Selecting Boat Wiring.


This discussion is generally based on US Code of Federal Regulations 33CFR183. However, standards and regulations can change over time. For this reason, no discussion, and certainly not this discussion, should be followed without checking to ensure the standards and regulations are still current at the time of your project. You assume all risk with any project you undertake from this website, including the failure to completely understand and heed all safety issues, standards and regulations, or to ensure the standards and regulations are current at the time you begin your project.

There are two major authorities concerning selection of boat wiring. One mandatory and one voluntary:

• The mandatory authority is the US Coast Guard, armed with federal law.
• The voluntary is the ABYC, which publishes a set of standards with the goal of improving boat construction.

The mandatory USCG regulation (US Code 33CFR183) is primarily concerned with safety, while the ABYC standards are concerned with operational issies. As well, the ABYC standards incorporate the safety issues presented by the USCG. So important are the safety issues that the USCG regulation is actually federal law.

In other words, USCG regulation has requirements within it to make the boat wiring safe, not necessarily operationally functional, while the ABYC does both, since it incorporates the USCG safety regulations.

There are a few other standards, such as UL and SAE that publish requirements for wires, and are actually referenced within the USCG regulation.

General Requirements.

Wire means a single, stranded length copper wire, with an insulation that carries electrical current.

Cable means one or more bundled wires, typically with an overall jacket of insulation. Cable Assembly, or just "Cable" can also mean a specific length of cable with connectors attached to it. Generally, the requirements include both wire and cable.

Specifications that boat wiring must adhere to:

• All boat wiring must be stranded copper wire.

• All boat wire insulation must meet minimum USCG requrements for oil and water resistance, as well as be fire resistant as specified in 33CFR183.430. This regulation lists several industry specifications, that if the wire meets, means it is OK for use in a boat. One specification is UL 1426 (Electrical Cables for Boats). Ancor for instance indicates on their website that their marine grade wire exceeds the requrements of UL 1426, so you would be safe in using this wire on your boat.

• While Welding cable is sometimes used for heavy gauge boat cable, such as battery cable, it must meet meet the requirements listed in 33CFR183.430. Use of any wire or cable not having specifications listed in 33CFR183.420 is a violation of the regulation (and federal law). However, I have found that staying with good quality marine rated cable is no more expensive, and may be less prone to corrosion and other factors and would recommend this rather than using welding cable.

• The minimum premissible AWG rating of wire that can be run through the boat, according to 33CFR183 is 16AWG. However, 18AWG may be used if it is bundled with other wires. The idea here is boat vibration and such over time may snap a smaller wire. There are exceptions to this requirement in 33CFR183.425. This standard is in conflict with the use of some NMEA-2000 Micro backbone cable, as its power carrying leads are typically 22AWG. However, work is being done by the NMEA 2000 standards bodies to remedy this conflict (NMEA-2000 Backbone Cable Status). At any rate, I recommend the use of the thicker mid cable, as it has larger diameter power carrying conductors. While these larger conductors will meet current USCG requirements, their chief advantage is that they are less prone to voltage problems vs. the micro cables.

Any wire, regardess of its size, can only carry a maximum amount of current. The current it can carry varies with the cross-section of the copper as well as the temperature rating of the insulation. The amount of current that can be carried by the wire is also determined by the ambient temperature of the area in which it passes. Two classes of ambient temperature are used by the USCG regulation. Inside engine spaces (50°C) and outside engine spaces (30°C).

What this means is that you must reduce the amount of current you intend to carry through the wire if it passes through engine spaces. If a wire passes through both inside and outside of an engine space, the allowable current it can carry must use the more restrictive engine space rating. Even if a 30ft section of wire only has 5ft inside an engine space, that 5ft section is prone to overheating, so the whole wire must have the lower current rating.

As higher amounts of electrical current passes thorugh a wire, it can heat the wire up. If the wire heats sufficiently, the insulation will melt, and at some point, even the wire itself can melt. The combination of internal heating and external heating by the ambient surroundings is where the USCG temperature restrictions comes from.

