Electrical installations in container-based houses must follow both:
- National electrical standards (NEC in US) and
- Local building codes.
After all, such houses must receive the certificate of occupancy from Local Authorities, confirming that the place is safe and sound as well as hazard-free. At first it may seem to be nothing new as all houses must meet such requirements. In practice however, given the nature of container-based houses (metal boxes), their electrical safety standards are stricter than those of traditional housing.
For example, in traditional houses, NM-type (Non-metallic sheathed) cables are commonly used for lights and outlets circuits. They hang (more or less freely) between drywalls and pass through wooden studs via bored holes, because wood and drywall are isolators. The point is that the damaged insulation of a Romex cable hidden behind the wall won’t have effects on the operation of the electrical installation and its safety. Yes, it may cause open circuit, short-circuit (fuse will be blown), but there is very low probability of exposure to the high voltage (110Vac or 220Vac) and subsequent electrocution. The biggest problems with electrical installations you may face in traditional houses are “hot spots” (usually poor-quality connections) causing overheating and possibly – a fire.
The same effects (and potential fire) may also happen in overloaded electrical installations in container houses, although unlike traditional wooden structures, they will be less prone to turn into a “torch” in split seconds. However, unlike in traditional residential housing, in containers any contact of bare electrical wire with the wall or stud will expose the whole structure to the high voltage, and inhabitants to electroshock.
No wonder then, that when it comes to electrical installations, the National Electrical Standard (NEC) is the “bible” to strictly follow. On top of NEC, also local building codes may add some extra requirements and restrictions (especially for containers). At the end – of value is also involvement in the design and installation of electrical system of a certified electrician experienced in the field of metallic (container) houses. Do not underestimate the importance of experience in the matter, because especially electrical installations in off-grid container houses bring a range of challenges not seen in traditional houses connected to the municipal grid. Dealing with dual power installations (DC and AC power), renewable power sources, converters (Battery Chargers and Inverters), grounding and earthing etc… may quickly surpass the ability of home-grown DIY electricians.
Note: The following text applies to single or split-phase AC residential networks with Neutral wire as well as low-voltage DC networks. The situation is quite different in 3-phase (usually balanced) networks used by industry, but such networks are not subject of this article.
It’s important to know that from the point of view of safety of electrical installations not all container houses are equal. Different building codes and regulations may apply to:
Container house connected to the municipal grid
Connection to grid gives you benefits of unlimited access to electrical energy. It’s an “elegant” and mostly painless solution (if we put aside bills), however the final connection must be done by a certified electrician and then “blessed” by the Power Utility Company (installation of the power meter). In the case of single dwellings, the Power Meter should be installed in an easily accessible area outside of the home (usually on the exterior wall). While recently introduced Smart Meters can electronically communicate their status (reading of energy use) without human intervention, the easy access and “visibility” is still a valid requirement allowing for verification of Meter’s integrity by the supplier (preventing bypassing and modifications leading to the theft of energy …).
Grid – sometimes may look ugly and unappreciated, but these days it’s hard to imagine life without electrical energy. Source: engineeringworldchannel.com
When the container house is located next to your residence (itself connected to the grid), you may DIY-connect its electrical installation to your residential one. It can be done by using a permanent (buried) cable, but also by a temporary, flexible cable certified for outdoor use with adequate power rating. In many cases, it be considered as an “extension” to your residential installation.
In both scenarios, your exterior connection must meet relevant requirements while an interior electrical installation in the container house (including exterior socket) must meet usually stricter safety requirements than your traditional house! (more on that below).
Off-grid container house fed by solar, wind or hydro DC power with interior 110Vac (220Vac) installation fed by an inverter or diesel generator
When generating your own electrical power, you are the master of your own “electrical” destiny (in other words no power meter, no Power Utility inspections and no monthly energy bills). It does not mean however that your electrical installation is out of scope of NEC and relevant building codes. Note that converting your DC-generated power to high-voltage compatible with home appliances (110Vac or 220Vac) brings you back under the umbrella of related safety standards. Additionally, you will also face new challenges related to lightning and surge protection of your Power Harvesting System as well as safe operation of your Power Storage System (bank of batteries).
Complexity of the Off-Grid power system including Solar Panels and optionally Wind Turbine as well as Backup Generator. Source: ECAI 2017 – “Hybrid renewable energy sources management system” by Viorel MIRON ALEXE, Iulian BĂNCUȚĂ and Nicolae VASILE (Romania)
Off-grid container house fed by solar, wind or hydro DC power running at Low DC-Voltage (48Vdc or lower (24V or 12V)).
