Hopscotch: Stop Worrying and Love MLPE

Here is the summary!
1) Improved installation logistics on the ground and roof
2) Improved cable management when combined with U channel racking
3) Mounting the module BEFORE DC optimizer improves workmanship
The Hopscotch Process:
On the Ground:
a) Select a rail with a top U channel.
b) Pre-assemble the rail, rail splices, and grounding lugs.
c) Mark where the L-feet and module frames will land.
d) Make up your home run and lay-into the U channel.
e) Mount the DC optimizers on the other frame of THE NEXT MODULE OVER, with two optimizers under the last module at the end of the row.
f) Tie wrap the DC optimizer whips into the U channel on each side of the optimizer.
g) Lift the entire rail up in one piece.
On the Roof:
a) Attach the rail to the L-feet, square and level the rack, and complete your wiring.
b) Mount one module onto the rack.
c) Reach under the module to grab the module whips.
d) Plug the “tight whip” into the optimizer. There should be very little slack.
e) Wrap the “loose whip” around the “tight whip” to remove the slack.
f) Repeat.

If you’re a NABCEP solar installer, you are probably already engaged in a love-hate relationship with Module Level Panel Electronics (MLPE). MLPE has clear design and performance advantages to offer solar owners. But MLPE exponentially increases the cabling and electronic failure points on the roof.  At the very least, MLPE means addition installation time. Under the worst case scenario, rodent-destroyed cables or otherwise faulty optimizers can result in numerous trips back to the job site which are not always supported by the manufacturer warranty. 

There is no way to fully eliminate this risk, as individual panels are plugged into their adjacent neighbors during installation. Cable management techniques such as tie-wraps and cable clips have their shortcomings too. Even the best cable management system does not address the exposed length of cable between the module junction box and module frame, which will inevitably droop over time.

Every solar installer has their favorite cable management technique, and I’ve finally honed in on a MLPE-mounting technique I’d like to share.

Do the Hopscotch!

1). Select any racking system with an “open U” top channel. I like the Everest Cross-Rail system, but even Unistrut will do. 

A U-shaped top channel assists easy cable management.

2) Everything except the L-foot attachments and module mounting is done on the ground. For example, you will fully assemble the solar rail and then lay the home-run cable into the U channel, before lifting up to the roof.

Pre-assemble the entire rail length on the ground (including rooftop home-run cable).

Liberal use of tie-wraps mitigates failure and vanishes exposed cable.

3) Measuring and marking the rail is critical for ground pre-assembly. With practice, much of this work can even be performed at the office before getting to the job site. The goal is identify where every single component, down to the zip tie, will land on the rail in order to avoid any conflicts up on the roof when mounting the modules. You want to mark the following:

•  Module End Clip •  L-foot attachments

•  Module Edges •  DC Optimizer Locations

•  Module Mid Clip

4) The Hopscotch!

Traditional Method:

DC Optimizers are commonly located on the rail underneath the solar module, located to not interfere with the module edges or L-foot attachments. Before mounting the module, the optimizer is plugged in. Then the module is lowered in such a manner that the loose cable whips remain atop the optimizer and below the module. Combined with pre-assembly, this can result in a very “clean” looking installation. However, this process requires multiple hands to install, and cables remain loose underneath the array, which could be problematic down the road. 

Traditionally, the optimizer is located under the module with awkward cable management to prevent drooping.

Hopscotch Method:

Locate the DC Optimizer underneath the next module over. This may sound like heresy. But consider the advantages of the next few installation steps.

5) Back on the ground, mount the optimizers on the rail and plug them into their next-door neighbors. Make liberal use tie-wraps, at least one on each side of the optimizer, with additional tie-wraps to keep any slack in the U-channel. Doubling up the cable-whips provides cheap insurance that the cable will remain in the rail over the life of the project.

6) Lift the entire rail assembly in one piece up to the roof. Attach the rail to the previously mounted L-feet and square, completing the rack installation. At this time, you will also want to finish wiring your transition box and home-run cable run down to the inverter.

7) BEFORE plugging the module to the optimizer, mount the module onto the rack. If you used the Hopscotch layout, you no longer need to juggle the module and optimizer connections, substantially improving the installation time of this process.

Hopscotching allows mounting the module BEFORE cable management.

8) Reach underneath the module and grab the loose module whips. Plug into the exposed optimizer whips. There should be almost no slack in the “tight” module whip. Wrap the “loose” module whip around the tight cable to eliminate any slack. Compared to the traditional method, you will be able to see the cable whips running underneath the module frames. But because there is no slack, the cable is secure and will not shift over time.

Wrap the 'Loose Whip' around the 'Tight Whip'

9) The final module will have two optimizers underneath. After “hopscotching” the second-to-last module, the final module is mounted in the traditional manner.

And that’s it! Through careful planning of every system component, ground-mount pre-assembly is utilized for a quick and clean installation on the roof. The additional planning time is more than made up for through reduced time spent on the roof, eliminating all sorts of project risks such as installer fatigue or unexpected rain (which can damage “unplugged and exposed” optimizer). Any misplaced optimizers can be easily adjusted AFTER mounting the module. Because you no longer have to fight MLPE mounting and cable management obstacles, you can finally learn to stop worrying and love your module-level panel electronics!

'Vanished' cable with hopscotched MLPE

Blog Author John Cromer is a NABCEP-certified solar installer, mechanical engineer, master electrician, and residential builder. He’s on a mission to convert Mississippi to 100% solar power. Because if a “bad solar policy, low electricity pricing” market like Mississippi can flip to solar, there will be no more excuses for carbon-heavy electricity and the rest of the USA will follow.

If you liked this post, why not support his mission by registering for The Glass-Slapper Guide to Solar Power for continuing education? The program meets 100% of NABCEP continuing education requirements and includes unlimited course access and updates.