There are different ratings of insulation, depending on the quality and intended purpose of the wire. Obviously, lower insulation temperature ratings cannot carry as much current as higher insulation ratings. Many of the "Marine Grade" wiring insulation is rated at 105°C. Also some have Wet and Dry ratings; for example 70°C Wet or 105°C dry. As always, consult with the manufacturer of the wiring you intend to use for its specificications.

Wire is further restricted in the current it can carry if it has more than 50 Volts (AC or DC) on it, and if it is bundled with other wires. The reason for this is any current flowing through wire creates a magnetic field around it, and the magnetic field increases with current and voltage. When the voltage approaches 50 Volts, this magnetic field, when bundled with other wires also generating a magnetic field, can interact and further heat the wire.

In a DC circuit, the length of the wire introduces a voltage drop across the wire. This can be significant enough to render the device to be powered by the circuit inoperable. While the USCG regualtions are silent on this matter, the ABYC recommends either a 3% or 10% maximum voltage drop rating to be used. For circuits that are voltage sensitive, such as electronic equipment, the 3% rating is recommended. For circuits that are not as sensitive, such as motors and lighting, a 10% rating can be used.

In the short lengths of wire found in a boat, AC powered circuits will not exhibit the voltage drop problem.

So now that you know the background, lets go thorugh a couple of examples on choosing the correct wire. To do this, we need three things.

1. Table 5 from 33CFR183 - Temperature ratings vs. Ampacity chart.

2. Table 1 from 33CFR183 - Circular Mils to AWG conversion chart.

3. Formula for determining voltage drop, which is:

CM = K x I x L/E

CM=circular mils
K=10.75 (a constant)
I = current in amps
L=round trip voltage in feet
E=voltage drop in conductor, in volts

If you don't want to bother with the formula in item 3, you can substitue items 2 and 3 with 3%-10% voltage drop charts, which are available from several sources on the internet. However, it is pretty easy to figure out with the above formula, and you can also calculate the specific voltage drop with it.

At this point, it is probably just easier to illustrate the concepts with a couple of examples.

Wire Selection Examples.

Example 1:

We want to install a spot-light on the bow of the boat, which requires a set of wires to the battery from the spot-light. The spot-light does have a remote, but since the remote cable came with the light, we are only concerned with specifying the wire for the spot-light itself.

The first thing we must do is to determine the voltage drop to properly size the wire:

The manufacturer has specified that the light requires 25 Amps @ 12 VDC to work.

We have to run 30 ft of wire from the battery within the engine room to the light. The boat is not 30ft long, but by the time we properly route the wire, securing it bulkheads and such, that is the amount of wire we need. This is sometimes called "cable-feet"

We decided we can live with a 10% voltage drop, since we are dealing with a light.

So we plug in the values (25 Amps, 60 ft round trip, 1.2 Volts) for the formula for Circular Mils:

CM = 10.75 x 25 x 60 / 1.2

returns 13,437.5 CM, which means we must specify a copper wire having this minimum cross section.
Refering to Table 1 in 33CFR183, we find this figure falls in between 8 AWG and 10 AWG. Therefore, we must chose 8 AWG wire.

We must now determine the insulation temperature rating specification for the wire. Since we are running part of the wire through an engine space, we need to ensure we reduce the current carrying capacity by that amount.

Refering to 33CFR183 Table 5, part 1 - we find that for 8 AWG wire, depending on the insulation temperature rating, we can run 55 to 100 Amps through the wire.

Since a higher rated insulation costs more, we found some 8 AWG wire at a discount store with lower temperature rated insulation - 60°C. We also checked 33CFR183 and found that this wire meets the required SAE and UL standards. Even at 60°C, this wire will allow 55 Amperes of current flow.

Since the wiring will be going through an engine space, we use Note 1 in the table to find that for this insulation temperature rating within the engine room, we must offset the current rating by multiplying the current carrying capacity of the wire by 0.58. The resultant current handling capacity of the wire will then be 31.9 Amperes - which is sufficient for our 25 Amp lighting requirement.

While the 8 AWG wire, having a 60°C rating is sufficient, what if we decided to use more common 105°C marine grade wire? Could we then use a smaller AWG wire? Well, no. Although a smaller AWG wire with a higher insulation rating may be safe, excessive voltage drop will be experiended along the wire, which may render the spot-light inoperative.

