DC Branch Circuit Notes for the Earthship

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 …


Electrical Rough-In Wiring of Earthship Tire Walls

It has been a few days since we finished our rough-in wiring, but I thought I would post about it before I forget the details.

As mentioned previously we spent some time after getting our electrical permit figuring out what type of wire would be required for the tire walls; the electrical inspector initially wanted us to use metallic or non-metallic conduit to protect against corrosion and moisture.  Based on our research of other buildings of cob construction in British Columbia we were able to show the building inspector that standard wiring and boxes have been used in buildings with similar construction techniques.

Last week we finally started the rough-in wiring of the tire walls.  I started working on the ends of the interior tire walls; we have wire running in channels at the tops of these walls and there will be switch outlets on the ends of all of these walls.  We decided to frame a wiring chase on the end of every wall rather than cob over this wiring.  There were two reasons for this choice; I was worried that there would not be a lot of cob covering the wire runs on the ends of the walls, and we intend to finish these chases so that they look like posts at the ends of the interior walls  … we hope that it will be aesthetically pleasing.  Making these boxes was slow work as the 2×6 forming the box had to be custom fit to the tire stack.  Eventually we will cob over the custom edges of the 2×6 when we finish mudding the walls.

On each of the end walls I put a 3 or 4 gang electrical box approximately 4′ above finished floor height.  These boxes will hold the light switches for the main room lights.  The intention is for all of the lighting circuits to be on DC power.  I ran some of these branch circuits using #10 AWG wire due to the length of the runs and number of lights on the circuit; I did not want any issues with voltage drop due to insufficient wire sizes.  I also placed a single gang box approximately 1′ above finished floor level in these chases.  This allows me to have a DC plugin in every room, something I was not sure how I was going to accomplish otherwise.  I did not include these DC plugins when ensuring that there was no point along the wall of the room further than 12′ from an electrical outlet; I ensured that you are always within 12′ of an AC plugin.

Once these end wall chases were finished we moved on to the wiring beside the east and west exterior doorways.  Here again electrical wiring for room lights travelled in a channel at the top of the tire wall.  This wiring was brought down to approximately 4′ above finished grade into a switch receptacle gang box by running it in the recessed grooves between the tires. The wires were attached to the tires using 3/4″ screws and CSA approved wire clips.  Here again, a single gang plugin was placed underneath the switch box.

Finally, all of the branch circuits were run for the plug receptacles on the tire walls.  All of these gang boxes were mounted on plywood screwed over the cavity between two tires in the wall.  A box was cut out of the plywood using a jigsaw and the gang box was mounted and screwed to the plywood.  Before mounting the gang box 6 mil poly and stucco lathe was stapled over the exposed face of the plywood.  The lathe extends beyond the edges of the plywood.  As described above the electrical wire was stapled in the cavities formed between the tires.

We passed the wire between rooms by drilling through the cob packed between the tires.  This worked well and reduced the wire runs as we did not have to go right out to the end of each wall.  We made sure the wire was stapled within 6″ of entering a gang box and at least every 5′ feet along its length.  At least 6″ of conductor was run into the gang boxes.

All of this work took two of us three days.

Too busy to post here

I have recently posted pictures to our FB page, The Darfield Earthship. Both Chris and I have been busy working in Kamloops and also on the house with James Hornett’s help.  Very little time to sit down and do more than click buttons to upload photos.  Chris has especially been wanting to post, but now that we are working on the planters, I think it will still be awhile before he gets around to it.  We are noticing that this component is the most taxing on the mind and on us!  See you on FB for the next little while.  Sandra.