Ungrounded Array Wire type question

[seaton]

690.35(D) specifies the use of PV wire for ungrounded pv systems. My question is this.
My past installers have always just run EMT, and run pv wire from roof (soladeck) all the way to the inverter(s). They included a bare wire #6AWG copper ground.

My new installer is running 10AWG PV wire to the Soladeck.
10AWG MC from the soladeck across the attic to the JBOX in the EVE.
Running 10AWG THWN from the Jbox down the side of the house in EMT into the inverter.

Is this sound and permitted ? I know emt can be a pain, but it almost seems better to me to just stick with PV wire the whole way. I know that is compliant, and less connections etc.
I know anything on the roof has to be PV rated, but MC and THWN are permissable for a 600V ungrounded system ?

Thanks,
Steve

 

[Carultch]

690.35(D) specifies the use of PV wire for ungrounded pv systems. My question is this.
My past installers have always just run EMT, and run pv wire from roof (soladeck) all the way to the inverter(s). They included a bare wire #6AWG copper ground.

My new installer is running 10AWG PV wire to the Soladeck.
10AWG MC from the soladeck across the attic to the JBOX in the EVE.
Running 10AWG THWN from the Jbox down the side of the house in EMT into the inverter.

Is this sound and permitted ? I know emt can be a pain, but it almost seems better to me to just stick with PV wire the whole way. I know that is compliant, and less connections etc.
I know anything on the roof has to be PV rated, but MC and THWN are permissable for a 600V ungrounded system ?

Thanks,
Steve

There is no rule against using THWN-2 or any cable assembly built with it, for an ungrounded PV system, once you are inside a raceway or inside a cable assembly suitable for the environment where installed.

PV wire is only specifically required for ungrounded systems where the wire is open air and exposed. By contrast, their grounded system counterparts can use USE-2/RHW-2 wire in place of the added standards of PV wire. Once it is inside a raceway, any wire is permitted for either grounded or ungrounded systems, if it is suitable for the voltage, ampacity, temperature, wetness where applicable, and other Chapter 3 rules. If you have a system in excess of 600V, chances are, the only 1000V rated wire you could get in practice, is PV wire, even though the insulation would be overkill for being used inside a raceway.

As for inside the building, your DC circuits have to be enclosed in metal. So the EMT, FMC, MC or other metal raceway/wiring method would be part of the installation, regardless of whether you are using continuous PV wire or spliced THWN-2 wire.

One thing to consider is that your array wiring will vary in length among all the source circuits. So you'd need to leave dissimilar lengths of wire (or unified excess wire) on the roof, as part of your pull, if you don't intend to install splices. Splicing allows you to do separate actions of pulling a group of wires uniform in length through the building conduit in a separate task from arranging the array wiring on the rail system.

 

[electrofelon]

If you have a system in excess of 600V, chances are, the only 1000V rated wire you could get in practice, is PV wire, even though the insulation would be overkill for being used inside a raceway.

Are you saying that PV wire has a thickness of insulation, beyond the dielectric requirements, for physical/environmental protection ?

 

[peterlarsen.suntec]

I did a one day training session with the guys from Solectria. They specified 1000v rated (PV, USE-2) wire for runs all the way to the inverter. The reason was to avoid isolation faults.
The SolarEdge inverters test for isolation as part of the inverter startup. A test from the DC current carrying conductors to ground of 600 Kohms or less will trigger an isolation fault 25. I had experiences last year of going in behind an installer that used 10-2 solid MC cables on DC home runs. I found multiple isolation faults. Some were just nicked insulation at the jboxes used to transition from MC to EMT/THWN at the eaves. I replaced lots of MC with EMT or FMC and pulled THWN. My current employer will not let me use PV, USE-2 all the way home. Its 3X the cost. If it was up to me I would run a single pull PV wire all the way home with a MC4 connection in the Soladeck. I learned a practical lesson repairing all those isolation faults. I will never use 10-2 MC for DC source circuits. As I understand it, the ground in a 10-2 MC cable is undersized for a PV set. The guys running MC would run a bare #6 in free air all the way to the AC disconnect and then bond it to the GEC. The ground is to be no less than a #8 but you cant leave any less than #6 in free air. Just seems bass ackwards to me. Comments?

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[jaggedben]

Having a bad experience with MC doesn't mean you have to pull PV wire to the inverter. We've done hundreds of installations with THWN-2 in EMT and FMC from the roof to inverter and never had an isolation fault attributable to wiring method. The only isolation fault I've had to deal with was on an array and it was the PV wire that got nicked somewhere - god knows where - and when it got wet the moisture created a 1 megaohm fault to ground. I've dealt with a couple other faults that were splice failure or installer error. Thanks for sharing your experience though, I will stay away from MC.

 

[jaggedben]

Are you saying that PV wire has a thickness of insulation, beyond the dielectric requirements, for physical/environmental protection ?

