Balancing system configuration: Wire Size vs Extra Costs

[Murby]

Putting in a 6.9KW ground mount grid tie system.
Looks like there's going to be about 200 feet of wire between my solar panels and my utility meter / breaker box.

If I mount the inverter at the panels and run my 240vac back, I have to run some sizeable wire to the house in order to avoid large voltage drops.

I could avoid the large voltage drops by running higher voltage DC to the house and put the inverter in the basement or on the side of the home, but then I have to deal with the New NEC codes requiring something called Rapid Shutdown..

What are you folks doing?

[ButchDeal]

We generally install SolarEdge which has rapid shutdown built in and do the run with DC around 350 plus volts.

[adoublee]

Putting in a 6.9KW ground mount grid tie system.

I could avoid the large voltage drops by running higher voltage DC to the house and put the inverter in the basement or on the side of the home, but then I have to deal with the New NEC codes requiring something called Rapid Shutdown..

In regards to 2014 NEC code, rapid shutdown only applies to PV system circuits installed on or in buildings. Putting the inverter outside and at the house should be an option that doesn't require anything except a grid-interactive inverter (if you keep the DC conductors off the building I suppose).

[Murby]

In regards to 2014 NEC code, rapid shutdown only applies to PV system circuits installed on or in buildings. Putting the inverter outside and at the house should be an option that doesn't require anything except a grid-interactive inverter (if you keep the DC conductors off the building I suppose).

I think you missed the point of my question.. Maybe I didn't word it correctly.

I understand the requirements.. What I'm saying is that running 240vac such a long distance will require much larger wire to minimize voltage drop.. The cost is significant for the wire. Have to wonder if installing inverter close and running high voltage DC through smaller conductors is worth the extra cost of a rapid shutdown system.

[adoublee]

I think you missed the point of my question.. Maybe I didn't word it correctly.

I understand the requirements.. What I'm saying is that running 240vac such a long distance will require much larger wire to minimize voltage drop.. The cost is significant for the wire. Have to wonder if installing inverter close and running high voltage DC through smaller conductors is worth the extra cost of a rapid shutdown system.

I'm only making the point that even with high voltage DC from array to house, rapid shutdown may not actually apply if you are creative and keep the uncontrolled (DC) PV system circuits close to but technically off of the building. In general for residential/rural ground mounts where a installer is calling the shots I tend to see the inverter at the array.

[tyab]

690.12 has some confusion - first the title is "on Buildings" but then it goes on to say PV circuits starting on sentence one. From my reading on mainly mikeholt forums some are taking this to mean that a ground system is exempt from rapid shutdown but the majority is saying even ground systems need rapid shutdown if you have more than 5ft of conductors in the building - 690.12(1). Now what about ground mount with conductors only on the outside of a building for more than 5'? Now you are rolling the dice on how your AHJ is going to consider those PV circuits on the outside of a building. Given that the reason for rapid shutdown is for the safety of firefighters whom may have to cut stuff - we want to make sure that our installations not only comply with code but are safe for first responders. Thus I would not try and split the hairs on rapid shutdown - just have it if the inverters is more than 10ft from the array - especially for your home.

Now to answer the OP question - lets figure what wire you need for that setup if the inverter is at the panels. I'm figuring you need a 40amp OCPD for that inverter. A 40amp circuit will need at a minimum #8 (avoiding the 240.4(D) restriction) if you are using something like THNN/THWN for that run. However your voltage drop for that will be close to 4% (assuming in conduit PVC). Going larger is then you deciding if the cost difference is worth the lower loss. The inverter may itself recommend a lower resistance loss so it is able to track the grid reliably and you need to look that up. Lets say you want to keep that run under 2% loss. That would require #4 cu. So you are looking at the cost difference of 600' #8 vs #4. I'm assuming in both cases you would be using oversized conduit just to make the pull easier and it would be mostly likely the same size as if it were dc circuits - so that is not a factor Knowing that cost then you can figure out what it would take to implement a rapid shutdown and make a decision which way to go.

