We have always planned to use DC LED lights and some DC appliances in the earthship.

I have been scratching my head a lot trying to figure out wire sizes for these branch circuits.  The lower voltage in a DC circuit means higher currents have to be allowed for in these circuits and larger wires must be used so that unacceptable voltage drops do not occur.  The following image is a screen snapshot of the spreadsheet of branch circuit calculations that I performed to figure out wire sizes.

The actual spreadsheet can be found  by clicking here.

As can be seen from the spreadsheet I am planning to use 24 volts DC allowing for at most a 2% voltage drop.  I assumed that my light loads would not exceed 13 watts per installed light fixture (which I believe is much more than I will need with LED’s) and I allowed extra capacity on the circuits for items like ceiling fans.  Finally, I was very conservative when estimating the actual length of my branch circuits.

This led me to install mostly 12 gauge and some 10 gauge wire for these circuits … an initial extra cost that will hopefully pay off down the road.

Some of the outstanding questions that I still have:

• Can I install my smoke detectors on a DC branch circuit?  There are a few issues here; are DC detectors built like AC detectors, will the inspector have any problems with this?  My initial thought is that it makes the most sense to install the detectors on DC branch circuits, but I am still learning and investigating.
• What style of plugs do I use for DC plugin receptacles?  Obviously, they need to be distinct from AC plugins, but is there any standard here?

The two most useful resources I have found while working on all of this are:

• Photovoltaics: Design and Installation Manual (By Solar Energy International?)
• Electrical Codes Simplified (by P.S. Knight)

I think my next step in the electrical is to go back to the distribution panel and start figuring out the DC side of the system …