Fronius Inverter Power Classes US Technical Article [EN]

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THE BRIGHT IDEA BEHIND POWER CLASSES

TECHNICAL ARTICLE

Authored By:
Brian Lydic, Standards & Technology Integration Engineer, Fronius USA

[e-mail]

Adam Gentner, Group Leader, National Technical Support, Fronius USA
[e-mail]

EXECUTIVE SUMMARY

/ 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

/ The Fronius Primo is available in power
classes ranging from 3.8 to 8.2 kW. Why
are these numbers so specific? Read on
to find out more!

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 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 (I

panel

the panel’s main breaker (I

main

/ To size a breaker for a typical load panel, NEC 705.12* is used.

).

) and

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:

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

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

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

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.

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LET’S DO THE MATH

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

I

Therefore, I

x 125% + I

inv

main

= I

= I

inv

panel

panel

x 120%

x 120% - I

main

125%

/ 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

I

= 100 x 120% - 100

inv

maximum I

= 16 A

inv

125%

For a 200 A panel with a 200 A main breaker

I

= 200 x 120% - 200

inv

/ Article 690 of the NEC dictates the rules specic 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

maximum I

= 32 A

inv

125%

For a 300 A panel with a 300 A main breaker

I

= 300 x 120% - 300

inv

maximum I

= 48 A

inv

125%

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 o of the Continuous Output Current rating of

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:

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

Example b)

POWER = CURRENT X VOLTAGE

Maximum inverter output power = maximum I

x 240 V

inv

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

If the 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).

*Continuous output current is listed as nominal output current in the manual ratings tables.

CONCLUSION

/ The maximum current on a given AC panel is the reason
for some odd looking inverter power classes. In order to avoid

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?

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

/ 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

can be calculated with the maximum continuous output
power of the inverter.

CONTACT US

/ If you have any questions about this or if you want to learn
more about our solar solutions, do not hesitate and contact
us at [phone] or [e-mail]. We

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

are happy to help you!

REFERENCES

/ * For all references to the NEC, the 13th edition, 2014 version was used:

Earley, Mark W, Christopher D. Coache, Mark Cloutier, and Gil Moniz. Nation-

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.

al Electrical Code Handbook. Quincy, Mass: National Fire Protection Associa­tion, 2014. Print.

Fronius USA LLC
6797 Fronius Drive
Portage, IN 46368
USA
www.fronius-usa.com
[e-mail]

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