What is the 20/20/20 rule for generators?

The 20/20/20 rule for generators does not exist as a standard from the NEC, manufacturers, or any listing body. The famous 20-20-20 rule is actually an eye-strain guideline: every 20 minutes, look 20 feet away for 20 seconds.

When the phrase gets attached to generators, the asker usually means one of the real rules wearing the wrong name: the 80 percent loading rule, which reserves 20 percent headroom, or a monthly exercise schedule of about 20 minutes.

The 20 percent headroom is the real engineering rule, and it is worth following whatever you call it.

Treat any tidy numeric slogan as a memory aid, then size the machine from your actual load list, because that number is the one that matters.

Build that load list with the generator size calculator and let the rules of thumb retire.