I believe that is correct, I've been told that numerous time. But I haven't seen the standard.

 

[Carultch]

Are you saying that PV wire has a thickness of insulation, beyond the dielectric requirements, for physical/environmental protection ?

Yes. Compare 600V PV wire to the same size in its counterpart of XHHW-2 that has insulation made out of the same base material, but only thick enough to withstand the voltage rating with a standard safety factor. PV wire is significantly thicker.

XHHW-2 is designed for use inside a raceway, while PV wire is designed for direct burial, passing a flame test, and additional physical abuse that would be expected when installed exposed along a PV racking system.

 

[electrofelon]

Yes. Compare 600V PV wire to the same size in its counterpart of XHHW-2 that has insulation made out of the same base material, but only thick enough to withstand the voltage rating with a standard safety factor. PV wire is significantly thicker.

XHHW-2 is designed for use inside a raceway, while PV wire is designed for direct burial, passing a flame test, and additional physical abuse that would be expected when installed exposed along a PV racking system.

Makes sense, I guess I always was focused on the voltage rating as determining the insulation thickness but
i just looked at a bunch of specs and saw how variable it is. For example the insulation thickness changes dramatically just as the wire size changes. The insulation on a 4/0 XHHW is almost twice as thick as a #12, 55 and 30 thousandths respectively. USE-2 #12 is .045, and 600V Pv #12 is a bout .060.

 

[Zee]

PV Wire
spliced to MC cable
spliced to THHN/THWN-2 in EMT
......seems like too many splices. That equals too much work, extra boxes and splicing..... AND more potential failure points, also, to me.
Plus now you gotta pull wire in j-boxes inside attic. UGGGGHHH.:rant::rant:
Or mount a box outside and pull to top of ladder on ext. wall...not too goodlooking and difficult.

1.
Why not combine the last two runs? You can just run flex metal conduit instead of MC cable to get same ease of install benefit.....then run that straight into EMT. Then you can pull a single THWN-2 thru the flex and EMT. Done. Just my two cents.
(or avoid the attic altogether and go all EMT over and off of roof - if not too ugly)

I do not own MC cable

2. In rare, and lucky, cases, Go ahead and run PV wire all the way home if it's real close.
E.g. single-story home with inverter on wall, right below array.
Bonus: Don't even have to bother with a j-box on roof. Just bend EMT-end into drip loop, strap it to rail, add an RT conn., and add grounding bushing to that.

 

[Zee]

I second or third what the guys said: PV wire inside EMT is overkill.
(EMT is a tougher shell than any plastic insulation anyways.)

Do it only if it saves you TIME.

(possibly the solaredge issue that was brought up..... but i have never experienced that on resi., 600 vdc systems)

 

[Carultch]

I second or third what the guys said: PV wire inside EMT is overkill.
(EMT is a tougher shell than any plastic insulation anyways.)

Do it only if it saves you TIME.

(possibly the solaredge issue that was brought up..... but i have never experienced that on resi., 600 vdc systems)

It is overkill, as in it has more material than you really need for the application.

In sub-600V applications, THWN-2 or XHHW-2 are good default wire types to specify for use inside raceways.
In 1000V applications, KV PV wire is more than you need for inside a raceway, but it is all that you can get in practice, since there has yet to be a THWN-KV or XHHW-KV product developed.

 

[Carultch]

As I understand it, the ground in a 10-2 MC cable is undersized for a PV set. The guys running MC would run a bare #6 in free air all the way to the AC disconnect and then bond it to the GEC. The ground is to be no less than a #8 but you cant leave any less than #6 in free air. Just seems bass ackwards to me. Comments?

#8 is the minimum for the ground only when it is a GEC. Not for an ordinary EGC.

The #6 requirement is specific to the case of "subject to physical damage". Exposed ground wire in a PV array is one of those cases where half the people will say it is, and half will say it isn't, "subject to physical damage".

For inside a raceway, the EGC only needs to be as large as 250.122 dictates when there is an applicable OCPD. When there isn't, it is 1.25*Isc that governs this sizing from Table 250.122. Or max continuous current for a power optimizer system.

 

[peterlarsen.suntec]

#8 is the minimum for the ground only when it is a GEC. Not for an ordinary EGC.

The #6 requirement is specific to the case of "subject to physical damage". Exposed ground wire in a PV array is one of those cases where half the people will say it is, and half will say it isn't, "subject to physical damage".

For inside a raceway, the EGC only needs to be as large as 250.122 dictates when there is an applicable OCPD. When there isn't, it is 1.25*Isc that governs this sizing from Table 250.122. Or max continuous current for a power optimizer system.

Thanks for clearing that up. I have been pulling a #8 EGC. I agree that PV wire is overkill in a metallic raceway. Just repeating what the Solectria trainer specified. Funny we are having this conversation. Yesterday I repaired an isolation fault on the a PV wire under the array.

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