Overcurrent Protection Device, Wiring, and End-Use Equipment Design and Selection for AC and DC Circuits

Electrical branch circuits? What does that have to do with the design and development of sustainable human habitats?!?

In our Introduction to Appropriate Energy, we present a process for designing efficient (and thus cheaper and more ecologically-sound) energy systems for our homesteads which, simply put, is to match the energy source (ideally a locally abundant one) to end-use in the fewest conversion steps. There is a lot of research and innovation being done to develop more efficient energy sources and end-uses, especially sources and end-uses that can move or utilize electricity. Machines and materials that convert the energy contained in fossil fuels, wind, water, and solar energy continue to become more efficient at moving electricity, and components devices, and appliances such as LED lighting and Energy Star-rated refrigerators and washing systems continue to become more efficient at transforming that electricity into the cold air, light, etc. that we desire. And while we don’t believe electricity is the universal solution to our every energy need (see Introduction to Appropriate Energy linked above!) that it’s currently touted to be by the puppets on TV and in office, it is very appropriate for certain uses, and we believe can be used sustainably in our human habitats.

However, as the electricity sources and end-use components evolve, not much attention seems to be currently paid to increasing the efficiency of the conversion steps between them. Most notably, as both our electricity sources and end-uses continue to move towards utilizing direct current (DC) electricity (solar PV panels, batteries, LED lights, computing and communications devices, cordless tools), the power distribution systems and wiring between the power supply and end-uses and end-uses (called “branch circuits”) in nearly every residence and structure are still typically designed and installed as if utilizing a grid-supplied alternating current (AC) form of electricity. Getting these two different systems to work results in several levels of inefficiency (again, see Introduction To Appropriate Energy!).

While our AC power distribution systems, branch circuits, and appliances are certainly still very appropriate where AC power is the primary energy source (wind energy, hydroelectric, fossil fuel), we believe that in the homes or communities that are increasingly utilizing decentralized and off-grid solar and battery power, DC power distribution systems, branch circuits and end-use equipment should be utilized wherever possible. This article presents our design recommendations for branch circuits that can be used more efficiently in our modern electrical systems, with a special focus to DC branch circuits.

*Note: the information contained below includes recommendations derived from research performed by 7th Generation Design in designing efficient residential DC circuits, for which there is little-to-no existing standard.  Some of the information provided below may not agree with what National Electric Code (NEC) recommendations do pertain to residential DC circuits and parallel DC and AC circuits.  Anyone considering utilizing the recommendations provided below should understand that they are doing so at their own risk, and follow what local codes do exist.  7th Generation Design is not responsible for any loss of property or government intervention that results from the application of the information below.

Branch Circuits

A branch circuit is a complete wiring path from the output terminal of an overcurrent protection device (OPD, such as a “circuit breaker”) at a electrical load distribution panel to at least a single load (such as a light bulb or appliance, hardwired or via an outlet) and back to a return point at the panel, such as a neutral bus for an alternating current (AC) circuit or negative bus for a direct current (DC) circuit.  A branch circuit is distinguished from a feeder circuit, which is the circuit that supplies power to a load distribution panel from a source (such as a grid transformer, inverter, solar PV array, or battery bank).

A branch circuit can either include a single load or multiple loads. 

A branch circuit that includes a single load is called a dedicated branch circuit. Dedicated branch circuits are often used for any loads that have a large motor (refrigerator, washing machine, dryer, air-conditioner), as they are typically items that draw high current levels relative to lighting and communications/computing devices.

Loads like lighting and outlets for small appliances and devices are typically combined in a branch circuit. There are many ways to configure lighting and outlet circuits. A single branch circuit could serve the lighting and outlets in a single area (such as a room), or a single branch circuit could serve just the lighting in several areas, and a separate branch circuit serve the outlets in several areas.  We typically recommend the latter so that lighting is still available in case one of the outlet circuits is switched off. 

Branch Circuit Overcurrent Protection Devices (OPDs)

An overcurrent protection device is sized to protect electrical equipment and wiring by interrupting the current flowing through a circuit when the current exceeds the rated ampacity of the wiring and loads.

