Estimating the Sun

By Stan Shook | Feb 15, 2007
generic image




You’re reading an outdated article. Please go to the recent issues to find up-to-date content.

Make sure you don't get burned:

The future is getting brighter as solar power systems become more common on commercial and industrial projects; for electrical contractors, this growing market offers a money-making opportunity. However, it adds another costly system to estimate—one that carries the potential for losing money if not done right.

There are three basic parts to most solar and similar alternative power systems.

1. The Power Distribution System (PDS): We encounter this system on most projects. It is the main switchboard, distribution panels, transformers, disconnects and their associated feeders. This system, unless existing, is always part of the electrical contractor’s scope of work.

2. The Power Conversion System (PCS): This is typically the AC/DC power inverter. Sometimes this is a complete, single point-of-connection, packaged unit. It can also consist of several pieces of equipment, which is often an isolation transformer and disconnect switches on both the line and load sides.

3. The Power Generation System (PGS): This includes the solar or photovoltaic (PV) system, the windmill or whatever is doing the generating.

Systems 2 and 3 are usually up for grabs by either the certified EC or a specialty contractor. If an EC is not certified or experienced with installing solar power systems, or is not a manufacturer’s representative and therefore can’t purchase the system components, it will probably need to get a subcontractor quotation for a complete, “turnkey” installation. The EC would only be responsible for primary voltage (120V to 600V) connections with the actual installation of the PCS and PGS being done by the specialty subcontractor. However, the EC still needs to establish with the sub who is responsible for furnishing and installing the AC/DC inverter unit and any related components.

Knowing your company’s exact scope of work versus your solar subcontractor’s scope of work is critical. When you solicit a sub’s quotation, you need to make sure it includes everything you are responsible for. You must also coordinate and clarify each other’s exact scope of work and what each of you will or will not be furnishing and installing. For example: you wouldn’t want the sub to omit $50k in labor for equipment you assumed it was installing.

More questions to ask your sub: Who is responsible for furnishing and installing the wires from the combiner boxes to the solar arrays? Who is responsible for terminating the wires at the combiner boxes and at the solar arrays? Who is responsible for making sure the system is bonded?

The PDS 1-Line will typically consist of the primary power equipment with feeder(s) to each AC/DC inverter. At the inverter, there should be a disconnect switch on both the line and load sides. These will most likely be furnished and installed by the EC.

Remember, some inverters are complete packed units, so you must know whether or not to include these disconnect switches. Again, confirm this with your subcontractor or equipment vendors.

Leaving the inverter, there will be a DC power feeder heading out to each combiner box. Sometimes this is a short distance, sometimes a few hundred feet or more. From the combiner boxes, a series of wires heads out to connect to each of the solar panel arrays. Usually, these wires are run in conduits to metal wireways that run the width of the solar panel arrays. Typically, this wire is a No. 10 gauge copper, usually two for each array series.

The solar panel arrays are last. These are usually installed by the specialty subcontractor (unless that’s your company). They come action-packed with mounting brackets, stanchions, specialty cables, clips and a whole bunch of nuts and bolts. They are often installed on the roof of a high building or parking structure. Factor the labor into your estimate.

Often, the branch circuitry and feeder systems for the PGS are not properly designed. Sometimes the designs come from a boilerplate design-spec issued by the PV system manufacturer, and many electrical engineers in the industry just don’t have a clue as to what they are doing.

As solar panels generate DC voltage (two-wire system), a ground wire is not always required. However, you should always check the specifications, wiring schematics and verify any special requirements with the solar power system manufacturer or subcontractor.

Know your National Electrical Code (NEC) and the local codes. Check the specs and notes to find out if the engineer has asked for more “spare capacity” than the NEC requires. Also, make sure you check wire and conduit sizes, distances and installation environments. Perform some voltage-drop calculations. Per the NEC, you may have to increase your wire size, which may require you to increase your conduit and wire-way sizes.

Solar power systems are pretty simple to estimate once you get familiar with them. If you do your homework, ask the right questions, coordinate the scope with your sub and cover all necessary materials and labor costs—your future should be bright, sunny and free of any burns.     EC

SHOOK is the president and chief estimator for his estimating company, TakeOff 16 Inc. He has worked in the electrical construction industry for more than 18 years. Reach him at 707.776.0800 or by e-mail at [email protected].

About The Author

Stan Shook was ELECTRICAL CONTRACTOR's estimating columnist from 2005 to 2012. He works as an electrical estimator in California. Read his blog at or contact him directly [email protected]





featured Video


New from Lutron: Lumaris tape light

Want an easier way to do tunable white tape light?


Related Articles