Choose The Correct Wire Size

One of the common questions DIY boaters ask is what size wire they need when performing an electrical accessory installation. This is a good question to ask for safety as well as for performance reasons. Electrical equipment simply cannot deliver its best performance if it's not getting enough power, and wiring too small for the task is one surefire way to ruin an otherwise neat and orderly installation and limit the capability of the equipment. From the safety side, wire too small for the job creates excessive electrical resistance. The primary side effect of excessive electrical resistance is heat, and enough heat can start a fire.

With these thoughts in mind, let's walk through the steps to selecting the correct size wiring for that new electrical gizmo you absolutely must have.

Is It AC Or DC?

The first thing you'll need to identify is whether the electrical circuit in question is running from DC battery power, a solar panel or wind generator, or supplied by AC shore power, a generator, or DC-to-AC inverter. The criteria for determining wire gauge size is completely different depending on these power sources.

With direct current (DC) circuits, one of the primary concerns is what we refer to as "voltage drop." Because we typically are dealing with either 12 or 24 volts — comparatively low values — we're concerned about any voltage loss due to electrical resistance. With shore power and the like, we're typically dealing with either 120- or 240-volts AC — much higher values — so a little loss has a less dramatic impact on equipment performance. That said, because we are dealing with higher voltages and often higher amperage demands from the AC equipment in question, electrical resistance caused by loose, faulty connections and wiring that is too small for the job can generate significant, dangerous heat more quickly than a similar fault with a DC circuit.

Sizing for AC circuits

Because AC circuit wire sizing is a bit simpler than DC, we'll start there. Again, with AC, our primary concern is heat buildup as cables get routed through your boat. Sizing of individual conductors is based on three things: (1) how much amperage the circuit is going to have running through it, (2) whether the cable is routed through a hot engine room space, and (3) the temperature rating for the cable insulation. An additional concern is when multiple cables are bundled together, which increases the potential for even more heat.

AC wiring is primarily what we call "triplex" and sometimes "four-conductor" cable. The green grounding conductor is not counted when considering bundle size. You only need to count normal current-carrying conductors. So a single piece of triplex cable has two current carrying conductors: the hot wire and the neutral, or the black and white wires. In the case of 240-volt four-conductor cable, there are going to be three current-carrying wires per cable: two hot wires and a neutral. Again, the green grounding wire does not get counted. (Note: If you're dealing with a foreign-built boat, color coding and conductor count may be a bit different than described here.)

Another important issue has to do with AC cables routed through engine room spaces because of the potential for more heat. This heat may require "derating" to a larger wire size. I often get questioned this way: "The cable run is 14 feet long, and all but 8 inches of it is routed outside the engine room. Do I need to derate as if it were all routed through the engine room?" To comply with standards, the answer here is "yes." You must always rate based on the weakest link in a circuit. Cables are also rated for their insulation temperature rating, or how much heat they can tolerate. Most boat cable today is rated at least 105 C, and this temperature rating should be found on the outer jacket insulation on the cable.

There are two ways to find the correct AC wire size: The first is to consult the American Boat & Yacht Council (ABYC) AC wire sizing charts found within ABYC Standard E-11. Boaters can access ABYC standard E-11 by going to the ABYC website. Navigate to the recreational boater area, and click on the "become a member" link. Sign up for the free trial membership, and use that five-day window to gain access to the E-11 standard. In E-11, you'll find a series of tables to help you properly size AC conductors using the criteria outlined here.

A second, much easier alternative is to download the ABYC Wire Sizer app ($4.99) for iOS or Android smartphones. The app does a great job of getting you to the standard compliant wire size for both AC and DC installations and, unlike other apps available, will also convert to metric wire sizing for those of you with European-built boats. Keep in mind that the search does not realize that the minimum wire gauge size the U.S. Coast Guard will allow is 16 AWG (American wire gauge), with some exception for the use of 18 AWG under certain standards-specified circumstances. This applies to both AC and DC circuitry.

Sizing For DC Circuits

As already mentioned, we're particularly concerned about voltage drop with DC circuits, so part of the circuit design criteria is determining how long the circuit is. By that, I mean the distance from the power source to the load or appliance, and back to the source of power. Remember, all wire has inherent electrical resistance, and the longer the run of wire and the smaller the wire, the more resistance will be present. Also remember that the inherent resistance is less for heavier wire versus thin wire.

System voltage and amperage make a difference, too. With higher voltages, smaller gauge wire can be used to conduct higher amounts of amperage. This creates a compelling reason to use higher voltages, but that's a story for another day.

The other issue that needs consideration when sizing DC conductors is what level of voltage drop is going to be acceptable for the circuit in question. This is an area that the Coast Guard addresses in its regulations for recreational boats. ABYC mirrors the regulations in its E-11 Standard. The choices here are 3% and 10%. Consider that a 10% voltage drop means that with a fully charged battery, rather than 12.6 volts, only 11.3 volts will be present at the device. Will it operate correctly at that voltage? If not, you'll need a larger wire.

The 3% maximum voltage drop level is reserved for what the Coast Guard and ABYC believe are "mission critical" circuits. These circuits are identified as battery-to-distribution panel primary conductors, navigation lights, critical electronics for communication and navigation, and bilge blower circuits.

Many people ask why bilge pump circuits are not on this 3% list. It's important to understand that the Coast Guard does not even require bilge pumps, and in the eyes of ABYC, bilge pumps are intended for removing incidental accumulation of water in the bilge from rain, washdown, and the dripping stuffing box on a propeller shaft. A bilge pump will not save you if you put a 6-inch hole in your boat below the waterline! All other circuit wiring can be wired to a 10% maximum voltage drop level.

So to summarize, when sizing DC conductors, you need to know:

• operating voltage
• current draw in amps
• acceptable level of voltage drop
• length of the circuit

Once all this information is collected, it's back to one of several tables found in the ABYC E-11 standard or the wire sizer app. Plug in the data and get your wire gauge size. Several important notes on that: (1) Remember the smallest wire gauge size allowed by ABYC and Coast Guard is 16 AWG with some exceptions for 18 AWG, and (2) using the app is more precise than using the ABYC tables. With the tables there is a mathematical rounding effect, and the user should always round up in size, meaning that you will often end up using larger diameter wire than you actually need to get the job done safely.