Solar-ready electric service specs
dustin_7022224
| Posted in Mechanicals on
We are building a new home and I’m installing the electrical and plumbing. We will have 200 amp service and I plan to place a 100 amp subpanel in the garage. There garage roof is largely south facing and we want to plan for a future PV array on this roof as well as EV chargers and maybe a battery in the garage, which will be insulated and partially heated in the winter.
Any advice or suggested references for specs, wiring details, etc?
-Dustin
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A 100A 120/240 service is good for 24kVA. Assuming you have minimal load out there, and a good power factor in general, you’re good for about 15-20kw which is a BIG solar array. A battery in an unheated space is not going to perform particularly well. Capacity will be reduced, and charging voltages will be differently (slightly). Batteries are specified at 77*F, any higher or lower than that and the capacity in ampere hours changes as does the optimal charging voltages.
I would use copper wire, and I’d make sure the busbar in the panel is also copper. Copper bussed panels don’t cost much different from aluminum bus and the copper bus is much better.
If this is an unattached garage, I would run PVC conduit and THHN or XHHW wire in the conduit. I’d also run a seperate PVC conduit of at least 1” diameter for control wiring (Ethernet, etc). The conduit is cheap, And you can always run the wire later. You do NOT want to trench again! You may want to run a third conduit for some three way lighting circuits so that you can turn outdoor garage lights and and off from your house.
If this is an attached garage, I wouldn’t bother with the 100A subpanel unless the main panel is a significant distance away. If the main panel is far away, then I’d run the subpanel the same way as for an unattached garage, except that I’d use EMT conduit instead of PVC. I do not like running SE cable for heavy services indoors, I always prefer to run conduit for large circuits like subpanels. I would make sure there is a ground WIRE in that conduit too — don’t let the electricians use the conduit itself as the grounding conductor, it’s just not reliable for that purpose.
If you plan on doing a lot of electrical add ons later, don’t recess the panel into the wall — Mount it on the surface for easier access. If you want to make a good, clean job of it, mount the panel on a piece of 3/4” “noncom” (fire retardant) 3/4” plywood.
You might also consider preinstalling a conduit run to the roof for future solar wiring too. I’d use EMT again here for a bit of extra safety. Check with your solar installers as to sizing and placement.
Bill
Do you have an idea how much PV you will want? Panels have limitations on how much PV can be "back-fed" into their busbars so it is a good idea to have a plan.
For example a 100 amp panel with a 100 amp main breaker can only be back-fed with 20 amps
A if you get a 125 amp panel and keep the 100 amp main breaker you will be allowed up to 45 amps of "back-fed solar"
A way around this is to "tap" the feeder wires but some jurisdictions do not allow this.
Battery backups are a whole other ballgame and often require "backed up loads panels" separate from the main panel.
There is a lot to think about so best to have a general plan as to not end up doing a lot of rework
Dustin,
For residential AC connected PV and batteries is fast becoming the cheapest/easiest setup. This means that most PV setups only need a larger conduit from your main panel to near the roof to connect up a string of micro inverters.
You do need to watch the 120% rule that Bill mentioned if you go with a standard panel. The ways around it is to get a larger panel and put in a smaller main breaker (ie 200A panel, 175A main, now you can go to 60A breaker so 10kW of solar) or get one of the solar ready panels that has a dedicated power connection from the solar to the main breaker.
Lithium batteries are very much like that uncle that likes to keep his house uncomfortably cold all year around. A nice cool unheated basement or enclosed insulated crawlspace is perfect spot for them. You don't want them in something like an outdoor shed.
The garage will be attached, but only at the corner and the exterior walls of the house are ICF all the way to the roof. The ICF, minimizing/sealing penetrations between the living space and garage, and ease of future wiring in the garage are the biggest reasons I plan to install the sub.
Reading comments I'm now thinking about a 200 amp subpanel with 100A main breaker, #3 THHN and #6 copper EGC in 1-1/2" EMT. The run will be about 35'. I'll sleeve the EMT through the concrete with 2" PVC.
If the PV array connects directly to the sub panel, can I put the batteries in the conditioned basement and connect them to the main panel (future purchase/install)?
A 200 amp panel with a 100 amp main is a good option and will allow you all the solar you will likely want.
If you go with the suggested "AC panels" which come from the roof as 240VAC there will be groups of modules in parallel that won't exceed 20 amps after a multiplication factor. All of these "circuits" will come into a junction box on the roof and then make there way to a "combiner panel" which is typically a smaller panel where all these groups of modules overcurrent protection land.
