Need advice on wire sizes

[donb108]

Last year, I installed a completely stand-alone system on my shed. The shed is insulated, wired, has ground rods, inverter, battery, etc.; it works great. But, I don't use most of the power it could be making, so I'm changing my setup and rewiring the panels to connect to a new system I'm planning for the house. I'll be runnnig the sheds PV wires to the new inverter, and power wires from the inverter back to the shed. We also want to add cameras to the shed, so I'll include a fiber optic line in the conduit.

The wire run is about 235', with 135' in underground PVC conduit (house to shed). I think I need 7 conductors (power, PV, fiber), but I don't have much experience with long runs, so I used online calculators to size wires and conduit.

Do these results look reasonable?

Panels:
All 8 shed panels are connected in series (panel VOC is 41.75Vdc). Typical running PV voltage is 230Vdc-280Vdc and Isc is 11.25amps. They're currently wired with 12 gauge PV wire.

-The calculator says to use 10 gauge, so I'll use (2) 10 AWG THWN stranded (19 strands).

Power:
The existing breaker box is feed with 10 AWG, directly from the EG4 3K inverter (at 120Vac) to a 30 amp main breaker in the panel. I'll replace those wires with a feed from the new inverter. I'll also add a conductor and change to 240Vac. Plugging 240Vac, 5%, and 30 amps into the calculator:

-The Southwire calculator says to use 8 gauge, so I'll use (3) 8 AWG THWN stranded (hot, hot, neutral) + (1) 10 AWG THWN stranded (ground).

Conduit:
The fiber optic cable is 5mm, which is similar to 8 AWG. I calculated for PVC (sch 40), with (4) 8 AWG and (3) 10 AWG.

-The Southwire conduit calculator says to use 1" PVC, with fill of 24.48%.

For reference:
Panels on a shed:

D

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• donb108
• Replies: 13
• Forum: DIY Solar General Discussion

Raising panels:

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• donb108
• Replies: 17
• Forum: DIY Solar General Discussion

• 

Solar Wire Size Calculator

The Solar Wire Size Calculator

solarwiresizecalculator.com

Southwire: Calculate Your Voltage Drop

Voltage Drop Calculator | Southwire

www.southwire.com

Southwire: Calculate Your Conduit Fill Percentage

Conduit Fill Calculator | Southwire

Get accurate conduit fill percentages with Southwire's Conduit Fill Calculator. Perfect for electricians, engineers, and contractors.

www.southwire.com

 

[robbob2112]

Don't forget an EGC in your run - you will disconnect and abandon any ground rods out at the shed.... there can be only one place where you connect to earth ground.

For PV wires you can afford a little loss due to the length and it won't hurt anything. The larger the wires the less loss there will be.

Do use a SPD at both ends to protect from nearby lighting strikes -- won't protect from a direct hit but will protect from picking up voltage on the long antenna

You can't generally run AC and DC lines inside the same conduit. And the fiber optic should be either direct bury or in its own conduit separated by 12" from the other two. Best practice is to have the AC and DC conduit separated by 6" in the trench.

The southwire voltage drop calculator is for AC wire typically... but it is a good idea of DC

 

[donb108]

Sorry for going dark on you, I got distracted.

Don't forget an EGC in your run

I just want to be clear here; is the EGC simply the ground conductor? If I bought romex 12/2 with ground, it's "that" ground...right?

you will disconnect and abandon any ground rods out at the shed

I assumed that would be the case. There's currently a ground bar that has several things landed on it: the inverter EGC, battery EGC, a conductor coming from the PV panel frames, and a 6 AWG bare conductor that goes to the ground rods. If I remove the 6 AWG from that ground bar, the ground rods are abandoned...correct?

For PV wires you can afford a little loss due to the length and it won't hurt anything. The larger the wires the less loss there will be.

Right, I'm going with one gauge larger to account for the extra length.

Do use a SPD at both ends to protect from nearby lighting strikes

I don't currently use them, but it's probably a good idea. I was looking at these:

Amazon.com: MidNite Solar Surge Protection Device MNSPD-300-AC: Electronics

Buy MidNite Solar Surge Protection Device MNSPD-300-AC: Surge Protectors - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

You can't generally run AC and DC lines inside the same conduit. And the fiber optic should be either direct bury or in its own conduit separated by 12" from the other two. Best practice is to have the AC and DC conduit separated by 6" in the trench.