In this example, we found that the AWG rating of the wire we needed had more to do with a performance issue (voltage loss across the wire) and not the safety issue regarding the temperature rating of the wire's insulation. For this reason, we can use a more economical wire that has a lower temperature rated insulation.

Example 2:

We want to install a new 120 VAC powered battery charger in the engine room. Since this is a small boat with restricted room, we must run the wires along the same path as existing wires that go to the water heater.

The Battery Charger will draw a maximum of 10 Amps.

Since this is AC and not DC, we do not have to worry about voltage drop across the wire.


Often, wire is rated for both dry and wet temperatures. If the wire is subject to becoming wet - for instance in bilge areas - safety and common sense dictate using the lower wet rating.

The first thing to check is Table 5 in 33CFR183. Again, we will start out with the lowest temperature rated insulation for economy reasons.

Table 5 indicates that with 60°C insulation, we can use a 18 AWG wire for the 10 Amp requirement. While normally 16 AWG wire is the smallest sized wire that can be used, since this wire is to be bundled with existing wires, 33CFR183 suggests 18 AWG may be permissible.

However, we then must apply Note 1 since we are using the wire within an engine space. This results in a reduction of current carrying capacity, and after applying the reduction factor, we now find that we can only run 5.8 Amps - oops!

Although we already know that this wire is not large enough, for the purposes of the exercise, we must also take into account that we are bundling wire carrying more than 50VAC with other wires carrying over 50VAC (remember the water heater), so we must also apply Note 2 for 4 conductors.

We now find we need to offset the current carrying capability even more, by a factor of 0.6. This now gives us a total current carrying capacity of only 3.48 Amps. What appeared at first to be a suitable wire size was not the case, since the environment the wire would be in might cause it to overheat dangerously.

Since this wire size is not adequate, we need to look at a wire size to handle 10 Amps after the required reductions. Using the reductions of 0.58 for engine room space and 0.6 for bundling the wire, we find that we need 10 AWG for the 10 Amp battery circuit.

The short amount of math shows that a 10 AWG wire with 60°C insulation can handle 10 Amps, which includes reductions for engine room spaces and bundling with other wires. Therefore, this is the appropriate sized wire. At first, a 10 AWG wire seems overkill for 10 Amps, but this is due to the restriction factors involved with a wire having a low temperature rating.


We used the cheapest wire that would satisfactorly work in a marine environment we could find. Hoewver, what if we used a more common marine-grade wire having a 105°C insulation temperature rating, which is a bit more expensive than the cheap 60°C stuff we started with?

For a 105°C rating, we find that we can use 14 AWG wire, which after reducing for environmental losses, provides with a capacity of 17 Amps, more than enough to power our battery charger.

The lesson here is that the more expensive marine rated wire might in the end prove to be the cheapest, because we can use a 14 AWG wire rated at 105°C, vs 10 AWG wire rated at 60°C. The 14 AWG wire will likely prove to be cheaper.


The two excercises presented here describes the process that one must go through to select the proper sized wire, as well as illustrating trade-offs from using different grades of wire (i.e. temperature ratings). However, in actual practice, it may be advisable to further increase the wire size above what is required to facilitate the addition of more devices to the wire in the future.

Before leaving this discussion, the sharp reader will realize that when we bundle the wire, we not only reduce the current capacity of the battery charger circuit by 40%, we are actually also reducing the capacity of the water heater wiring by the same factor (remember we ran the battery charger wire along side of the existing water heater wiring).

What if the water heater wire was sized marginally to begin with. It could be possible that by simply running the new battery charger wire along the same path as the water heater wire, we could inadvertantly cause the water heater wiring to overheat.

Therefore, the task of determining what wire to use is not complete until we alwo determine the AWG, temperature rating of the insulation, and current demands of the water heater cable. Otherwise, we would do well to avoid placing the battery charger wiring near the water heater wiring.

In conclusion, some of this may seem too much bother than worth, and the temptation might be to simply throw any old wire in the boat, but realize that federal law dictates these requirements since they are safety oriented.


      USCG Boat Builder's Handbook (incorporating 33CFR183)
      Wire Sizing Calculator

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