Code Corner: AC Modules

AC module
This AC module has a maximum continuous output current of 0.94 A at 240 VAC (for a maximum output rating of 225 W AC) and comes with a 25-year warranty.
AC module
This AC module solution has the DC source cabling exposed.
AC module
AC module

Section 690.6 of the National Electrical Code covers AC modules—those that output AC current to tie directly to the grid. These modules are a fairly new technological development. Although this section does not cover all of the Code-related rules, there are some key points that designers and installers need to be aware of.

As with many sections within the Code, it is worthwhile to read the definitions. Section 690.2 defines an AC module as “a complete, environmentally protected unit consisting of solar cells, optics, inverter and other components, exclusive of tracker, designed to generate AC power when exposed to light.”

There are some products that look a lot like an AC module, but they are actually a microinverter connected to a PV module. This only can lead to confusion and misapplication of the Code. A true AC module is one with the inverter integrated into the module design at the factory—not added after the fact. Some industry people even contend that a true AC module will not have any exposed DC wiring; that the only wiring external to the module is the inverter’s output conductors (since there is nothing in the definition that prohibits exposed DC conductors, this is not an absolute requirement). But the presence of exposed DC conductors opens up parts of these NEC sections to interpretation. There are listed AC modules that have exposed DC wires. Since these are listed as an AC module by a nationally recognized testing laboratory (NRTL), my opinion is that the DC wiring can and should be considered internal to the listed product. In contrast, some AC modules integrate the inverter within the module’s junction box, keeping all DC conductors concealed.

Conductors & Overcurrent Protection

Section 690.6 (A) states that, since any DC circuits are considered internal wiring, the requirements for PV source-circuit conductors do not apply to AC modules.

DC conductor sizing and maximum voltage calculations are not required by Section 690.6(A), since they are provided by the AC module manufacturer and are covered in the module’s listing. According to the NEC’s second subsection in this section, the output from an AC module is considered an inverter output circuit and as such must follow those rules for circuit sizing and current calculations in 690.8. This results in calculations and conductor sizing that is the same as for any other inverter output circuit used in grid-tied PV systems.

Read the 690.8 subsection titles carefully, as the requirements set within vary based on the circuit you are considering. For example, 690.8(A)(3) applies to AC module circuits, but the other three current definitions within 690.8(A) do not. The maximum inverter output circuit current is defined as the continuous output current rating. To properly size the conductors and overcurrent protection, refer to 690.8(B)(1) and (2). This process is similar to the conductor and sizing calculations used for string inverters, as the Code does not differentiate between inverter technologies. You should also follow the manufacturer’s listing instructions for this portion of the installation, which may provide important details like the maximum number of AC modules per branch circuit and maximum ampacity of the overcurrent devices.

Section 690.6(E) provides allowances for overcurrent protection per 240.5(B)(2). This section deals with the ampacity and allowable overcurrent protection for fixture wiring, the factory-supplied conductors connected to the AC module. This section deals specifically with the fixture wire but does not specifically address the size of the branch-circuit conductor. Designers and installers do not have control over the conductors attached to the modules but are responsible for the branch circuits from the point of interconnection to the PV installation. When sizing conductors and overcurrent protection for these circuits, use the established methods outlined in 690.8(A) and (B).

Comments (4)

tpacyna's picture

Hey Ryan,

I have a question about the calculations for an AC module system in residential. So I understand that an AC system is looked at from an inverter output viewpoint but I am confused as to the safety/de-rate factors I should be using.
At this point I have been calculating it with the following formulas:
AC Max continuous output @ 240V X # of modules in largest string X 1.25(roof reflectivity= PV load sub-panel breaker.
(1.33A X 10 X 1.25 = 16.63A) 20A breaker
From the PV Load Sub-panel do I use another 1.25 (Continuous Load) to calculate ampacity?
sub-panel output = 78.17A
(78.17A X 1.25 = 97.71A to main panel)
So we would size the OCPD to 100A breaker.

I am just really confused on whether or not we use the 1.56 factor for AC modules. If I didn't use the 1.25 for reflectivity then I would size by main panel OCPD an 80A, which is a huge difference.
Please let me know what the correct formulas are for AC module output.

Ryan Mayfield_2's picture


This is something that comes up a fair amount. Since the output of the ac module is considered an inverter output circuit, you are only required to use the continuous use multiplier, you don't need the first "roof reflectivity" multiplier.

I'm not clear on the total number of modules in your array, but you can use the same method for the combined OCPD as you did for the branch circuit: Max output current x # of ac modules x 1.25 = minimum OCPD rating. So if you have 47 ac modules: 1.33A x 47 x 1.25 = 78A so round up to an 80A OCPD.

tpacyna's picture

Hey Ryan,

Thank you for your response!
Why do we use "roof reflectivity" for DC modules but not AC modules?

So to clarify, total system output into the main panel, regardless if there is a PV load Sub-panel, will be:
Max output of module X total # of AC modules X 1.25 = OCPD minimum.

This differs from DC modules with micro-inverters, correct?
Inverter Output X # of inverters X 1.56= OCPD minimum rating.

Thank you so much for your time, I have been looking for this answer everywhere!

Ryan Mayfield_2's picture

First off, "roof reflectivity" is probably better called something like "high irradiance conditions" given than any number of things can cause increases in irradiance, not just reflection from something like roofs. And the reason why we use them on the module output is because the current produced by modules is directly related to the intensity of the sunlight but the modules do not have a way of limiting that current (at least for the current "dumb" technology - as module-level power electronics are incorporated directly into modules, this very well may change). So, under high irradiance conditions, the modules are capable of producing more current than they are rated for.

Inverters (and ac modules by definition) do not have the ability to produce more current than they are rated for. If an ac module says the maximum current output is 1.33A, then that is its hard limit. Increases in irradiance may allow more dc current into the inverter, but the power electronics within the inverter will not allow more than the 1.33A (or whatever the inverter limit is) out on the ac side. So once you are on the ac side of your PV system (micro, ac module, string inverter or central inverter) the OCPD is calculated by the rated max output current by 1.25 for the continuous use factor.

1.56 is only used on the PV source and output circuits (dc) and only when dc optimizers are not used. Take a look at Code Corner in issue 159, there is more on the subject there.


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