25 kW Solar power project in Iowa.
- Thread starter Redneck in training
- Start date
OP
Redneck in training
Elite Member
There are always surprises due to inexperience. I decided to build the supporting structure out of treated wood but I just found it will corrode aluminum. Have to rethink what material I will use. I think I will use treated wood on most of the structure and non treated wood (such as cedar) on everything that comes in contact with aluminum.
They make adhesive material that can be used between aluminum and treated lumber to prevent the corrosion. I've seen it used on decks to allow aluminum flashing to be used on ledger boards. But...
Using treated lumber has become such a hassle to get the right fasteners, hangers, flashing, etc I've started priming and painting regular wood for everything above ground, I only use treated for ground contact. The other problem with treated is it's generally not kiln dried so it twists and warps all over the place if you don't get it up really quick. I have 40 year old fascia and trim made out of painted pine and it's been hanging right out there in the sun and rain all that time. I'm not sure it would have survived any better if it had been treated.
OP
Redneck in training
Elite Member
Thanks for the info. I was thinking about plastic between the aluminum and the wood too but have not decided how I will resolve it.
MotorSeven
Elite Member
Do you weld? If so have you calculated the cost of making your frames out of steel? You would still need something between the steel an Al, but at least the steel will out live any wood frame by a lifetime.
OP
Redneck in training
Elite Member
I was looking at galvanized metal but it would significantly increase the cost. In general any metal structure would add significantly to the cost. The system is a big monster consisting from about 1700 sqft of panels. I decided to use untreated wood for all parts that will contact aluminum. Those will not be part of the suporting structure but be bolted to it. This way I could replace the parts as necessary without tearing things apart.
JeffInCO
Bronze Member
Redneck,
I'm curious about where/how your tying into your existing house/farm electrical system.
In my case, my 5.5kW PV system is tied into a panel on my barn that is rated for 200 amps, that already has a 200 amp main breaker (the house panel is cascaded off of this panel, hence the need for a 200 amp main). The 2008 NEC allows a PV system to go 20% (I think?) over the panel rating (so 240 amps, in my case). In other words, I can have a 40-amp backfed breaker supplying power to the panel in addition to the 200 amp main breaker. My central inverter is rated to supply a maximum of 30 amps @ 240 volts (7200 watts, which I'll never hit because my panels can't make that much). However, the NEC requires that I use the 30 amp number and a 40 amp breaker because of the inverter rating. So my panel is already maxed out.
The quandary for me is that I'm inspired by your project and the really low cost of panels right now, and am thinking of a DIY expansion to my system... One option is to increase the size of the array on the existing inverter by about 50%, which pushes me right to the limit as to what my existing inverter can handle, but doesn't change the backfeed amps from a code perspective. Another option is to use micro-inverters for the expansion, but then I'd need to decrease the main breaker in my existing panel from 200 to 150, which would allow me to backfeed up to 90 amps, rather than 40 amps. However, it means I'd only have 150 amps available for my farm when the sun's not shining (that might be okay...).
In your case, you have 100+ amps coming from your array. If you have 400 amp service, then you're probably set... If you have 200, it seems that you might have an issue. I know you said that inspections aren't necessary for you, but my take is that this rule in the NEC is one of those that makes sense... I don't expect that you want to overcurrent your panel by a substantial amount.
Just curious -- I'm definitely not trying to throw a wrench in the works for you.
Jeff
I'm curious about where/how your tying into your existing house/farm electrical system.
In my case, my 5.5kW PV system is tied into a panel on my barn that is rated for 200 amps, that already has a 200 amp main breaker (the house panel is cascaded off of this panel, hence the need for a 200 amp main). The 2008 NEC allows a PV system to go 20% (I think?) over the panel rating (so 240 amps, in my case). In other words, I can have a 40-amp backfed breaker supplying power to the panel in addition to the 200 amp main breaker. My central inverter is rated to supply a maximum of 30 amps @ 240 volts (7200 watts, which I'll never hit because my panels can't make that much). However, the NEC requires that I use the 30 amp number and a 40 amp breaker because of the inverter rating. So my panel is already maxed out.
The quandary for me is that I'm inspired by your project and the really low cost of panels right now, and am thinking of a DIY expansion to my system... One option is to increase the size of the array on the existing inverter by about 50%, which pushes me right to the limit as to what my existing inverter can handle, but doesn't change the backfeed amps from a code perspective. Another option is to use micro-inverters for the expansion, but then I'd need to decrease the main breaker in my existing panel from 200 to 150, which would allow me to backfeed up to 90 amps, rather than 40 amps. However, it means I'd only have 150 amps available for my farm when the sun's not shining (that might be okay...).
In your case, you have 100+ amps coming from your array. If you have 400 amp service, then you're probably set... If you have 200, it seems that you might have an issue. I know you said that inspections aren't necessary for you, but my take is that this rule in the NEC is one of those that makes sense... I don't expect that you want to overcurrent your panel by a substantial amount.
Just curious -- I'm definitely not trying to throw a wrench in the works for you.
Jeff
OP
Redneck in training
Elite Member
My system (Actually it is 24kW DC or around 20KW AC) will produce 80A AC but I will have room for 32 more panels on the rails. I plan to connect directly to the tombstone by the transformer where the main meter and main disconnect are. I will have 150 or 200A fused disconnect and a meter at first array. Each of nine strings of microinverters (10 or 11 of them) will have its own 15A breaker. There are three arrays. One 3 rows by 10 columns and two 3 rows by 11 columns.
OP
Redneck in training
Elite Member
There is a lot of money in the system so I don't want to take any risk. It should last at least 20 years with minimum maintenance. I will err on the side of reasonable overkill.