Solar · Wiring and code
Everything else in this section can be planned at a kitchen table. This part cannot. A property battery bank stores enough energy to start a fire in seconds, direct current does not put out its own arcs, and the rules that govern it change from one county to the next.
What this page is
The other pages in this section give you arithmetic you can run yourself. This one does not, and the omission is deliberate.
Electrical requirements in the United States derive from the National Electrical Code, principally Article 690 for photovoltaic systems and Article 706 for energy storage. But the code is adopted state by state, often amended locally, and different editions are in force in different places at the same time. There is no single national answer to publish.
So this page explains what the system needs and why, well enough that you can hold a real conversation with an electrician and recognise work that is being done badly. It does not give you wire sizes or fuse selections, because the correct ones depend on your equipment, your distances, and your jurisdiction.
Self-reliance is knowing which jobs are yours. A household that can size an array, specify a bank, and explain why its disconnects are where they are is not being passive by hiring a licensed electrician for the connections. It is being the kind of client who gets good work.
The parts
Every one of these exists so that a person can work on the system safely, or so that a fault stops before it becomes a fire.
Switches that isolate a section so it can be worked on. An array disconnect, a battery disconnect, and disconnects around the inverter. Their job is to make a circuit provably dead.
Fuses and breakers sized to protect the conductor, not the appliance. A wire that can carry a given current safely must never be able to carry substantially more.
Gives fault current a deliberate path back rather than an improvised one through a person or a structure. Array frames, racking and enclosures are all part of it.
Unglamorous and genuinely important. A firefighter arriving at night needs to know there is stored energy on site and where to shut it down. Code requires it for that reason.
Direct current
Alternating current reverses direction many times a second, passing through zero volts each time. An arc drawn across a gap tends to extinguish itself at that instant. Household switches and breakers quietly depend on this.
Direct current never crosses zero. An arc that starts has nothing to interrupt it, and it can sustain itself across a widening gap, burning at temperatures that melt metal.
This is the single most important thing to understand about a solar and battery system. It is why switches, breakers and fuses must be rated specifically for DC service at the voltage in use, and why AC-rated equipment cannot simply be substituted because the amp numbers look similar.
A device rated for a current at one DC voltage may be rated far lower at a higher one, because higher voltage sustains a longer arc. When someone quotes a rating without the voltage it was measured at, the rating is not yet information.
Fault current
The array is not the dangerous part. An array is current-limited by physics, and a shorted string delivers only a little more than its normal output. The battery bank is a different matter entirely.
A lithium bank has very low internal resistance, which is precisely why it delivers power so willingly. Into a short circuit, that same property lets it deliver current far beyond anything the system sees in normal use, and it arrives within milliseconds.
Every fuse and breaker carries two separate numbers. The current at which it opens, and the largest fault current it can interrupt while opening. The second is called the interrupting rating, and it is the one that gets overlooked.
If available fault current exceeds a device's interrupting rating, it does not clear the fault. It can fail violently and leave an arc burning across its own terminals, which is a worse outcome than having fitted nothing at all. Battery protection is therefore selected against the bank's prospective fault current, a figure that comes from the battery manufacturer.
A battery datasheet should state prospective short-circuit current. If a supplier cannot provide it, that is information about the supplier.
Fuses sold for vehicle and marine use are rated for lower voltages and lower interrupting capacity than a property-scale bank presents. Similar shape, different job.
A correction worth making
A specific claim circulates widely in off-grid material: that battery overcurrent protection must sit within seven inches of the positive terminal. It appears in vendor guides, forum posts and videos, usually presented as an electrical code requirement.
It comes from ABYC E-11, a standard for boats. It is quoted correctly as far as it goes, and it is the wrong document for a dwelling. It also carries exceptions that extend the distance considerably under stated conditions, and those exceptions are almost always dropped in the retelling.
A house in the United States is governed by the National Electrical Code as adopted and amended where you live. Marine practice is thoughtful and often stricter, but importing one rule from it while ignoring the framework it belongs to produces confident, wrong answers.
Not because the seven-inch figure matters to you directly. Because it shows how confidently wrong off-grid electrical information can be, in sources that otherwise get a great deal right. Treat any specific requirement you read, including anything on this page, as something to confirm against the code in force where you live.
Voltage drop
Conductors have resistance. Current flowing through them loses a little voltage along the way, and that loss becomes heat rather than useful work. Unlike most losses, this one is chosen at installation and then paid every day for decades.
More amps means more drop over the same wire. The 125 amps calculated on the controllers page is a large number to move any distance.
A ground mount placed for sun rather than convenience can sit a long way from the equipment room, and every metre counts twice, out and back.
The same power at four times the voltage moves as a quarter of the current. This is the practical reason property systems use a 48 volt bus and long, higher-voltage panel strings.
Where the array sits, decided on the mounting page, is therefore also a wiring cost decision. Moving an array fifty metres for slightly better sun can cost more in conductor than the extra sunlight is worth. That tradeoff is worth putting in front of your electrician before the trench is dug rather than after.
The permit path
A common assumption is that a system with no utility connection is nobody's business. That is rarely true. Electrical permits generally attach to the work and the structure, not to whether a meter is present.
Requirements vary widely, so the only reliable answer comes from your own authority having jurisdiction. Four things are worth asking before any equipment is bought.
Jurisdictions adopt the National Electrical Code on their own timetable and amend it. Requirements differ between editions, sometimes substantially.
Some places permit homeowner work on their own dwelling, some do not, and many draw the line at the point where the system connects to the house.
Energy storage carries its own siting requirements covering location, separation, and sometimes the total capacity permitted in a given space.
Asking early turns an inspection into a checklist you can build toward, rather than a verdict delivered after the walls are closed.
Insurance. Undocumented electrical work on a property can affect a claim, and a fire that starts at an uninspected battery installation is exactly the case where that gets examined closely.
Selling. Unpermitted systems surface during a sale, and resolving them then is far more expensive and slower than permitting the work when it was done.
Next
Both are in progress and will link here as they publish.
Short days, snow cover, cold open-circuit voltage, and the temperature limits on charging a bank.
The point where roof space, shade or winter load pushes a household into property-scale planning.
Back to Solar at Property Scale, or to battery banks for the storage decisions that set the fault current discussed here.
Sources
This page is written to help you understand and specify the work, and to hold a competent conversation about it. It is not installation guidance, and nothing here substitutes for the code in force where you live or for a licensed electrician.