Now if you decide to put the inverter at the home - then you have to decide what wire is needed for that DC circuit. Look at 690.8 to figure that out. Then use voltage drop to see if you "want" to oversize that DC run to have greater output. Now you will have five data points.

1. Cost for 600' of #8 carrying AC
2. Cost for 600' of #4 carrying AC
3. Cost for 400' of xxx carrying DC (minimum required)
4. Cost of 400' of oversized carrying DC.
5. Cost to implement rapid shutdown.

Then think about the convenience of having the inverter near the home and that may be worth whatever it is you. Now you have 6 data points to make a decision.

[Murby]

690.12 has some confusion - first the title is "on Buildings" but then it goes on to say PV circuits starting on sentence one. From my reading on mainly mikeholt forums some are taking this to mean that a ground system is exempt from rapid shutdown but the majority is saying even ground systems need rapid shutdown if you have more than 5ft of conductors in the building - 690.12(1). Now what about ground mount with conductors only on the outside of a building for more than 5'? Now you are rolling the dice on how your AHJ is going to consider those PV circuits on the outside of a building. Given that the reason for rapid shutdown is for the safety of firefighters whom may have to cut stuff - we want to make sure that our installations not only comply with code but are safe for first responders. Thus I would not try and split the hairs on rapid shutdown - just have it if the inverters is more than 10ft from the array - especially for your home.

Now to answer the OP question - lets figure what wire you need for that setup if the inverter is at the panels. I'm figuring you need a 40amp OCPD for that inverter. A 40amp circuit will need at a minimum #8 (avoiding the 240.4(D) restriction) if you are using something like THNN/THWN for that run. However your voltage drop for that will be close to 4% (assuming in conduit PVC). Going larger is then you deciding if the cost difference is worth the lower loss. The inverter may itself recommend a lower resistance loss so it is able to track the grid reliably and you need to look that up. Lets say you want to keep that run under 2% loss. That would require #4 cu. So you are looking at the cost difference of 600' #8 vs #4. I'm assuming in both cases you would be using oversized conduit just to make the pull easier and it would be mostly likely the same size as if it were dc circuits - so that is not a factor Knowing that cost then you can figure out what it would take to implement a rapid shutdown and make a decision which way to go.

Now if you decide to put the inverter at the home - then you have to decide what wire is needed for that DC circuit. Look at 690.8 to figure that out. Then use voltage drop to see if you "want" to oversize that DC run to have greater output. Now you will have five data points.

1. Cost for 600' of #8 carrying AC
2. Cost for 600' of #4 carrying AC
3. Cost for 400' of xxx carrying DC (minimum required)
4. Cost of 400' of oversized carrying DC.
5. Cost to implement rapid shutdown.

Then think about the convenience of having the inverter near the home and that may be worth whatever it is you. Now you have 6 data points to make a decision.

Looks like I'm going to be able to get away with 150 feet after some careful consideration.. (and a concrete saw ) That should put me at #6 CU direct burial at less than 2% drop.. I can live with that..

[bcroe]

The 2% AC line loss will increase the probability that high line voltage will trip out the inverters. At 4% it could
be fatal. With the inverter near the meter, this problem goes away. DC loss will not affect this. Bruce Roe

[foo1bar]

Looks like I'm going to be able to get away with 150 feet after some careful consideration.. (and a concrete saw ) That should put me at #6 CU direct burial at less than 2% drop.. I can live with that..

Look at aluminum direct burial.
You can probably have less voltage drop for less money using aluminum.
You'll need to be sure to use anti-ox stuff on your connections - but that stuff isn't that expensive.

