Conduit size for future solar install

[MeasureTwice]

I am doing some work on my house and would like to run conduit in the walls for a future solar install. It's a 2-story house with basement using 2x4 wall construction. If I'm planning on a 5000 watt solar system what size EMT conduit would you recommend to bring the power from the roof down to the electrical panel in the basement? Should I install a 2nd conduit for low voltage signal wires? Thank you in advance.

 

[FilterGuy]

If I'm planning on a 5000 watt solar system what size EMT conduit would you recommend to bring the power from the roof down to the electrical panel in the basement?

We don't have enough info to answer your questions.

Will the panels be a single serial string or lots of parallel strings Will you have combiner boxes on the roof? Will you be using micro inverters at the Panels and bring AC down to the basement? You have to know this to know the number of wires and the size of the wires. (Note: since the wires are in conduit, you have to oversize them.... and even that changes based on how far off the roof the conduit runs. Once you have the number and size of the wires you can go to a conduit fill chart to determine the size:

https://www.stateelectric.com/resources/conduit-fill-table

Note: I would go a bit over-sized on the conduit to 1) cover contingencies and 2) make it easier to pull wires.

Should I install a 2nd conduit for low voltage signal wires?

I do not know the NEC requirements, but I would run a seperate conduit for the signal wires.

 

[MeasureTwice]

Will the panels be a single serial string or lots of parallel strings Will you have combiner boxes on the roof? Will you be using micro inverters at the Panels and bring AC down to the basement? You have to know this to know the number of wires and the size of the wires.

Is there one kind of system that is more prevalent that the others? I thought most systems used micro inverters because it was inefficient to transfer the DC very far.

 

[FilterGuy]

Micro inverters are rather new. They are easy to install and with all the new requirements around rapid shutdown they make it easy to meet the requirements. However, they tend to be more expensive than traditional grid tie systems with a central inverter. Furthermore, if you want to add battery storage, you are pretty much stuck with AC coupling. (AC coupling is not necessarily bad, but it can be less efficient). Micro-inverters don't seem to be popular with the DIYers due to cost, but they are very popular with installers because they are so easy to install. If I were to install grid-tied solar without battery backup today..... I would seriously consider micro inverters.

I am not experienced with micro inverters, but we can still work through what the needs will likely be. The AC from the roof will have at least 4 wires. Two hots, a neutral and ground. For 5000W, that is nominally 5000/240V=~ 21 Amps. That means at least 4 10AWG wires. However, unless you know the system specifics and characteristics, I would plan on 4 8AWG wires. You could probably get away with a 3/4" conduit, but I would do 1" just to be sure I could handle all possible eventualities.

As far as the low voltage wiring goes..... I am not familiar enough with the micro inverters to definitively say what you will need but you may not need to rout any low voltage signals to the roof.
* Since disconnecting from the grid will shut off the inverters, I *think* the rapid disconnect is covered.
* Most modern inverters will tie into the internet but it appears most of the micro inverters send data over the AC line to a box that will capture the data and put it onto Wifi or a hard wired ethernet port. Consequently you will need internet in your basement but may not need it to the roof.

Central Inverter: Typical grid tied residential solar used to always be one (maybe two) long serial string(s) of panels. This meant very high voltage and low current so there was not a concern about DC power loss leading to the inverter. (A 5000 W array would easily have an open circuit voltage of 500V or more) This is typically the lowest cost way of doing grid-tied solar, but, with the newer disconnect requirements the equipment that has to be at and/or around the panels eats into the cost savings and makes the system a lot more complicated. However, if you bring DC down from the panels you can do DC coupling to a storage system. Assuming your design ends up with two series strings, you will need a ground wire and two sets of two DC wires to the roof (5 total). You could probably get by with a 3/4 inch conduit.... but like before, I would go with 1".

For low voltage wiring, you will need at least two wires for your Rapid Disconnect system (plus ground). I would plan on 4 to be safe.

Commercial Home storage systems. A lot (most? all?) of the pre-packaged DC coupled residential battery systems use high voltage batteries and high voltage PV strings. Consequently, as far as wiring to the roof, they are much like the central inverter systems

Last but not least DIY with storage: Some of the all-in-one inverters discussed here on the forum will not take really high voltages on the PV. Consequently, you will have to start doing more parallel strings of wires. A 1" conduit can have up 16 10AWG or 9 8AWG wires so the 1" conduit is probably OK.... but you would have to get more specific to be sure.


Conclusions:
1) You could probably get away with a 3/4" conduit but I would do a 1"conduit for the power wires. (The cost of 1" is not that much greater, it is about the same difficulty to install and the flexibility is much greater).

2) I don't know if you are allowed to put the low voltage wires through the same conduit as the power wires so you should find out. If you can, the single 1" conduit is probably fine. If you can't, I would run a separate 3/4" conduit. (or just run the extra conduit and not worry about it).

