ELECTRICAL SYSTEM  Wire Size
Electrical regulations and standards
that apply to boat builders and recreational boats. Calculating
Wire Size .
The following are not verbatim from the regulations. It is my own wording. Get a copy and read the regulation!
The US Coast Guard Electrical Regulations
33 CFR Subpart I sec 183401  183.460
https://newboatbuilders.com/pages/33CFR_electrical.html
How to Calculate Wire Size: These are only examples and each
installation will differ.
Before I start, I will define a few terms
that you need to know.
Volts: A unit of
measurement of force, or pressure, in an electrical circuit. The common
voltage of an AC power line is 120 volts of alternating current
(alternating directions). Common voltages used on boats are 12 volts DC
and 120 Volts AC.
Amperage(amps): the
strength of an electrical current measured in amperes.
Watts: A measure of power.
Watts are found by multiplying volts times Amps: For example 120 volts
times 1 amp equals 120 watts, which is about the amount of power to light
a 100 watt light bulb.
Ohms: A measure of
resistance to the flow of electrical current. One ohm equals one volt
divided by one amp. 120 volts divided by 1 amp would be a
resistance of 120 ohms.
Load: The load is the electrical
equipment on the circuit. Each item draws a certain amount of amps. The
amperage is the load for that circuit. If you add up the amps for each device
the total is the total load.
So what does all this mean to you? If
you have a small boat much under 25 feet, it probably has a 12
volt electrical system, which means one, or two batteries.
Most small boats have one battery for lighting the navigation lights, starting the engine,
running the depth sounder, the instruments, and bilge pump. If
you have a bass boat you may have a second battery for running the electric
trolling motor. Each of these pieces of equipment runs on 12 volts, but
they draw different amounts of current or amperes. How do you know how
many amperes they need? Most electrical equipment is rated in Watts.
So you can easily find the amperes by dividing watts by volts. Example:
A 300 watt depth sounder needs, 300/12 = 25 amps.
Wire sizing:
You need to know the amperage of a circuit
to calculate the wire size for that circuit. If that circuit has
only one load, or piece of equipment on it then selecting the wire size is
easy.
First select the temperature rating of the conductor. What is
temperature rating? Wire comes in different temperature ratings
depending on what temperatures it can be exposed to. This is the
rating for the insulation on the wire, not the wire itself. It is rated in
degrees C (Celsius). But it is
simpler than that. Most boat manufacturers use 105 degree C wire. So stick
with 105.
Next, will the wire be traveling through
the engine room, where it can get pretty hot? If so then there is a
factor that needs to be figured in to compensate for the elevated
temperature.
Suppose you have one
circuit, with one load, a depth sounder, and the wire doesn't go through
the engine room. Look at
the table
and find the column for 105 degrees. https://newboatbuilders.com/pages/elect2.html
Our depth sounder needs 25 amps. Look down the 105 deg. column for 25 amps
under the column labeled "outside engine space". If 25 amps is not
found go to the next higher number. Then move over to the left to
the first column that says at the top, conductor size. In this case,
16 gauge. If the wire had been in the engine room the wire size
would have been 12 gauge. Under each column for temperature
rating is a column that says "outside engine space" and
"inside engine space". The people who made this table have done
the calculation for you that compensates for the hotter engine space.
Now suppose
you have more than one load on this circuit. Suppose you have the
depth sounder, your instrument panel and a marine VHF radio.
You simply add up the amount of amps for each device. Depth sounder
25 amps. Instrument panel 3 amps, radio 5 amps. 25 + 3 +
5 = 33 amps. Then add 10% to the total. That is 33 + 3.3 = 36.3.
Round it up to 37 amps. Consult the table again. This time the
wire size would be 12 gauge. Why?. Because 30 amps is a 14 gauge
wire, but since you are drawing more than 30 you use the next larger wire
size which is 12 gauge.
Now, what about the length of the wire? First you need to know the
length of the conductor from the power source to the device. If there is more than one device, then
measure along the wire the distance to the one that is farthest away
and back. For this you need another table
that calculates a 10% voltage drop. https://newboatbuilders.com/pages/elect4.html
Suppose the total length of the wire is 25 feet. Then the table would show
that you should select 10 gauge rather than 12 gauge, that is, one size larger.
I have only done a very simple example.
If you to create a more complex system then you need to hire a marine
electrician or a consultant to help you design the system. If you are going to have both
a DC and AC system you need to consult ABYC
http://abycinc.org
and hire an electrical engineer to design the system for you.
See
Electrical Planning.
https://newboatbuilders.com/pages/electrical_planning.html
There are online calculators that do wire
size calculations, but you should know why it is selecting that wire
size before you use them.
Energy Matters DC Cable Sizing Calculator
http://www.energymatters.com.au/climatedata/cablesizingcalculator.php
Energy Development Cooperative LTD
Cable Size Calculator Tool  DC Power Cables
http://www.solarwind.co.uk/cablesizingDCcables.html
© 2010
newboatbuilders.com All rights reserved. revised 12/01/201