While it is a good engineering practice to follow well established rules for traditional electrical installations, it’s worth to remember that any dc (direct current) voltage lower than 50 V is considered as safe for human beings (in other words there is no risk of electrocution by accidently touching bare wire connected to 12, 24 or 48Vdc). Note that 48Vac (alternating current) has an amplitude of 68 volts so it does not fall into the range of safe voltages).
Operating your interior installation at low dc voltages brings new challenges, due to much higher currents and as a result – to overheating of wires. High operating currents put extra requirements on wires (must meet power transmission ratings) and quality of all inter-connections in circuits.
Shortly speaking – at fixed voltage, the level of current is determined by consumed power. Given the fact that Power = Voltage x Current, at lower voltages the transfer of the required power to the end-user implies higher currents. For example, the 1kW heater at 110Vac will require the current of about 9 Amps, but at 12Vdc it will be staggering 83 Amps.
Also, batteries used for storage of energy have their own safety requirements!
Commercial container structures
Electrical installations in container-based commercial and industrial structures are subject to more rigorous requirements and are out of the scope of this article focusing mainly on container houses.
For better understanding we will subdivide requirements and practical aspects of electrical installation in container-based houses as follows:
a) Interior wiring installation (to this group we also include exterior wiring for porch lights and eventual exterior outlet).
b) Exterior Power hook-up
c) Grounding system protecting from electroshocks
d) Earthing system as part of the protection from electroshocks, but also as the system assuring that container’s metal structure is equipotential and reliably stays at the potential “close” to the earth (considered as the “zero” reference level).
1. Interior wiring
Electrical conduit is a sort of tubing protecting exposed wires from mechanical damage and/or water (rain, humidity…). Typically, conduits are used in industrial “environments”, but they are also mandatory in exterior and (in watertight version) underground installations. Conduits can be also used to route electrical wires along selected paths (in other words to prevent wires from freely hanging between the walls). If documented, it helps to avoid damage to wires when drilling holes for objects to be hanged on the walls.
However, even if not required by NEC standards or building codes, conduits may be used as an extra level of security following the good engineering practice. And this is exactly the case of container houses. As the bonus – conduits also allow for an easy upgrade of installation (if required) or replacement of defective wires.
Cantex Industries A53AE12 Schedule 80 PVC Electrical Conduit, 1/2-Inch. Source: Amazon
Among many types of conduits, the most popular are rigid Schedule 80 PVC pipes. They are characterized by dark gray color which helps to distinguish them from water pipes (white). When used with glued fittings, PVC conduits can be watertight so also suitable for burial in the ground. They are relatively inexpensive and easy to install.
There seems to be some controversy regarding the use of NMC (Non-Metallic sheathed cables like Romex) in rigid conduits due to possible overheating. To make it clear – the NEC standard does not prohibit routing NMCs in rigid conduits. In fact, it addresses the internal heat problem by providing the guidelines for proper size of wires and diameter of conduit to reduce heat to safe levels for given applications (currents).
In conduits you can also use individual, insulated color-coded wires like THHN or THWN where letters – correspondingly T (Thermoplastic), H (Heat-resistant), W (rated for Wet locations) and N (Nylon coated for extra protection) indicate wires’ properties. Hot wires come in Black, Red and Orange colors, neutral in White and Brown and Ground wire in Green or Yellow-Green colors. Thanks to lower amount of insulation (no common sheathing), individual wires will have better heat transfer to air, and so better thermal performance. Such solution will be rarely used in 110Vac systems (when overheating is not a real problem, it is easier to deal with just one cable than with few individual wires). However, individual wires may be required in low-voltage DC-Power systems., when heat is actually the main safety problem.
Flexible conduits (corrugated pipes) do not provide any mechanical protection, however they may be used for routing and will allow to upgrade the wiring in the future (if needed).
From the practical point of view – PVC conduits can be easily used on walls intended for spray-foam or blanket-type of thermal insulation.
Touching the subject of thermal insulation – in container-based structures, conduits also prevent exposure of electrical cables to the water (moisture). Note that at low exterior temperatures (cold climate zones), the vapor generated by cooking, hot shower, breathing etc… will condensate at metal walls making them wet (even dripping). This will put all electric wires located next to the walls into “wet environment” – conditions for what they may be not designed.
Rigid conduits may be inconvenient on exterior walls where rigid block-foam insulation like polystyrene (EPS), polyurethane (PU) or Phenolic Foam (PF) will be used. Insulation panels will have to be cut to accommodate conduits adding extra work and jeopardizing continuity of the thermal insulation.
Fortunately, industry addressed this challenge by offering block-foam panels with raceways designed for electrical wiring. Good example is a series of InSoFast’s CX 44 panels. They have built-in horizontal and vertical raceways meeting NEC standards for Romex wires (main requirement is their location at the distance of at least 1 ¼” from the surface). They are especially designed for cargo shipping containers to fit walls’ corrugation pattern. They offer continuous thermal insulation with an R-factor reaching 11 as well as vapor and moisture control.