Depending on what brand you go with you may have 12 modules per one 20 amp 2-pole breaker. Often these are 2 wire without a neutral.
So after all the circuits have "combined" in this panel, you then leave with your bigger(connected to main lugs or breaker of panel) wire to feed into main panel or sub.
In this example typically only a #12 ground is needed and you don't need to use EMT. With a lot of the AC module system the grounding is very integrated.
For the battery......what battery do you have in mind? Lots of different options here. Are you wanting to backup the entire house or just some dedicated circuits?
And don't forget there is often a "production meter" involved so your utility can keep track of how much PV you produce. Around here these have to be exterior to building.
This is all Utility specific. If there is not net metering where you live than there is not much point to this meter.
But for planning wire runs this is all good stuff to know.
OK everyone: I want to clear up a misconception I’m seeing here regarding some electrical sizing rules. This is important stuff.
There is in the trade what is commonly known as the “80% rule”. This rule is basically saying “anything running continuously must not exceed 80% of a circuits rated capacity”. This applies to everything, the wiring AND THE BREAKERS. You CANNOT put a 100A breaker in a 200A panel and get around the 80% rule — the 100A breaker isn’t supposed to carry more than 80A continuously. The reason is that common breakers are only rated for 80% in continuous service. There are “100% rated” breakers available, but I’ve never seen one for residential service and they are MUCH more expensive than normal breakers. The price difference is so much that it’s cheaper for me to put in an oversized 800A switch at work instead of a 600A switch and use smaller fuses. Fuses are different than breakers, it is the switch mechanism that the 80% rule applies to. Breakers are different, so you can’t just use a smaller breaker and be ok. “Continuous” per code is loads of 3 hours or more duration. This is in the NEC in 384-16.
Downsizing the breaker with respect to the wire ampacity is done when exceeding the conduit fill limitations and dealing with the derating tables. If you look closely at the ampacity tables in the NEC, you’ll see everything says “not more than 3 current carrying conductors”. “Current carrying conductors” are phase (hot) conductors and sometimes the neutral, depending on the circuit configuration. The ground conductor does not count. If you want to run 6 current carrying conductors in one conduit run, you have to refer to the derating table and derate the ampacity for each conductor from the 90 degree table in the ampacity table. Yes, you get to use the 90 degree table here if you’re using 90 degree wire (like the common THHN). You are only limited to the 75 degree column in the ampacity chart for the max per-conductor loading due to the 75 degree rating of the wire connection to the circuit breaker.
All of these ratings and codes are based on thermal rise (heating) of the wiring and devices. The intention is to allow for some cooling time between heavily loaded periods to avoid any localized overheating.
If you go with a 100A breaker, you don’t gain anything using a 200A panel. You can use a 100A panel with a 100A breaker. You do get to use a so-called “main lug” panel though since you’ll be feeding it with a 100A breaker in the main panel. That saves you the cost of a main breaker in the Subpanel. Even though #3 wire is OK for 100A, if you’re expecting to heavily load the circuit using #2 wire gets you a little cooler operation for very little additional cost.
Bill
My local utility requires the power from the cells pass thru a disconnect switch that is located next to the power meter before going to any load. My utility would require you to run all the solar power back to the house on one set of wires and power the loads in the garage on different set of wires.
The requirements vary from state to state, county to county, fire district to district and each utility may have separate requirements. What is allowed on this side of a road may not be allowed on the other side of the road. Internet knowledge is interesting but may not be useful locally.
Walta
The general stuff that I posted is from the national electric code (NEC) and applies pretty much everywhere in the US as a minimum. There are locals that are more strict in some areas, Chicago is a good example for their conduit requirements. Canada is different, although surprisingly similar in many ways (their “T90” is nearly the same as our “THHN”, for example).
Local utilities may well have special requirements if you’re going to be doing net metering. I would not be surprised if they would want a separate disconnect that was easily accessible to their personnel. It would make sense to check with your local utility for any specifics in that regard.
Bill
Downsizing the main breaker in a panel is a very common and code compliant method for being able to back-feed more PV onto said panel. We do it all the time.
At this point I agree with where Walta was headed and suggest talking to a local installer, maybe pay them for a consult or something.
Grid-Tied solar is very easy, but integrating a battery adds a layer of complexity worth exploring with a professional.