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Stranded Copper Wires (Conductors) are responsible for carrying the electrical current (Amperage) throughout the Tiny Off-Grid House to power appliances and lights (Loads) . . . 

Since the Tiny Off-Grid House will generate and store 100% of its own electrical power it is of great importance that the electrical components are designed to maximize energy production and prevent energy loss.  The right type and gauge of electrical conductors are key to this strategy.  Highly conductive Copper (Cu) (like gold and platinum . . .) wires have very low resistance and will be used exclusively compared to less efficient Aluminum wires. Copper wires have less resistance to the flow of electrical current due to the mobility of electrons in their outer shells (Conductance band).  Aluminum is conductive, but has fewer available electrons in their conductance band resisting the flow of electrons. This resistance causes the wire to heat up; like the tungsten filament in a conventional lightbulb, or the restive wire found in a toaster, hair dryer. But the selection does not end there because wires come in different forms.

The focus is on alternating current (AC) carrying wires shape and gauges.  Wires come in two forms: Solid and Stranded:

Stranded wire are made of multiple strands of wire that are concentrically wound together in a helix.  The multiple strands are all covered in a single nonconductive sheath; while, a solid wire is made of a single core of copper covered in a nonconductive sheath.

The American Wire Gauge (AWG) is the unit of measurement for wires.  A low gauge number corresponds with a larger diameter (cross-section) wire.  Conversely, a higher gauge number corresponds with a smaller diameter wire. A 10 AWG Copper (Cu) wire has a diameter size of 2.588 mm while a 20 AWG Copper wire has a 0.812 mm diameter size. The larger diameter 10 AWG wire has less insertion loss than higher gauge wires that have a thinner diameter.

To determine which American Wire Gauge (AWG) is needed one must determine the length of the wire and the anticipated amperage it intends to carry. (Length & Amps = AWG).  It is standard in residential settings for 14 AWG conductors, sheathed in white, to be used with 15 Amp circuit breakers to power lights . . ., while 12 AWG wires, sheathed in yellow, to be used with 20 Amp circuit breakers to power receptacles . . .  Then there are 10 AWG wires, sheathed in orange used with 30 Amp circuit breakers.

The length and thickness of the wires will be determined by its ability to handle a large current (Amp) load; with atleast a 35% margin increase, along with how long the wire has to travel (wire run), with less than 3% voltage drop (Attenuation) which can be exaggerated by longer run lengths, temperature of the wires, the gauge of the conductor and higher frequencies.

When selecting the proper wire gauge one should also consider the right size circuit breaker/fuse that are designed to protect the wire and indirectly, the appliance (Load).  If the thermal rating of a wire is lower than a circuit breaker amperes rating the wire can overheat and become a fire hazard before the circuit breaker activates.  

Solid wires are great for corrosive environments but this should not be a concern with the Tiny Off-Grid House; this also rules-out the need for metal armored cable or conduit. As the gauges get smaller, larger size solid wire become more rigid and harder to work with.  This is important when routing wires through grommets or corners.  Solid wires are more compact in diameter than stranded wires.  However, the smaller diameter size does not reduce its current carrying capacity compared to similar gauge stranded wires.  The low tensile strength of a solid wire can be impacted by constant flexion or vibrations of a Tiny Off-Grid House on wheels which can cause breaks in the solid wire resulting in a series arc.

Stranded wires have greater flexing abilities that should enable it to handle movement and vibrations better than  a solid wire can.  This same feature makes it easier to work with when routing the wire through the walls and ceilings during the roughin stages.  Stranded wires have lower current carrying capabilities compared to solid wires.  The greater surface area in the individual strands of wire makes it carry less current compared to similar gauge solid Copper wires.  This is due to the Skin-effect.

The skin effect is the tendency of high frequency alternating current (AC) to confine travel (conduction) along a thin layer on the surface of the wire diameter; avoiding the inner core of a solid conductor.  This increases the resistance of a wire resulting in voltage drop.  The multiple air gaps between individual stranded wires amplify the skin effect created by the magnetic fields along the wires surface creating higher resistance.

After careful consideration and extensive research, it is determined that UL listed 12 AWG—for its high conductivity— Southwire Romex®, aka SIMpull, non-metallic (NM), branch (B), stranded Copper —no dissimilar metals— wire will be used for all circuit runs (30 Amps or less) to mitigate voltage drop (Attenuation). Romex is lightweight, heat & fire-resistant. Romex is made of multiple Thermoplastic High Heat-Resistant Nylon-Coated (THHN) wires bundled together and sheathed in polyvinyl chloride (PVC). The sheathing protects the conductors from abrasion, cuts and chemicals. Operating temps go up to 194 °F / 90 °C. Romex is designed for use in dry and damp indoor areas; no outdoor use. Compared to solid wire, the lower current carrying capacity of stranded wire is compensated by increasing the wire gauges from 14 to 12 AWG. The ability of stranded wire to function in environments with movements and vibrations, without compromising the tensile integrity of the wires, was one of the main selection points.

However, Just because the 12 AWG wires are overrated does not mean the over current protection circuit breakers should be also (2020 NEC 210.21 (B3).  The appliances (Loads) connected to the overrated wires are still designed, by their manufacturers, to operate safely at their respective amperages: In most cases, 15 Amps for lights and 20 Amps for most residential electrical outlets. 

WIRE SAFETY

Predetermined holes are cut in the metal studs as electrical wires conduits and some for the plumbing; while still maintaining the structural integrity of the metal studs.

Convenient pre-punched service holes are in the center of the steel studs, with protective plastic grommets, that allows for the safe passage of Romex wires from the sharp edges of the incision holes in the steel studs. 

The 2020 NEC Sections 300.4 (D) and 334.17 require protection for Romex wire when it is tunneled through holes in metal studs.  A metal plate 1/16” (1.6 mm) thick (2020 NEC Article 300.4(D)) must be installed on the stud infront of the grommet; for protection from perforations by nails or screws.  

Also, the Romex wires tunnel through plastic grommets which offers protection from the sharp edges of the perforated metal studs.  These perforations are centered atleast 1 1/4” (32 mm) from the front edges of the studs. NEC Sections 300.4(D).   

Firestop grommets and fire barrier sealants (NEC Section 300.21) are forms of passive fire protection installed to seal openings created by perforations through walls.

The number of conductors passing through the holes of the metal studs may seem to be determined to avoid capacitance and bundling that can impede airflow needed to dissipate heat. However, the number of conductors carrying current at the same time is unlikely. The other concern of bundling is mute since the conductors will be encapsulated in closed cell spray foam insulation; inside the wall cavity.

Spray Foam Insulation Impact on Electric Wiring: An Underwriters Laboratory of Canada (ULC) study from 1998 on the impact, if any, of SPF on electrical wiring shows “The results of this laboratory study supports the prevous findngs indicating that teperatures on conductors surrounded by polyurethane insulation can exceed the maximum service temperture for the wire insulation. These test included two most prevalent residential wire types, 14 AWG and 12 AWG, both rated at 90° C (194° F) This study and the literature suggest that eiter intermittent temperatures over the rated maximum of the wiring is acceptable to both the wiring manufacturers and the rated maximum of the wiring is acceptable to both the wiring manufacturers and the various electrical codes or that the electrical code should change to reflect reality.”

The NEC does not provide any recommendation on the number of conductors allowed to use the metal stud as a raceway—not to be confused with 2023 NEC 300.17 applicable raceway Articles 358.22 for EMT conduit & 376.22(A) for metal wireways. The primary concern with the number of conductors using the metal studs as a raceway is professional installation without damaging their insulation.

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