CX-44 insulation panels for container houses with supporting studs and raceways for electrical cables. Source: InSoFast
Here is the video:
c) NM-type cables
Conduits are not always the best solution for wiring. The main problem is substantially reduced heat exchange between potentially warm wires and surroundings. While overheating should not be the problem in 110Vac installations (and even less in 220 Vac ones), it may be the serious problem in low DC-voltage Power Transmission systems.
To give you some idea, let’s compare the amount of generated heat from the previous 1kW example. The energy (heat) dissipated in any wire is proportional to its resistance R (it is very small, but always non-zero) and square of the flowing through current (I). The 9 Amps current (in 110Vc system) will generate heat proportional to I2 (“81” in this case), while the heat generated by the 83 Amps current flowing through the same wire will be proportional to “6,889” (so 85 times higher). This may cause a serious overheating of cables and all poor-quality connections.
Obviously, we took here the worst-case scenario assuming the 12Vdc system to visualize potential problems. They can be largely mitigated (but not fully eliminated) by using higher dc-voltages. For example, at 48Vdc the required current for the transfer of 1kW of power will be about 21 Amps, and dissipated heat proportional to “434” (so only 5.4 times higher compared to 110Vac system).
Note that in traditional houses with 110Vac systems, most wiring is done with Romex 12-2 cables. The cable is rated for 20A and includes black (hot) and a white (neutral) wires as well as a bare copper wire for ground.
For completeness, here are the most popular Romex cable sizes in xx-2 configurations used in residential housing:
Note: The NM-type cables with 3 active wires: hot (black), hot (red), neutral (white) will be marked as correspondingly 14-3, 12-3 etc. Their most popular application is in circuits with 3-way switches. They can also be used in 220V/110V circuits (220Vac between two hot wires and 110V between any hot wire and the neutral one).
Romex cable 14-3 with two hot wires (Black and Red), Neutral (white) and GND – bare copper. Source: Alamy
It’s obvious that these popular Romex cables cannot be used in low dc-voltage systems (especially 12V or 24V), unless the circuits are designed only for lights and low power appliances.
In many states, the use of NM-type cables (like Romex) in walls of container-based houses is not prohibited. However, to be allowed, the wirings must meet some extra requirements. The most common are:
- Wires must be installed at the distance of at least 1 ¼” from the walls’ face to protect them from penetration by screws and nails (it’s also valid for cables in traditional housing). If this is not possible, then a metal protection plate has to be installed over areas where wire is located at a smaller distance from the wall’s face.
- – Wires passing via holes in metal studs and walls as well as floors must be protected from abrasions (sharp edges) by “sleeves”
- Wires must be secured within 8” of boxes (light switches, outlets etc), and at no more than every 4.5ft along their length.Note : due to the metal structure, traditional cable clamps cannot be used for that purpose (expanding adhesive foam can do the job). NM stands for cable with Non-Metallic casing (usually vinyl). Such cables can be installed ONLY in enclosed areas (within walls, ceiling or under the floor). One of the most popular brands of NM-cables is Romex. In contrast, BX-type cables have flexible metal casing (armor) protecting them from mechanical damage. The casing is grounded to earth – protecting from electrocution in the case of any (inside the armor) short-circuit.
2. Exterior Power Connection
a) Connections to Grid
When a container structure is located on your property next to your permanent residence you have several options. The easiest will be to connect it to your residential network by an appropriate aerial cable (rated for the required current and outdoor use as well as including Ground Wire). In fact, if your container structure is not considered as permanent, most likely an aerial cable will be regarded the same way as RVs’ shore connections on campgrounds. For security, the cable should be suspended well above the ground level (you may use posts or trees’ trunks to secure the cable). If exterior sockets will be used, then these have to be water and weatherproof models designed for outdoor use. For security, the source socket should have a GFCI (Ground Fault Circuit Interrupter)/ (RCD – Residual Current Device) protection.
Note that an aerial connection must be built with just one, continuous cable connecting the source (residential installation) with the load (container house).
Utilitech Pro 50-ft 12-Gauge 3-Prong Outdoor Heavy Duty Lighted Extension Cord. Source: Lowe’s
However, for permanent container structures, it is recommended to bury the electric wire in a trench using rigid, non-metallic conduit (typically Schedule 80 PVC). The NEC specifies the minimum burial depth at 1 ½ feet (determined as depth out of reach of typical garden-digging). In cold zones, it is suggested to increase the burial depth below the freezing line to avoid mechanical stress related to freezing and thaw cycles (subject to local codes).