I was pretty surprised by this, so did some reading on the forum. It seems to be a grey area, and highly influenced by the practices of the AHJ. Here's a thread discusssing it:

T

Thread 'AC/DC Buried Conduit Separation'

Jun 17, 2024

How much separation does there need to be for buried PVC conduits containing PV DC circuits and 220 V AC circuits?

Thinking ahead towards winter and the limitations of my setup with my panels being on a roof with 4/12 pitch. My wife suggested tying in the panels from my first solar setup that are mounted on a shed 250' away that are mounted on the vertical part of a gambrel roof. If I'm trenching I might as well run power over there as I'll be disabling the solar setup that's there currently. My plan is 4 wires for PV (Do I need a ground wire in both conduits?) and 5 wires for AC (red...

• teal95
• Replies: 17
• Forum: DIY Solar General Discussion

I started this thread thinking I could simply run some wires in preparation for my new system, but then realized I may need a permit, and inspections, etc, etc. Thanks for the help, but I'll park this part of it until getting a handle on that stuff.

 

[robbob2112]

Sorry for going dark on you, I got distracted.


I just want to be clear here; is the EGC simply the ground conductor? If I bought romex 12/2 with ground, it's "that" ground...right?

If you are running romex between the mppt and the panels the ground would be the EGC. But, let's say you plan 2 strings and will home run them to the house. You could pull 5 strands of THHN in conduit. Two pairs of current carrying conductors and a single EGC. The only rule is the EGC is required to be in the same conduit as the pv wires. So if you need to use 2 conduits, one for AC and one for DC, you will end up with separate grounds in parallel.

I assumed that would be the case. There's currently a ground bar that has several things landed on it: the inverter EGC, battery EGC, a conductor coming from the PV panel frames, and a 6 AWG bare conductor that goes to the ground rods. If I remove the 6 AWG from that ground bar, the ground rods are abandoned...correct?

Rights you disconnect from the ground rod.... now don't let this confuse you... but you could put a SPD out at the shed and tie that ground and only that ground to the rod you have there.... this is because the ground on the SPD is not connected unless there is a high voltage arc through the MOV...

Right, I'm going with one gauge larger to account for the extra length.


I don't currently use them, but it's probably a good idea. I was looking at these:

Amazon.com: MidNite Solar Surge Protection Device MNSPD-300-AC: Electronics

Buy MidNite Solar Surge Protection Device MNSPD-300-AC: Surge Protectors - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

I was pretty surprised by this, so did some reading on the forum. It seems to be a grey area, and highly influenced by the practices of the AHJ. Here's a thread discusssing it:

T

Thread 'AC/DC Buried Conduit Separation'

Jun 17, 2024

How much separation does there need to be for buried PVC conduits containing PV DC circuits and 220 V AC circuits?

Thinking ahead towards winter and the limitations of my setup with my panels being on a roof with 4/12 pitch. My wife suggested tying in the panels from my first solar setup that are mounted on a shed 250' away that are mounted on the vertical part of a gambrel roof. If I'm trenching I might as well run power over there as I'll be disabling the solar setup that's there currently. My plan is 4 wires for PV (Do I need a ground wire in both conduits?) and 5 wires for AC (red...

• teal95
• Replies: 17
• Forum: DIY Solar General Discussion

I started this thread thinking I could simply run some wires in preparation for my new system, but then realized I may need a permit, and inspections, etc, etc. Thanks for the help, but I'll park this part of it until getting a handle on that stuff.

Very much AHJ and how they closely they follow the NEC.

But, there is a practical reason why you run 2 conduits and separate them. The same voltage can magnetically couple across to the dc wires if they are in the same conduit. So you end up with an AC voltage on your PV wires... do this all the time and it can fry the SPD....

 

[donb108]

If you are running romex between the mppt and the panels the ground would be the EGC.

No, not using romex, just used that as an example; I should have been more clear. I'll be using THHN/THWN in conduit.

So if you need to use 2 conduits, one for AC and one for DC, you will end up with separate grounds in parallel.

Good point, I wondered about that.

So you end up with an AC voltage on your PV wires... do this all the time and it can fry the SPD....

Ohhh, didn't realize that, thanks for pointing it out.