 

[Hedges]

Not sure if EMT is OK outside. I use rigid or IMT outside, and I have used EMT for indoor commercial work.

Even if a conduit fill chart says a certain number/gauge of wires is allowed, doesn't mean you can get them through. Maybe if a short straight run (like a nipple between boxes.) Make the conduit way oversize and it'll be easer to pull wires. I keep adding more. My 3/4 is stuffed. My 1 1/4 is stuffed. My 2" is stuffed.

With more than 3 current-carrying conductors in a conduit, ampacity is reduced; at some point you'll need larger gauge.

Low voltage? Just run it in separate conduit.

DC for high voltage PV string inverters is typically as high or higher than AC, so no savings there.

Microinverters avoid rapid shutdown, because only one PV panel per inverter is already low enough voltage and wattage.
If RSD is required, any other inverter will require a box per panel (or per several panels).
I prefer high voltage string inverters. Lower cost, more efficient, easy to access. But RSD pretty much negates the cost savings.
Microinverter (or optimizer/RSD if smart enough) provides module-level diagnostic reporting. It also tolerates multiple panel orientations.
Either microinverter or big inverter is fine for partial shading in most cases.

If you want battery backup, plan for it upfront. Your selection of inverter can support that or include it (some are batteries optional.)
If battery is UL listed for AGM it can go in the house. Any DIY or non listed lithium battery plan to put in a bunker more than 5 feet from the house. If that isn't code yet, looks like it is going to be.

You might want to put standoffs and flashing in the roof for panel mounts. Find out what code says regarding leaving open space for fireman, and for mounting. (My area, if < 40 pounds per attachment point then no mechanical permit. That probably means screwed into sheathing is acceptable, but I would want it into rafters.)

 

[MeasureTwice]

Thank you both for your detailed responses. It is far more than I expected. I have read both responses multiple times and looked some terms up on the internet to better understand what you are saying. There are more options and possiblities than I realized (happens every time I get into something!).

I was thinking of including battery backup and, as I understand, the batteries do better inside (I live in Michigan) so I would expect to use UL listed AGM type. I'd rather have the battery in the garage but for weather reasons the basement is probably better.

I don't know if the local codes require RSD but it seems likely where I live. So I guess that makes the electronics on the roof more complicated and expensive.

The EMT will be run in the wall and I believe that is allowed by code. A 1" EMT conduit seems reasonable. The distance the wires would run from the roof to the basement I would estimate at 25 feet.

The low voltage lines don't need a conduit so I was just going to run them through the wall. I am thinking of running both a 6-wire thermostat cable and an Ethernet cable from the basement to the attic with a bunch of extra wire on each end.

Thanks again for helping out a newbie!

 

[Hedges]

UL Recognized AGM batteries, or UL Listed complete energy storage system with lithium (which isn't a guarantee they won't catch fire, just that it won't give your insurance company any excuses).

https://www.cpsc.gov/Recalls/2020/l...e-energy-storage-batteries-due-to-fire-hazard

AGM has cycle life of a few hundred cycles to 75% DoD (varies with brand.) I used SunXtender, cost $5000 for 20kWh gross capacity, 14 kWh usable. You might get by with $1400 for 5 kWh. Since it is for grid backup, I don't expect enough cycles to wear it out over a decade. Large PV array, relatively small battery, runs A/C and everything else during the day but have to turn off most things at night.

For an off-grid system cycling every night, lithium batteries might last 5x as many cycles so could last 10 years of nightly cycling. If they are reliable, which 95% of brands tested are not.

DIY LiFePO4 might cost $2000 for 15 kWh gross capacity, which does make it an attractive alternative to AGM.

Most rooftop systems are wired outside to a disconnect. Maybe with RSD at the panel and a control outside, it's OK. to have the conduit disappear through the roof to interior. I would use rigid for the exposed portion with a service entrance cap.

Batteries are the most expensive and least reliable or shortest lived part of the system, so if they can be minimized or eliminated that would be better. SolArk is a hybrid that I think can operate without batteries, so could be added later. SMA Sunny Boy can generate up to 2kW 120VAC direct from PV while grid is down (confirm that is compatible with RSD). Addition of (high voltage lithium) Sunny Boy Storage or (low voltage AGM or lithium) Sunny Island makes a backup system. Instead of SolarEdge, StorEdge allows addition of a battery. Some other brands are much lower priced.

SolarEdge brand uses optimizers that provide RSD. SMA uses an RSD box:

SunSpec Certified Rapid Shutdown Technology | SMA America

Each PV system is unique. Different roof structures, system alignments and various types of shading influence the PV system's energy yield. Thanks to the innovative TS4-R module technology, energy yields can be increased.

www.sma-america.com

 

[MeasureTwice]

Wow, thanks for all of that great info. Mine will be a grid-connected system and I would like some way to run heat/fridge when the utility power is out. I will research what you suggested.