Recently, I prepared two estimates for Tiny Home communities for the unhoused. I had some learning to do on the first estimate, as this was a new type of construction for me. 

The homes, which are also called pallet shelters, were about 7 feet square, and were shipped to the project as a set of pre-assembled wall and roof panels. Most of the electrical work was factory-installed, including a small panel, three receptacles, one smoke detector, one air conditioner and one electric heater. 

The heater and air conditioner were connected to a selector switch so only one of the loads could be used at a time. We were required to add a small light to the front of each home. All the wiring was installed using NM cable. The original scope required fire alarm wiring from every pallet back to the main administration pallet. An addendum eliminated that work and added a requirement replacing every smoke detector with a wireless model.

A novel design

There were 82 residential pallets and several larger ones, including six hygiene, three administration, one storage and one administration/laundry pallet. The power distribution started with a 600A, 277/480V, three-phase, four-wire switchboard. The switchboard contained five 2-pole circuit breakers, which fed five disconnects. The disconnects were then connected to five 480V to 120/240V single-phase transformers. Each of the five transformers fed a panel.

Three residential pallets shared a 50A, 120V circuit. Each hygiene pallet was on a 200A, 240V circuit. The laundry pallet was on a 250A, 240V circuit, and the administration pallets were each on a 50A, 120V circuit. The final load was 11 site lighting poles, which were fed by a few 20A, 120V circuits.

The grounding requirements were mostly indicated on the single line diagram. Each disconnect, transformer and panel required grounding per the local electrical code. A detail sheet described the remaining grounding. Every shelter required a ground rod, and every ground rod was bonded together, creating a large grounding grid. There was also a requirement to ground the fencing.

The detail drawings for the feeders from the panels to the various pallets were confusing. They included overhead aerial wire, overhead galvanized rigid conduit and underground conduit with T-conduit bodies for rising to each shelter. I sent a request for information to the general contractor. The reply required us to use the underground conduit option.

What’s it all made of?

The material requirements for underground conduit contained several conflicts. The specifications called for Schedule 40 PVC, if approved. The specifications also called for slurry-encased Schedule 40 PVC, if approved. Both PVC specifications required the last 2 feet of underground conduit to be galvanized rigid conduit, as well as the conduit rising above grade. 

The most stringent (and expensive) requirement was shown on a detail plan sheet, and it called for all underground site electrical distribution to be galvanized rigid conduit with EYS seals (explosion-proof sealing fitting) at every riser. This requirement was due to the presence of methane in the ground. Eventually, the designers decided this was not a methane zone, and we could use Schedule 40 PVC, direct buried with no encasement. The riser conduits were still required to be galvanized rigid conduit. 

The feeders to the pallets were not laid out on the drawings. This was a bit of a challenge, but also an opportunity to lay out the feeders in the most economical pathways. I worked to minimize the length of each feeder, while trying to share trenches, as much as possible. It helped that the switchgear locations were arranged so the panels were close to the pallets they were feeding.

As I believe estimators must fully understand the projects they bid on, I did some research on these prefabricated pallet shelters. I called the manufacturer for some insight on how the electrical was installed at the factory. 

It turned out that most of the electrical work was concealed behind a plastic panel on the back wall of the structure, including the panel, receptacles, switches and wiring. The wiring to the smoke detector was concealed in a roof framing channel. Each of the residential units were shipped on a pallet and could be set up in less than an hour.

I estimated two of these tiny home communities. However, my customer and I were unsuccessful on both. We were not very close to the successful bidder. We did everything we could to lower our second bid, but could not get it low enough. My customer eventually discovered the reason—the successful bidder ignored the specifications. Their bid was based on all underground feeders being direct buried cable, instead of wire in conduit. Small wonder we could not match their price.