Bonding ends of DC Conduit?

[Fortunat Mueller]

Hello all,

I have a question about both Code and Physics as it relates to bonding of metal conduit used on the DC side of a solar system.

While the code is clear that any ferrous piping containing the Grounding Electrode Conductor needs to be bonded to the GEC at both ends 250.64(E) / 250.92 (B)(2)-(4), i don't find any similar requirement for the DC piping (which contains solar circuit conductors and an equipment ground but no GEC).

We have generally made it a practice to ground both ends of our DC EMT runs even though it does not appear to be a requirement of the NEC, based on the assumption that lightning induced induction is also possible on that section of pipe and inductive choking would be an issue there as well.

Is our practice overkill? Can anyone explain to me why there is no requirement for bonding metal conduit on the DC side of the solar system? Are those conductors less likely to have a lightning related surge in them that might be affected by inductive choking?

Thanks in advance for your thoughts and insight

~Fortunat

www.revisionenergy.com

 

[petersonra]

my understanding is that it is only required at a service because there is a good chance of getting a surge from the utility. an SDS like a PV system just does not have any external source of surges.


if lightning hits any of your electrical equipment it is toast anyway. does not matter much if it it is grounded or not or whether the GEC is bonded.

 

[SolarPro]

Bonding bushings are required for PV systems with a maximum system voltage over 250 V to ground per 250.97:

250.97 Bonding for Over 250 Volts. For circuits of over 250 volts to ground, the electrical continuity of metal raceways
and cables with metal sheaths that contain any conductor other than service conductors shall be ensured by
one or more of the methods specified for services in 250.92(B), except for (B)(1).

 

[Zee]

Yes, as SolarPro noted, they are required.

If circuit is over 250V and conduit terminates:
in air
or concentric
or eccentric knockout
or hub
(basically not a hammered-on lock-ring into original maximum sized or drilled hole)
===> you must bond with gr. bushings.

Be aware that the zinc-plated screws on the bushing (both the setscrew and the lug-mounting screw) along with the aluminum bodied lay-in-lugs provided by mfctr on the grounding bushings are not rated for our conditions of use - outdoors.
I highly recommend stocking and using (2) stainless steel screws (to mount lug and also as a setscrew on bushing) and direct burial rated lugs (tin plated copper lugs) to replace the original on the bushing.

When a copper wire is laid in the original lug the galvanic corrosion that arises in the presence of moisture between the copper wire, the lug and the steel set screw looks bad even within one year.

 

[petersonra]

Yes, as SolarPro noted, they are required.

If circuit is over 250V and conduit terminates:
in air
or concentric
or eccentric knockout
or hub
(basically not a hammered-on lock-ring into original maximum sized or drilled hole)
===> you must bond with gr. bushings.

Be aware that the zinc-plated screws on the bushing (both the setscrew and the lug-mounting screw) along with the aluminum bodied lay-in-lugs provided by mfctr on the grounding bushings are not rated for our conditions of use - outdoors.
I highly recommend stocking and using (2) stainless steel screws (to mount lug and also as a setscrew on bushing) and direct burial rated lugs (tin plated copper lugs) to replace the original on the bushing.

When a copper wire is laid in the original lug the galvanic corrosion that arises in the presence of moisture between the copper wire, the lug and the steel set screw looks bad even within one year.

I do not believe that if what you are saying is true that your solution is acceptable.

The bushing is a listed part. You have to use it IAW with the manufacturer's instructions. If they say it is not suitable for this use, you can't just modify the part and claim it now is suitable.

 

[petersonra]

Bonding bushings are required for PV systems with a maximum system voltage over 250 V to ground per 250.97:

250.97 Bonding for Over 250 Volts. For circuits of over 250 volts to ground, the electrical continuity of metal raceways
and cables with metal sheaths that contain any conductor other than service conductors shall be ensured by
one or more of the methods specified for services in 250.92(B), except for (B)(1).

I thought the OP was asking about the conduit the GEC is in and not the circuit conductors.

 

[jaggedben]

I do not believe that if what you are saying is true that your solution is acceptable.

The bushing is a listed part. You have to use it IAW with the manufacturer's instructions. If they say it is not suitable for this use, you can't just modify the part and claim it now is suitable.

One could probably argue 90.4 on this. IOW, show me a grounding bushing that comes with a lug that's suitable for exposed use.

 

[Zee]

I do not believe that if what you are saying is true that your solution is acceptable.

The bushing is a listed part. You have to use it IAW with the manufacturer's instructions. If they say it is not suitable for this use, you can't just modify the part and claim it now is suitable.

1. Yes, you are correct.

2. Is it also conceivable that grounding bushing manufacturers don't have a product for PV and it's weather challenges?

There is no acceptable means for bonding conduit on a roof.

Being able to withstand conditions of use is a fundamental req. that I will adhere to - your literal interpretation notwithstanding.

 

[Zee]

I thought the OP was asking about the conduit the GEC is in and not the circuit conductors.

He actually seems to conflate the two: DC and GEC in conduit.

 

[GoldDigger]

He actually seems to conflate the two: DC and GEC in conduit.

There is also the potential requirement for a separate DC GEC and/or DC Ground Electrode in a solar PV system.