In order to avoid costs for AC panel upgrades, it makes sense for inverter manufacturers to provide power classes that get the most solar power on an existing customer’s AC panel without upgrading it.

### The Bright Idea Behind Power Classes

Brian Lydic, Adam Gentner | Fronius USA

With the spread of residential PV power, more and more companies are making and selling inverters in odd sizes, namely 3.8 kW, 7.6 kW and 11.4 kW. But why is that?

The answer to that question lies within the AC service panel. At a 240 V grid, a 3.8 kW inverter is the largest inverter that is allowed on a 100A service panel, a 7.6 kW inverter is the largest allowed on a 200 A service panel and an 11.4 kW inverter on a 300 A panel. As the cost of upgrading a service panel can be very high, it is useful to fit the largest system possible on the existing panel. With most modern homes being supplied by a 200 A service panel, the 7.6 kW inverter is the largest inverter allowed on the home without updating the service panel.

To figure out why these odd sizes are the maximum for common AC panel sizes, we have to have a look on the inverter’s output current and how this is related to the AC panel size. First, we will need to figure out the maximum inverter output current that works for our panel and then check in which inverter power class that would result. After picking the power class, we can easily calculate the required breaker size for the inverter.

The Fronius Primo is available in power classes ranging from 3.8 to 8.2 kW. Why are these numbers so specific?

### THE MAXIMUM INVERTER OUTPUT CURRENT FOR YOUR AC PANEL

The National Electric Code (NEC) defines the relation between the inverter’s output current, the AC panel’s current (Ipanel) and the panel’s main breaker (Imain). To size a breaker for a typical load panel, NEC 705.12* is used.

### WHAT THE CODE SAYS

705.12

(3) Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards.

(a) The sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.

Informational Note: This general rule assumes no limitation in the number of the loads or sources applied to busbars or their locations.

(b) Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar

The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back-fed breaker from the inverter that displays the following or equivalent wording:

WARNING: INVERTER OUTPUT CONNECTION; DO NOT RELOCATE THIS OVERCURRENT DEVICE.

The warning sign(s) or label(s) shall comply with 110.21(B).

### LET’S DO THE MATH

Following (b), we can calculate the largest inverter output currents using the following formula:

With that we can easily calculate the maximum inverter output current for different AC panel sizes:

For a 100 A panel with a 100 A main breaker

For a 200 A panel with a 200 A main breaker

For a 300 A panel with a 300 A main breaker

### THE INVERTER‘S OUTPUT POWER

Once we know the maximum output current that the inverter can have on our panel, we can easily calculate the inverter’s output power. Given a grid voltage of 240 volts, we can use principles of electrical engineering to calculate the largest inverter allowed on these panels:

The main formula to calculate electric power is:

POWER = CURRENT X VOLTAGE

Maximum inverter output power = maximum I inv x 240 V

• 16 A x 240 V = 3840 W --> 3.8 kW is the max. inverter power allowed on a 100 A panel
• 32 A x 240 V = 7680 W --> 7.6 kW is the max. inverter power allowed on a 200 A panel
• 48 A x 240 V = 11520 W --> 11.5 kW is the max. inverter power allowed on a 300 A panel

### WHICH BREAKER SIZE IS REQUIRED?

Now that we know our inverter size, we can calculate the required breaker size. Breakers are Overcurrent Protection Devices (OCPDs) that protect circuits and equipment from currents that exceed their ratings. Breakers and fuses are the most common type of OCPD.

They are common in electrical installations of all types, and are sized according to Article 240 of the NEC*.

240.6 Standard Ampere Ratings.

(A) Fuses and Fixed-Trip Circuit Breakers. The standard ampere ratings for fuses and inverse time circuit breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.

(B) Adjustable-Trip Circuit Breakers. The rating of adjustabletrip circuit breakers having external means for adjusting the current setting (long-time pickup setting), not meeting the requirements of 240.6(C), shall be the maximum setting possible.

Article 690 of the NEC dictates the rules specific to PV Systems.

690.8(A) (3) Inverter Output Circuit Current. The maximum current shall be the inverter continuous output current rating. 690.9(B) Overcurrent Device Ratings. Overcurrent device ratings shall be not less than 125 percent of the maximum currents calculated in 690.8(A). Exception: Circuits containing an assembly, together with its overcurrent device(s), that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent of its rating.

For our use the important information is that the OCPD will be sized off of the Continuous Output Current rating of the inverter, multiplied by 125%. This number should be rounded up to the next highest standard size listed above.

Calculation Examples

Example a) If the max. continuous output current of the inverter would be 16 A, we multiply by 1.25 and get a 20 A circuit breaker.

Example b) If max. continuous output current of the inverter would be 28.9 A: 1.25 x 28.9 = 36.1 A. The next highest breaker is used (40 A in this case).

### CONCLUSION

The maximum current on a given AC panel is the reason for some odd looking inverter power classes. In order to avoid costs for AC panel upgrades, it makes sense for inverter manufacturers to provide power classes that get the most solar power on an existing customer’s AC panel without upgrading it. That’s why Fronius offers the Fronius Primo in the power classes 3.8 kW and 7.6 kW, or the Fronius IG Plus A with 11.4 kW.

The breaker size for the inverter is defined in the NEC and can be calculated with the maximum continuous output power of the inverter.