The UF-type cables can be buried without a conduit however, the trench must be at least 2 feet deep. To make it clear – conduits are mandatory at the places where the UF cable enters and leaves the ground (2 ft into the ground and all along exposed cable above the ground).
Southwire UF-B Underground Feeder Cable, 6/2 AWG. Source: GlobalIndustrial (Canada)
b) Off-grid power connections
Solar as well as small Wind or Hydro Power systems operate at low DC voltages (12 to 48Vdc). While it eliminates the danger of electrocution, it does not mean that low-voltage wiring does not have its own challenges. Transferring a reasonable amount of power from the low-voltage generator to the user involves high currents and so puts tough requirements on connecting cables (note that Power = Voltage x Current so lower the voltage, higher the current).
All wires have non-zero resistance so a flowing current will cause a drop of voltage along them while the lost electric energy (Voltage Drop x Current x Time) will be converted to heat. The drop of voltage will decrease efficiency and performance of the renewable power system, while overheating may cause serious safety issues (fire). What makes it worse, is the fact that typically a source of energy (wind or hydro-turbine and to some extent solar panels) is usually located at some distance from the home. Obviously longer cables will only magnify the scale of problems (higher voltage drop, lower efficiency and more heat).
The bottom line – selecting: Adequate wires for the transfer of DC power and Right operating DC voltage are the two most important steps when designing an off-grid DC power system (and by far not easy ones).
The DC-voltages are standardized – basically nominal 12Vdc is used in the automotive industry, while its multiplications, typically 24Vdc or 48Vdc are used in trucks and telecommunication industry. Due to the widespread commercial use of 12Vdc batteries it may be more economical to build higher voltage system (24V or 48V) by connecting in series 12V batteries, than buying compact 24V or 48V models. Similarly, typical PV Solar Panels generate 12Vdc voltage, but you can also connect them in series to deliver electrical power at 24Vdc or 48Vdc.
To give a better idea of challenges related to low-voltage wiring let’s take an example of 6 and 10 AWG (American Wire Gauge) cables.
- 100 ft represents the length of the single wire. Any connection between the source of electrical energy and load must include feeding and return wires. This means that the calculated above voltage drops and power losses are valid for the cable (distance) of only 50 ft (15.25m) long!
- 12Vdc at the source will be seen as only 9Vdc at the end of a 50 ft long AWG-10 cable carrying 30Amps! Obviously, it will be well below requirements of any nominal 12Vdc equipment.
- Dissipated heat will increase the wire’s temperature and so its resistance. This in turn will increase the voltage drop along the wire, power loss and dissipated heat. Wire’s temperature will stabilize thanks to natural cooling. It will depend however on cable’s insulation (from this point of view, single wires will perform better than Romex-type cable with insulation and sheathing) and ambient temperature. Low tolerance of wire’s insulation to temperature and direct exposure to sun may lower wire’s current rating in a given application! To be on the safe side, NEC provides two different current ratings for cables – one for a single wire exposed to air (Chassis Wiring) and another rating (always lower) for bundled, sheathed cables (Power Rating).
- Often, connecting few wires (or cables) in parallel to increase the current rating may be a better solution than using one high-current rated low-gauge wire. The latter will be heavy and quite rigid which makes it less practical (more difficult to make connections, pull through conduits etc…).
- When selecting wires for high-current, low-voltage applications, it’s common practice to limit the round-trip voltage drop to less than 4-to-5 % of the nominal voltage at the source. Note that higher the nominal voltage (24V or 48V instead of 12V), lower the current, voltage drop, power loss and wire’s temperature.
- Requirements for interior low-voltage wiring (lights and outlets for low-power equipment) are much easier to meet compared to the main cable (trunk) for power transmission from the source to a bank of batteries and an inverter. Nevertheless, the same rules apply – at 48Vdc, currents will be 4 times lower compared to that in 12V network while power losses 16 times smaller (loss of power is proportional to the square of the current and wire’s resistance (I2 x R).
The above article cannot and does not intend to replace the National Electrical Code or relevant local building codes. It is meant for informational purpose ONLY! Its goal is to reveal to potential DIY “enthusiasts” and home-grown electricians the scale and seriousness of problems related to electrical installations in container-based houses. It should give ideas as to what kind of problems you will face when designing and making your electrical installation, what you should know when hiring a certified electrician for this job etc… However, the “Bible” still is (and last words belong to) the NEC and local building codes. It’s your responsibility to follow administrative rules! All parties associated with the above text are NOT Responsible for the use and results of this information by any party. Any hazard created when building the electrical installation based on information contained in this article is the SOLE Responsibility of the USER!