When it comes to designing a PV system for any residential or even commercial system, the 120% rule is used to determine the limit to how much a building or structure can hold or how much energy the site’s service can handle.
When it comes to designing a PV system for any residential or even commercial system, the 120% rule is used to determine the limit to how much a building or structure can hold or how much energy the site’s service can handle. Yes, maybe the roof, ground or carport can support several kilowatts of PV, but can the service to the site handle it all? This is where you will hear whispers of the 120% rule. Well, what does that even mean?
This rule is meant to calculate how many amps can be backfed through the load side of the site’s existing service equipment with a measure of safety. In this instance, you have to consider that not only is the solar PV system of a certain amperage but also that the grid is present too and it is not sitting idle. With the two sources there to supply the loads with the energy they need, a system that squeaks by without consideration for the 120% rule is a time bomb just waiting to explode.
I’ll set up the scenario, but let’s first look at the calculation and define the NEC code that outlines the rule. NEC 690.64(B)(2), or 705.12(D)(2) if you're using NEC 2014, mandates that “The sum of the ampere ratings of overcorrecting devices in circuits supplying power to a busbar or conductor shall not exceed the rating of the busbar or conductor”. Further, it goes on to allow as much as 120% of a busbar’s rating to be exceeded. This is where the calculation comes in to play.
(Busbar Rating (A) x 1.2) - Main Breaker Rating (A) = Max PV (A)
Let’s start with an example. We have a 200 Amp bus rating for our service panel. In it, we have a 200 Amp main breaker.
200A x 1.2 - 200A = 40A
In this example, the maximum output of our PV system can be 40A or approximately 9.6kW. This would satisfy the busbar rating without an issue. However, in many instances there is a need or desire to have a much larger system, so what then?
If we exceed the rating of the busbar without regard to the 120% rule, we are creating a scenario where we are feeding a panel board with too much energy and have nothing in place to prevent the sum of the loads in that panel from drawing more current than the busbar can handle. Let’s say we have a 200A service with a 200A main breaker and have installed a PV system with an output of 60A. In this case, we have exceeded the bus bar rating of the panel board and could potentially be applying 20A of excess energy to it. This could be hazardous and lead to a fire. This may sound like a no-brainer and a something that couldn’t possibly slip by inspection, but it really does happen and hopefully a lot less frequently now with knowledgeable building inspectors.
What do we do in this case? Above all else, we need to have a PV system that is safe and installed to code. What can we do to remedy this and still retain the system size we have targeted? There are a few things that can be done that do not force a load or a line side tap to be done.
The simplest solution would be to downsize the main breaker in that service panel. So in our case above we could remove the 200A main breaker and replace it with a 175A main breaker to allow us to install up to 65A of PV. Call it good, right? Well, this approach is effective when taking careful consideration of all the existing loads in that panel board. There could be a potential for these loads to draw close to or more than the ampacity of that 175A breaker. We do not want nuisance tripping, so this approach must be done carefully. Refer to NEC 220 when considering this approach.
For another possibility, assume that you determined to downsize the main breaker is not going to work. For whatever reason, the loads' calculation proved to be a complete bust. Should we pack it up and go back to the shop? Certainly not! You have another solution in your bag of tricks. Shaving!
When downsizing a main breaker is not an option for the site, you should consider amp shaving. This adds a bit more labor and costs into the mix, but it will help get that PV system stuffed into that service panel. This works well in a system that is comprised of more than one string inverter or several branch circuits of micro-inverters. The result is that you can shave off a few extra amps before you connect the sub-panel output to the main panel board.
Consider that we are using two string inverters; an SMA Sunny Boy 2500HF and an SMA Sunny Boy 5000. The output of the SB 2500HF is 10.4A requiring it to have a 15A breaker. The SB 5000 has an output of 21A requiring a 30A breaker. The sum of these breakers is 45A and we already know we cannot land them in the main panelboard because it will only allow us 40A using the 120% rule. However, if we add our PV breakers into a sub panel that is dedicated to only the PV breakers we can land the sub panel into a 40A breaker in the main panel board (10.4A + 21A = 31.4A x 1.25 = 39.25A). Voila!
In a follow-up article, I will introduce Line and Load side taps which can also be considered in installations where the PV system supplying a service would exceed that of the service rating main panel board. These are most common in commercial and light commercial installations.
Note: There is an active discussion on "Do I need a service panel upgrade?"