Wire Sizing Plan

AWG wire gauge ampicity and resistance in milliohm per foot
AWG	current	mΩ/ft	bend rad		AWG	current	mΩ/ft	bend rad
2/0	231-330	0.08	5"		8	56-80	0.62	2⅛"
2	147-210	0.16	3½"		10	42-60	1.00	1½"
4	112-160	0.24	3⅛"		12	32-45	1.75	1¼"
6	84-120	0.40	2⅝"		14	25-35	2.53	1⅛"

A number of resource provide wire sizing information. The Blue Sea ABYC Ampicity Rating Table is a reprint of the American Boat and Yacht Council (ABYC) recommendation for minimum wire size to safely carry a current load. The Ancor catalog provides ABYC recommended wire colors (which we will ignore) and the often cited or reprinted tables of conductor size for 10% or 3% voltage drop. More useful is the table of AWG Wire and Cable Specifications. Using this table, the exact voltage drop and power loss can be computed for a given wire length and current. Unfortunately the URL for the catalog keeps changing. The Marine Grade Wire & Cable product specifications has the resistance per foot and other useful information and hopefully the URL will be more persistant.

An excerpt table is provided, giving the ABYC Amp rating for wire with 105°C insulation (Ancor marine wire) that is not exposed to the heat of an engine room (we no longer have a diesel engine). A current range is given for bundled wires and unbundled. This range takes into account variations in air flow around the wire and inability of heat to escape the wire due to bundling.

The Ancor recommended minimum bend radius is also given in the excerpt table, rounded up to the nearest ⅛" in this table. These recommendations are often not adhered to. The Ancor wire is made up of AWG #30 tinned copper strands. For example AWG 2/0 has 1330 strands. The recommended minimum bend radius is the radius needed to avoid the risk of breaking some of the individual strands in the wire. This is an important consideration in tight wiring such as no the high amperage breaker panel (see the High Amperage Panel and Breakers web page.

Computing Wire Power Loss

Even the very large AWG #2/0 has some resistance and when current is passed through this resistance some power is coverted to heat in the wire. Power is voltage in volts (V) times current in amps (A). This equation is $P=VA$. For the wire we know the wire's resistance (R). Voltage (V) is current (I) times resistance (R) in ohms. This equation is $V=AR$. Power loss in a wire is therefore $V=A^{2}R$

For example, the 48V battery wire can never exceed 300A (and likely will be a lot less in practice). If there is a 10ft length of wire the resistance would be 0.8 milliohm or 0.0008 ohm. The voltage (V) drop across this length of wire is 300 times 0.0008 or 0.24V. The power lost in the wire is 72W. If the voltage is 48V, then the power delivered to the load is 48V times 300A which is 14400W (14.4kW). The power loss in this one wire is 0.5%. If the ground wire is the same length and size then loss in the battery cables is 1%. If only 50A is being drawn then the power lost in each wire is 2W and the power delivered to the load is 2400W. Percentage loss is under 0.2%.

Looking at one load at a time the losses in each length of wire can easily be calculated and those losses summed. It is only slightly more complicated if there is more than one active load and some wires carry both loads. A simple example is the motor drawing 2.4kW (50A at 48V) and the charger drawing 12A to charge the 24V battery at nearly 25A (the current limit of the charger). If the motor has another 15ft of AWG #2/0 the 50A is passing through this resistance of 0.0012 ohm, so losing 3W. If the charger has 10ft of AWG #6, then the 12A is passing through 0.004 ohm of wire for a loss of about 0.576W. The battery cables are carrying 50A plus 12A so 62A. The battery cables are losing 6.15W. Total loss is 3W plus 0.576W plus 6.15W so about 9.7W. The useful power is 2976W so the percent loss is about 0.33%.

Some of the loads are 24V and could have been purchased as either 24V or 12V. Two examples are the autopilot and radar. The radar consumes 17W at either voltage and 7W in standby. If the round trip wire run up the mizzen mast and under the cockpit to the high amp wiring is 50ft and AWG #8 wire is used (to keep the wire size in the mast smaller) then the loss will be 0.12% at 12V and 0.03% at 24V. As expected loss at 12V is 4 times the loss at 24V. Among the larger autopilots considered the drive unit consumes up to 72W at 12V (6A) and up to 96W at 24V (4A) and the control unit draws up to 2W (0.167A or 0.083A). This is an unusual case since the 24V version can draw more power than the 12V version. If the wire to the stern is 25ft round trip distance and AWG #6 is used, then the loss would be 1.6% at 12V and 0.5% at 24V. Despite drawing more power at 24V, loss at 12V is 3 times the loss at 24V.

The losses using these large wires is low. Long runs such as to the windlass on the bow will incur larger percent loss but still well under 3%. Small loads are all over the boat. Use of AWG #10 even for small loads will keep this loss very low. AWG #14 has 2.5 times the resistance per foot as AWG #10 so will only be used for very low current loads such as individual LED cabin lights.

Wire Sizes Used

The four parallel 48V battery modules will each have a separate AWG #2 gauge wire from the module positive side to the 48V load bus bar and a separate AWG #6 gauge wire from the 48V module positive side to the charge side bus bar. With one module each, no bus bar is needed for the 24V or 12V bank. The 24V and 12V modules will also have AWG #2 gauge wire from the module but connect directly to a load side breaker and WG #6 gauge wire from the module to the charge side ProStar charge controller.

The 48V bank will require AWG #2/0 load side wire from the 600A bus bar where the 48V battery modules are connected in parallel to the battery switch for the motor and in the future to the circuit breakers for the galley stove inverter. The 24V and 12V banks can use AWG #2 load side wiring. A common 600A ground bus bar will serve all three battery banks. AWG #2/0 wire will be used for all wires used to connect the 48V bus bar to the fuse, the battery switch, the shunt, and then to the motor power terminals.

Wires from the 48V load bus bar to the breakers will be AWG #2/0. Wires from the 24V or 12V battery to the breakers will be AWG #2. Wires on the load side of the breakers will be AWG #6 for all breakers of 50A or less or AWG #2 for breakers greater than 50A. All feed side wires (5) to the cabin mounted breaker panel assembly will be AWG #6. The cabin mounted breaker panel assembly load side wires to the terminal blocks behind the breaker panel will be AWG #10, except loads with 30A breakers (water heaters) or 50A breakers (small inverters) which will be AWG #6.

Wires to the large inverter used for the electric galley stove will be AWG #2 or AWG #2/0 (if we have enough #2/0 left over). The AC/heat inverter and water heaters will use AWG #6 wire. The smaller inverters for the galley and vanity AC sockets will use AWG #6 wire. Wires that have to be strung up the mizzen mast to the radar and to the wind generator will be AWG #8 or AWG #10 if AWG #8 won't fit. Wires to the solar panels will be AWG #8 or AWG #10 if AWG #8 won't fit through the solar mount tubing.

All other loads are connected to 15A or smaller breakers and will use AWG #10 wire to the terminal block nearest the load and then a fuse in some cases and the size wire used by the load if smaller to the load itself. For loads with short wires a butt connector and either AWG #10 or AWG #14 wire will be used, appropriately sized for the butt connector. LED cabin lights or other low current loads using smaller than AWG #16 wire will use step down butt connectors to bring the wire size up to at least AWG #14.