And you'll need to make sure the terminals you connect to are labeled for copper or alum. (I think most larger breakers, etc that take #6 or larger are labeled CO/AL - but check. )

[adoublee]

You can probably have less voltage drop for less money using aluminum.
You'll need to be sure to use anti-ox stuff on your connections

I see this as more potentially hazardous than having a few feet of DC conductor "near" the house. Yes, it can be done safely but does have more potential to loosen over time.

Inverter monitoring connection or wifi proximity is another reason to want it close to the house. If not wanting to be creative (maybe put the inverter on a stand right of the house) and deal with the AHJ on what code says, a module level power electronics system like SolarEdge is you best bet.

Regarding firefighter, think about what their ability to shut off power to the utility meter before you let that scare you to much.

Are you fencing the array in?

[Murby]

I see this as more potentially hazardous than having a few feet of DC conductor "near" the house. Yes, it can be done safely but does have more potential to loosen over time.

Inverter monitoring connection or wifi proximity is another reason to want it close to the house. If not wanting to be creative (maybe put the inverter on a stand right of the house) and deal with the AHJ on what code says, a module level power electronics system like SolarEdge is you best bet.

Regarding firefighter, think about what their ability to shut off power to the utility meter before you let that scare you to much.

Are you fencing the array in?

Yup.. aluminum seems to do that.. You have to go back and tighten down the connections every year for the first five to ten years if using larger wire. It seems to somehow "mash" down into itself.
I'll probably stick with copper if the cost isn't prohibitive.

I will also be running a CAT5 line to the solar array.. While I understand a wifi signal can do the job, I'm already going to have to dig a ditch so I might as well spend an extra $30 for some cat5 wire.. That would also allow me to mount a camera on the array too.. Always wanted something to record the motions and travel patters of the deer around here..

[foo1bar]

. Yes, it can be done safely but does have more potential to loosen over time.

I would guess >90% of underground feeders from the utility are aluminum.
Properly torqued connections that are intended for use with aluminum aren't going to loosen over time.

You have to go back and tighten down the connections every year for the first five to ten years if using larger wire. It seems to somehow "mash" down into itself.

Really?
The vast majority of houses in the area here uses aluminum for it's feeder lines. And I have never heard of electricians coming back after 1 or 2 or 5 years to tighten the connections.

Aluminum branch wiring (ex. 12AWG) does have a reputation for working loose - but that's pretty different from large gauge.

[Murby]

Really?
The vast majority of houses in the area here uses aluminum for it's feeder lines. And I have never heard of electricians coming back after 1 or 2 or 5 years to tighten the connections.

Aluminum branch wiring (ex. 12AWG) does have a reputation for working loose - but that's pretty different from large gauge.

I have aluminum feeder lines on my box and have never had to tighten them either.. But I've seen it happen a couple times over the past 30 years..

[reader2580]

I have aluminum feeder lines on my box and have never had to tighten them either.. But I've seen it happen a couple times over the past 30 years..

I had to tighten the neutral and both hots in my breaker box when I bought my 35 year old house. The neutral had charred insulation on it. I pulled the neutral and put no-ox on it.

[Mike90250]

So, I'm helping someone put together a system, he has 24V. Adamant about staying at 24V. had actual fire in a 3 bay Li battery bank, fairly unconscious about batteries and their care.. (yes, I know, run away)

But, with the size of the main inverter breaker (250ADC) at 24V, 4/O wire is required to meet the breaker rating, and it's poorly laid out, he poured a new pad for a forklift battery bank, but it's not next to the power panel !!

But the 4/O wire, the 4/O lugs and stuff he has to have the shop in town (50 miles 1 way) crimp on - I don't have a 4/O crimper, he's up to $700 in cables when it should be only a couple hundred for all of it. And the mess of having to bend and get the 4/O cable and terminals into the breaker panel - OMG

From now on, it's only 48V systems for me, this is nuts, but he has a fat wallet and can't see the simple answer to change the inverter to a 48V one. (using an old 3500w Vanner) which he says is nice. I'm sure it's nice and toasty - even in idle.