Code Corner: AC Modules: Page 2 of 2

Beginner
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

Disconnecting Means

AC modules also have requirements for disconnecting, which are detailed in 690.6(C). A single disconnecting means for one or more AC modules can be used when in accordance to 690.15 and 690.17. 690.6(C) stipulates that each AC module in the system must have its own disconnecting means. This can be the module connector that is used to connect multiple AC modules together.

In many situations, the module wiring output connector qualifies as the disconnect required in 690.17, so this will meet the Code requirements. Although adding a rooftop disconnect at the array location is not a Code requirement for AC modules, some installers prefer to include one. This additional disconnect can provide an easier location to disconnect the array without having to access conductors under modules and can provide a good visual indication that the conductors have been disconnected.

Since the circuit is carrying AC, there are numerous options for disconnects that can be integrated into the installation. The challenge will be to make sure you install a disconnecting means appropriate for the NEMA ratings of the enclosure.

Grounding & Bonding

Grounding and bonding is considered one of the most contentious topics for PV installations. For listed AC PV modules, grounding and bonding is a little bit easier. When attempting to provide the proper grounding and bonding for an AC module array, the first place to begin is with the manufacturer’s installation instructions. Not only is this a requirement per Section 110.3(B), but the manufacturer will likely have the best understanding of the proper way to install their product within the scope of the NEC. You can then use these guidelines and recommendations to reference the Code and make the final determinations.

Under Section 690.6(D), which covers the ground-fault detection for AC modules, the Code allows for a single detection device and removal of AC power to the system. The Code language here is permissive, rather than establishing requirements. At this time, the most commonly used method to provide the protection is to use the circuit breaker in an AC distribution panel. This breaker will protect the conductors in the event of line-to-line or line-to-ground faults.

One aspect of the AC module installation not covered in 690.6 is the grounding and bonding requirements. As for equipment grounding, a method for bonding all of the equipment together and providing a low-impedance connection for ground-fault detection is necessary. The AC module manufacturer may supply an equipment grounding conductor (EGC) within the cable assembly used in conjunction with the modules. All of the modules and racks still need to be bonded together using grounding lugs and a separate EGC or using grounding clips that bond the modules and rails together. If an EGC is not included in the AC module’s wiring harness, you will need to bond all the PV and racking components together and continue that conductor through a disconnect or transition wiring box to the electrical distribution equipment. This conductor is not required to be unspliced and will be sized based on parameters set forth in 690.45.

System grounding for AC modules occurs at the service entrance. The DC conductors are considered internal to the AC module and therefore a grounding electrode conductor (GEC) is not required for AC modules. The requirements set in 690.47(A) dictate the grounding electrode system installation for AC modules, referencing 250.50 through 250.60—all of which deal specifically with the installation of a grounding electrode system and grounding electrode conductor. Since an AC module system is connected to an electrical distribution system, the grounding electrode is already part of the electrical system. You may need to examine and bring this portion of the electrical system up to Code, but, generally, you will not be required to bring an additional GEC from the PV array to the existing grounding electrode. For this portion of the installation, it becomes very important to refer to the manufacturer’s installation instructions. If the manufacturer does indicate that a GEC is required, you will need to follow those instructions and meet their requirements. And as with any installation, it is a good idea to bring your authority having jurisdiction into the conversation early for this portion of the installation.

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Ryan Mayfield is the principal at Renewable Energy Associates, a design, consulting, and educational firm in Corvallis, Oregon, with a focus on PV systems. He is an ISPQ-certified affiliated master trainer and instructor at Lane Community College.

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?
Example:
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

tpacyna,

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.

Ryan

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