extra lugs

I made a separate gland at my JBox for the #6. The glands for my JBoxes all sit under a panel so they are protected.

I had a pretty annoying layout because of random roof penetrations and skylights, and needing to switch portrait to landscape. For all gaps in the rails I would need to add some lugs + #6.