Disclaimer

This site is educational. It is not professional electrical advice, and it is not a substitute for a licensed electrician, a qualified installer, or your local inspector.

Off-grid power systems involve stored energy that can start fires and injure people. A battery bank can deliver thousands of amps into a short circuit, a solar array cannot be switched off, and a backup system wired incorrectly can energize a utility line that a lineworker believes is dead. These are not remote hypotheticals. They are the ordinary failure modes of this equipment.

Please read what follows before acting on anything here.

The information here is general, and your situation is specific

Every page on this site is written for a general reader. None of it accounts for your loads, your climate, your building, your equipment, or the code enforced where you live.

The calculators are the clearest example. They answer a narrow, defined question with the inputs you give them. They do not know that your battery box is in an unheated shed, that your conduit run is bundled with four other circuits, or that your inverter terminals are rated to a lower temperature than your cable. Those factors all matter, and they all make the correct answer more conservative than the one a calculator returns.

Treat every result here as a starting point to be checked, not as a specification to build from.

Codes and jurisdiction

Wiring guidance on this site follows the National Electrical Code (NFPA 70), as used in the United States.

• Codes vary by jurisdiction. Your state, county, or municipality may amend the NEC, adopt a different edition, or impose additional requirements. What is compliant in one place is not automatically compliant in another.
• Marine and vehicle installations are different. Boats and RVs fall under ABYC standards, which are more conservative than the NEC in several relevant respects. Applying guidance from this site to a boat or an RV will in places give you answers that are not adequate.
• Permits and inspection are usually required for anything permanently installed in a building or connected to its wiring. That requirement exists for good reasons.
• Grid-connected work is a separate matter entirely. Any system that can interact with utility service requires a utility interconnection agreement, equipment certified for the purpose, and in nearly all cases a licensed installer. Do not improvise here.

Accuracy

Considerable care goes into the figures on this site, and sources are cited so you can check them. Even so:

• Values are transcribed from standards and datasheets, and transcription errors are possible.
• Standards are revised. A figure correct at the time of writing may not match the edition enforced where you are.
• Products change specification without changing model number.

Verify anything safety-critical against the primary source before you rely on it: the current edition of the code, and the manufacturer’s own datasheet for your specific equipment. If you find an error here, please report it.

No warranty is given, express or implied, as to the accuracy, completeness, or fitness for any particular purpose of the information on this site.

Your responsibility

You are responsible for the safety and legality of your own installation. By using this site you accept that any work you carry out, or have carried out, is done at your own risk, and that neither the author nor this site is liable for any loss, damage, injury, or death arising from the use of this information.

If you are not confident you can do a piece of work safely and to code, hire someone who can. This is not a subject where an approximate answer is good enough.

Product recommendations

Products are recommended based on published specifications and general suitability. This site does not test equipment destructively, does not certify products, and cannot guarantee that any product is safe or appropriate for your installation. Read the manufacturer’s documentation and follow its installation instructions, which take precedence over anything written here.

See also the affiliate disclosure.