Balance of System Material

Internal Conduit

Figure 52. Planning the Home Run

Internal conduit runs go through the attic rather than across the rooftop. These runs are easy with AC micro-inverters, but DC solar output circuits must be inside metal when inside buildings per code.

Many installers will stay outside the building, because attics are not fun to work in. But flexible metal clad cable can often be used to keep the internal cable run simple. It’s expensive and a specialty

item that requires advanced planning, but in many jurisdictions it can run directly from a rooftop transition box. 

Figure 53. Rooftop Transition Box 

The one pictured is a Soladeck and it can be useful just as an empty shell as a transition box. The MC cable then runs through the attic can comes out the soffit on the other side of the home, where the inverter is located out of direct sunlight. There are some additional conduit requirements on that other end, but ultimately the internal cable run through the attic results in a very nice looking installation. I use the same technique to move from one subarray on the roof to another in a visually appealing manner.

Figure 54. MC Cable and Cable Glands

The problem with buying generic material at the local big hardware store, is that it is not designed to fit nice and tidy underneath a solar array. Advanced installers pride themselves in clean cable management and well-looking array layouts. These additional steps result in long term resale value, whereas an unprofessional installation may result in a clunker.


Figure 55. Supply Side vs. Load Side Interconnection

Regarding interconnection, there are two main strategies. Either the power lines into the home are intercepted and tapped onto, between the meter and the main breaker, or the solar inverter is landed on a breaker on the existing electric service panel at the bottom of the panel. When planning a panel layout, it is also useful to reserve the top of the busbar for an emergency power supplies. In other words, instruct electricians to leave the top and the bottom of the busbar free of breakers when planning future expansion, with the bottom having higher priority.

At any rate, there are flexible options to interconnect an array

on site. The existing infrastructure can almost always be optimized or improved upon. But having a solar array by itself is much less complicated than adding a battery and wanting the system to operate and power the whole building, while connected to the grid, but switch into an isolated offgrid mode automatically. Those advanced project solutions are still being figured out today, and our other classes will better prepare you to make those decisions. 

Backup Power

Figure 56. Battery Inverter Interconnection 

But customers pursuing backup power should be prepared to add an additional $15k or more in project cost to incorporate a rather modest battery. The batteries in a 120kwh Tesla car, for example, costs Tesla about $15k for the batteries themselves. Taking into account the complicated interconnection equipment required, running a whole house off grid can add over $50k in additional project cost. So there are advantages of a grid connection, and much of solar design is planning the array around the customer’s specific rate structure.

I think adding a 200A automatic transfer switch ahead of a supply-side interconnection can make sense, although it is much cheaper and easier to stick to only powering smaller critical loads during a power outage for most customers. When planning an electrical room, it is not a bad idea to leave physical space around the existing panel for expansion.

Figure 57. Generator Interlock Switch

There are “low budget” manual ways to backup an entire home during a power outage using listed, inexpensive parts. Here is a manual generator interlock switch which can be used to provide generator power to a home during a blackout, and it could be almost as easily connected to the critical load output of a battery inverter (with additional considerations to prevent the inverter from feeding itself). 

But that gets into the realm of solar design, and it turns out, we have a four hour class on solar design and computer-assisted design capabilities as a next step in your solar journey, so please reach out if you want more information.

[button outline=”true” url=””] Racking [/button]

[button outline=”true” url=””] Register [/button]

[button outline=”true” url=””] Design Your Solar Roof [/button]

Tags: ,
Not Permitted for Redistribution. Licensing available on request.

Posted July 2, 2020 by John Cromer in category "Material Selection