When to require bonding both ends of a home-run conduit to the EGC?

[LarryFine]

. . . we more often have plastic enclosures on both ends.

Now you tell us!

 

[photonboy]

You should have included that in your opening post. Why would anyone use metallic raceways between plastic boxes or enclosures? There may be bigger problems here than bonding.

It is very common in residential and small commercial PV installations to have plastic junction boxes on the roof, usually under the array with metal conduit jumping between mounting planes and taking the home-run down to the equipment wall. This is EMT across the roof or metallic flex if run through the attic. Most of the systems I review are either SolarEdge, which has plastic inverter wiring compartments, or Enphase which has plastic combiner boxes. So the majority of the conduit until we get to the AC disco is going from one unbonded enclosure to another.

Apologies for not clarifying that, it was in my head, couldn't you see that?

 

[LarryFine]

So the majority of the conduit until we get to the AC disco is going from one unbonded enclosure to another.

The NEC only requires that one end be bonded, wire EGC or not. Anything more is design choice.

GECs on the other hand do require bonding both ends to minimize choke-effect impedance.

 

[jaggedben]

I work for a solar financing company and I'm currently wearing two related hats, working on design standards and reviewing PV systems post install. ...

I see. Having been on both sides of this issue, my advice is to drop the both ends requirement. It is quite natural for the contractors to hate you for calling for a truck roll to fix such a minor and debatable item before they get paid (if I'm catching the nature of the situation), especially if your requirements that exceed code weren't put in 20pt bold font at the top of all your pre-install communications. If you're going to insist, then at least be willing to let each installer off with a warning the first time, and make sure your exact specs are clear beforehand. This is not something where you can appeal to 'standard industry practice' or some such phrasing; it's too debatable for that.

 

[jaggedben]

You should have included that in your opening post. Why would anyone use metallic raceways between plastic boxes or enclosures? There may be bigger problems here than bonding.

PVC is really unacceptable in rooftop installations in this area. It's been known to degrade in the sun, it droops and touches the roof, and it looks like absolute s---. (You can't paint it to make it look nicer either, the paint peels off.) EMT is industry standard. Plastic boxes on the roof are also pretty common because you don't get metal 4X boxes that are big enough and using 3R on the roof can lead to water coming down the raceway.

One code exceeding practice that *is* industry standard is to always run a wire EGC, so that bonding the array is not dependent on the EMT couplings. And installers know to install a bonding bushing on the EMT in the inverter or combiner. So we are really just talking about whether we have to bond the upper end of the raceway that might, in a fluke, get both separated from the lower part and energized. Not to jinx myself but I have yet to see that happen. Do I think it's good practice to put a bonding bushing at the top? Sure. Do I think payments should be held up over it? Generally no.

 

[Zee]

We and many others use "Bell" boxes on the roof. They have threaded openings for conduit. Just like an LB. No bonding bushing needed. Not even possible.
Also, they are more durable and last longer than PVC boxes.
On the other end, the bottom end of the conduit will alway have a bonding bushing as it enters a non conductive box.
As a result, if you insist on "Bell" boxes then problem solved: the conduit run will be solidly grounded on both ends.

(also a lot easier to install than unweildy NEMA3R gutterboxes.
Hides / fits better under array.
A HUGE advantage is the square opening can face up towards the sky where your installer is, not downhill like a typical NEMA 3R metal box. Less back breaking awkward work a few inches from roof surface.)