What is the 120% “Rule” in Solar PV Design?

If you’ve spent any time learning about solar PV systems, you’ve probably heard someone mention the 120% “rule.” It turns out that it’s not actually a rule at all. It’s one of several options for connecting solar to the utility grid. 

In this video, Sean White breaks down the 120% rule in simple terms, walking through the calculations you need to know to determine proper breaker sizing for solar installations. As you can imagine, it’s challenging to fully explain these electrical concepts in just three quick minutes. To build your comprehensive understanding of the 120% option and other critical design principles, sign up for Sean White’s courses where you’ll get the in-depth instruction and knowledge you need to build your solar expertise.

Transcript below.

So a lot of people are hooking up solar, and there are different ways to do it. One of those options is called the 120% rule. People love to call it a rule, however, it’s not really a rule – it’s an option. Just know that it’s one way of doing things.

People love to hook up solar on the load side of the main service disconnect, which is called a load-side connection. Our favorite way of doing a load-side connection is with what we affectionately call the 120% rule, and I’m going to explain it and keep it nice and simple.

So you have a thing called a bus bar. A bus bar is where you connect all your circuit breakers – it’s a common piece of metal where things are connected together. We take that bus bar and we get 120% of that. And that’s what all my grades were in school, of course. And that’s what your grade will be also on every NABCEP test you take if you take my class.

That 120% rule bus bar is measured in amps. So if we had a 100-amp bus bar, we get 120 – that’s 120 amps allowance. And we subtract from that the main breaker. So if you had a 100-amp bus bar protected by a 100-amp main breaker, you could subtract that 100-amp main breaker from that 120% of the bus bar. That leaves you with 120 minus 100, which is 20 amps. And that’s your allowance.

That 20 amps is the biggest backfeed breaker that you could have for your solar, for your energy storage, or whatever you’re going to be backfeeding with.

And then what is the most inverter that you could put on a 20-amp breaker? That’s the other question. So if we took a 16-amp inverter and we multiplied that by 1.25 – so inside of this 120% rule, we have a 1.25 or 125% just to keep you on your toes – I could take 16 amps times 1.25, which is 20 amps for that 20-amp breaker.

Another way to do that is doing it backwards. So we take that 20-amp breaker, we divide by 1.25, and that gives us 16 amps. That’s called algebra.

So to review what we have: a 100-amp bus bar, a 100-amp main breaker. We take the 100-amp bus bar times 120%, that’s 1.2. We get 120 amps. We subtract off the 100-amp main breaker. We’ve got a 20-amp allowance. 20 amps times 0.8 gives us a 16-amp inverter. And if that inverter is on your house, your house is going to take a 240-volt inverter. So 16 times 240 is 3,840 watts. That’s right – volts times amps equals watts.

Those are just things that you’ll learn in my classes. This is the kind of thing you might learn in my NABCEP PV Associate Bootcamp class, and you would definitely learn it in just about every other class I teach because it’s important to understand your 120% rule and all of these other options that we have for knowing where to connect breakers – especially if you’re going to be selling solar systems, designing solar systems, and getting yourself those golden tickets known as NABCEP certifications.