To Transition or not to Transition, PV wire to THHN

[Skypower]

I’ve found thhn to be much easier to pull in a conduit and less expensive than PV wire. PV wire is better for UV exposure than thhn. So for me logic dictates the use of a transition box to take advantage of each wire’s attributes.

 

[garybryan33]

PV cables when greater than 32 volt must be in metal conduit when on the roof and when entering and inside a building
PVC conduit can be used outside to bring the PV cables to the house JBox converting to metal

Could Liquidtight Flexible Metal Conduit be used throughout the installation? There seems to be some discrepancy as to whether MC Cable can be run exposed to the weather.

 

[timselectric]

Could Liquidtight Flexible Metal Conduit be used throughout the installation?

Yes

 

[seneysolar]

I’ve found thhn to be much easier to pull in a conduit and less expensive than PV wire. PV wire is better for UV exposure than thhn. So for me logic dictates the use of a transition box to take advantage of each wire’s attributes.

How is the transition made? What connectors are you using in the box?

 

[Zwy]

Could Liquidtight Flexible Metal Conduit be used throughout the installation? There seems to be some discrepancy as to whether MC Cable can be run exposed to the weather.

You can run PVC conduit outside of a structure.

 

[garybryan33]

How is the transition made? What connectors are you using in the box?

Din rail terminal block

 

[Zwy]

How is the transition made? What connectors are you using in the box?

What you can do it run PV wire into a box, either NEMA 4 and install a junction block or you can use something like a PV disconnect such as the IMO. Mount the IMO outside of the structure, then run metal conduit into the structure. This allows you to not only transition the wire but also the conduit.

The other choice is a box like the Vevor I used. You mount a DIN rail inside with a breaker and SPD's. You make the transitions there, mount it outside the structure and transition to metal conduit into the structure.

 

[garybryan33]

You can run PVC conduit outside of a structure.

I was under the impression I could not run it if the voltage was higher than 32 volts, had to be MC.

 

[Zwy]

I was under the impression I could not run it if the voltage was higher than 32 volts, had to be MC.

Outside the structure can be PVC.

Inside a structure metal.

 

[timselectric]

I was under the impression I could not run it if the voltage was higher than 32 volts, had to be MC.

It doesn't have to be in metal conduit until it enters the house.

 

[garybryan33]

What you can do it run PV wire into a box, either NEMA 4 and install a junction block or you can use something like a PV disconnect such as the IMO. Mount the IMO outside of the structure, then run metal conduit into the structure. This allows you to not only transition the wire but also the conduit.

The other choice is a box like the Vevor I used. You mount a DIN rail inside with a breaker and SPD's. You make the transitions there, mount it outside the structure and transition to metal conduit into the structure.

View attachment 194584

Thank you for your input. I was planning on running the PV wire into a NEMA 4 junction box to make the transition to THHN. The IMO disconnects are located as soon as the wires come into the structure. The run from the roof to the point where they enter the structure is about 15ft. Everything being equal I would just assume to run the Liquidtight Flexible Metal Conduit from start to finish. An spd did not cross my mind, I am in California. Should this be of concern?

 

[garybryan33]

It doesn't have to be in metal conduit until it enters the house.

Okay. It's becoming clear. It's better to keep asking and avoid a costly mistake. Thank you

 

[seneysolar]

Din rail terminal block

Ahh ok. Ive always used those midnite combiners w/breakers to make the transition. I just didnt have a better idea without long runs of PV wire.

Im glad you didnt say wire nuts

 

[garybryan33]

Ahh ok. Ive always used those midnite combiners w/breakers to make the transition. I just didnt have a better idea without long runs of PV wire.

Im glad you didnt say wire nuts

Even if I taped the wire nuts? kidding!

Those Midnite boxes are pretty nice, but in my case, the breakers on the roof are not readily accessible. I running IMO DC breakers inside the structure.

 

[timselectric]

Im glad you didnt say wire nuts

Why?
Perfectly fine.
That's what I have used.

 

[zanydroid]

Yes, MC will meet the NEC code for PV. I run MC to my charge controllers, it is shown in a video about the EG4 MPPT's I did last year.

Just remember to use the plastic bushing on the ends to prevent wire chafing.

MC Run on the roof though???

 

[zanydroid]

(Metallic liqudtite)

Yes

It will have a lower temperature rating though. 60C when wet. This will be quite relevant for temp and CCC derating

 

[seneysolar]

Why?
Perfectly fine.
That's what I have used.

I feel like i opened a can of worms here, but really??
Im just curious because I was taught to steer clear of wire nuts for DC

 

[timselectric]

I feel like i opened a can of worms here, but really??
Im just curious because I was taught to steer clear of wire nuts for DC

No idea why that would be.
It's the one connection least likely to loosen over time.

 

[garybryan33]

(Metallic liqudtite)

It will have a lower temperature rating though. 60C when wet. This will be quite relevant for temp and CCC derating

Another consideration!

Let's say the standard ampacity for a 10 AWG THN wire with 60-degree Celsius insulation is 30 amperes. Applying the derating factor:

Rated Current Capacity=Ampacity×Derating Factor

Rated Current Capacity=30A×0.70=21A

Each string is producing about 14 amps, so I should be okay. Am I missing something?