For alternating current (AC) load distribution panels, the OPD is almost always provided by a circuit breaker.  In direct current (DC) load distributional panels, a circuit breaker is also a good choice since if they are triggered by a short in the circuit, they can be simply reset once the problem has been addressed. There are fewer options for DC branch circuit breakers however – the Square-D QO Circuit Breakers are rated for up to 48VDC, and the Midnite Solar MNEPV breakers are rated for up to 150VDC.

Another option (and the cheapest) for low-voltage DC branch circuit protection is a fuse block, such as the one found in most cars – however, make sure you’ve got some spare fuses handy, as if one is tripped, it must be replaced!

The individual OPDs (whether breaker or fuse) should be sized for 125% of the maximum current anticipated in the branch circuit.  For example, if a single branch circuit from a 12VDC load distribution panel will serve six 6W light bulbs and two 30W USB outlets, the total wattage anticipated is 96 watts and the total amperage anticipated is 96/12 = 8A, so a 10A fuse should be used.

Branch Circuit Wiring

Typical AC branch circuits each utilize 3 or 4 wires: one or two “hot” wires (for 120VAC and 240VAC, black and red, respectively), a neutral (white), and a ground (bare copper or green). DC branch circuits each utilize 2 wires, a positive and a negative (typically either red (+) and black (-) or black (+) and white (-) – the standards are murky).

The wires of each branch circuit should be sized so that their rated maximum ampacity is larger than the OPD selected for the branch circuit, while minimizing voltage drop (<5%).  For example, 10AWG wires are rated for 30A, and thus can serve a branch circuit with a 30A OPD – however, if the distance from the load distribution panel to the load is significant, utilizing 10AWG wires could result in an undesirably large voltage drop, and thus larger gauge wiring (like 8AWG or larger) may be needed. An excellent tool for calculating minimum wire size for DC circuits is available through Wire Barn.

The combined loads in each individual AC and DC branch circuit must be connected in parallel, meaning the hot (AC) or positive (DC) and neutral (AC) or negative (DC) of each load in a branch circuit are all connected back to single supply and return points at the load distribution panel. The supply point for each branch circuit is the output terminal of that branch circuit’s dedicated OPD (breaker or fuse).  The return point is usually a common terminal bus – a neutral bus in AC circuits, or a negative bus in DC circuits.  In AC circuits, a ground wire is also utilized, connecting the ground terminal of each load to a common ground bus at the load distribution center.  This ground wires in AC circuits usually do not see any current – they are essentially a “backup” to the neutral wire, activating the OPD in the event that something the neutral becomes disconnected. Because DC circuits that are 48V or less are not considered dangerous to human life, this backup “ground” wire is not used in DC circuits.

The neutral and ground busses (return points) are typically bonded (connected together) at an AC load panel (unless they are bonded elsewhere in the system – they should only be bonded in one location!), and the ground bus is typically connected to a ground rod via an grounded electrode conductor (GEC). The negative bus (return point) in a DC load distribution panel is also typically connected to a ground rod via a grounded electrode conductor, or to the AC ground bus in an application with parallel AC and DC load distribution panels.

Since all of the neutral wires (AC) and negative wires (DC) are connected together in the system, in the case where multiple branch circuits are located near each other in an area, a single, larger neutral (AC) and ground (AC) and/or negative (DC) wire can be used to provide a single return path from the various branch circuits in the area back to the common busses. This is called a “multi-wire branch circuit”. The shared neutral, ground, and/or negative wires must be sized to accommodate the maximum current (amperage) anticipated by the combined loads in all branch circuits, and in an AC load distribution panel all of the circuit breakers serving the multi-wire branch circuit must be tied together so they can only be disconnected or reconnected simultaneously. In situations with both AC and DC circuits, the AC ground and DC negative return paths could potentially be shared by a single large wire, sized for the maximum current (amperage) anticipated by the combined loads of either the AC or DC branch circuits – whichever is largest.

An interesting case arises when AC or DC circuits are installed in a conductive metal structure, such as a vehicle or shipping container. Since the structure itself is one large conductor, it can typically be used as the common ground (AC) and/or negative (DC) return path for all of the AC and DC circuits contained (a separate neutral wire should be connected from the neutral terminal/wire of all AC loads back to the common neutral bus at the load center, as issues arise with having the ground and neutral of AC branch circuits connected in multiple locations).  Practically, this could mean connecting the ground (AC) or negative (DC) terminals of each outlet or wires of each fixture/appliance to the metal structure body itself at a nearby location (perhaps right outside the outlet or fixture, and connecting the ground or negative bus at the load distribution panels to the same metal structure.  This is done in all modern vehicles, and can save an immense amount of wiring in metal structures.

Branch Circuit Loads

AC Loads

Nearly every residence in the world that has electricity service utilizes AC branch circuits, and thus there is no shortage of information or options available when it comes to AC circuit connectors, lighting, appliances and other load components. We will not try to recreate it here – if you would like more information about AC branch circuit loads, we recommend starting at your local library!

DC Loads

Lighting

Electrical lighting is primarily provided by incandescent, fluorescent, and light-emitting diode (LED) bulbs – though, for a variety of reasons summarized well in this article, we never recommend fluorescent bulbs. Incandescent lamps can operate on either AC or DC, however the lamp should match the voltage of the source (a 120V incandescent bulb will work with either 120VAC or 120VDC, and a 12V incandescent bulb with work with either 12VAC or 12VDC, however a 120V incandescent bulb will not work well with a 12V source – though it will give off some light). The actual LEDs contained in all LED bulbs utilize DC. LED bulbs designed for use with DC branch circuits pass the DC supplied by the wiring straight through to the contained LEDs, while LED bulbs designed for use with AC branch circuits utilize built-in converters to convert the AC supplied by the wiring to the DC required by the contained LEDs (though there is some loss of efficiency in this process).

Conventional LED light bulb showing large portion of unit dedicated to converting from 110VAC to DC.

Because nearly all houses are still wired with AC load distribution panels (even those solely powered by a DC battery bank), most incandescent and LED bulbs found on store shelves are designed for 120VAC branch circuits. 12/24VDC bulbs with a variety of connection options, including hardwired strips and bases with sockets for replaceable lamps, are available online.  A good source for fixtures and lamps that can be used with DC branch circuits is www.12Vmonster.com.

Branch Circuit Outlets

Outlets for AC branch circuits are widely available – this is the typical 3-prong outlet found in the US (there are standard AC outlets in most other countries as well). 

Unfortunately, no nationally-standard residential DC outlets exists like that found in the AC world.  12/24VDC socket-type outlets found in automotive/marine applications and USB-A outlets are the closest thing to a “standard” DC outlet, though USB-C outlets are quickly gaining popularity and will likely become the most standardized DC outlet within the next few years. Besides some of the newest computing and communications devices, most DC devices come with either the socket-type plug or USB-A plug.

There are several good options for recessed and surface-mounted socket outlets that can be wired directly to a 12/24VDC load distribution panel. Our favorite option is manufactured by Blue Sea Systems:

As USB-A and USB-C powered devices become more prevalent, socket plug outlets (“car charger” style) with both USB-A and USB-C outlets that can be utilized anywhere a socket outlet is available can be purchased. This simplifies things by only necessitating socket-type outlets throughout a structure.

Many common household computing and communication devices designed with a plug for a 120VAC wall outlet in a grid-tied structure actually operate on low-voltage DC – the 120VAC is converted to the the device’s required DC voltage in a transformer (the plastic box or “wall wart” at the plug end, or bisecting the cord).  However, where a DC-supplied socket outlet is available, this unnecessary conversion step (and the resulting inefficiency) can be bypassed. For end-use equipment with a 12VDC input (this can typically be identified by looking at the “input” specification on the device, or the “output” specification on the transformer), this can be simply achieved by cutting off the “wall wart” and connecting the wires that remain to a socket plug that can be used with a 12VDC socket. Some common equipment that utilize a 12VDC input include many WiFi routers, Apple TVs and similar, and more. 

For DC loads with atypical input voltages (such as computers or tool batteries), an adjustable DC-to-DC converter can be used.  There are even some off-the-shelf converters for the most common devices, such as Macbook laptops (non USB-C powered ones), PC laptops, and cordless tool batteries.

---

That’s a wrap (for now!) on electrical branch circuit design. If you haven’t already, please check out our Introduction to Appropriate Energy, which provides a big-picture look at our relationship to energy and how we can use it more efficiently in our lives. And please reach out with any comments or questions!