Recommended Battery Cable Size

Electrical and electronic topics for small boats
jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Recommended Battery Cable Size

Postby jimh » Sun May 14, 2017 3:17 pm

Evinrude offers advice about the minimum size of the battery cables that connect their engines to the engine starting battery in their literature on proper installation and rigging of their engines. The recommended minimum wire size varies by the engine horsepower range and length of the cables.

EVINRUDE 2010 RECOMMENDED BATTERY CABLE SIZE FOR E-TEC ENGINES
Distance        CONDUCTOR SIZE by MODEL
in Feet 15 to 30 HP 40 to 300 HP

1 to 10 6-AWG 4-AWG

11 to 15 4-AWG 2-AWG

16 to 20 2-AWG 1-AWG


These recommendations are a good guideline to use if the cables between the engine and its cranking battery are to be lengthened, as often occurs when the engine cranking battery is relocated to the center console from the transom area. Note that the recommended cable size is for a cable made entirely from that wire gauge, not for cables that mix wire gauges. As a general rule, if you move the engine cranking battery to the console, you will have to abandon the original battery cables used for installations where the engine cranking battery was at the transom and a few feet away from the engine. You cannot splice on added conductors to the original engine cables and meet the recommended wire gauge.

For example, if the distance between the battery and the engine is 20-feet, the recommended wire gauge is 1-AWG. This does not mean that you can use original cables of 4-AWG for 6-feet of the run and splice them with 14-feet of 1-AWG. The whole run should be 1-AWG.

CALCULATING WIRE RESISTANCE IN RECOMMENDED CABLES

From this recommendation we can deduce the maximum allowed resistance in the cable. A wire of 1-AWG has a resistance-per-1000-feet of 0.1239-Ohm. The resistance of 20-feet of 1-AWG will then be:

0.1239-Ohm/1000-feet x 20-feet = 0.002478-Ohm

If part of a cable consists of 4-AWG wire, that segment will already have significant resistance. A wire of 4-AWG has a resistance-per-1000-feet of 0.2485-Ohm. The resistance of 6-feet of 4-AWG will then be:

0.2485-Ohm/1000-feet x 6-feet = 0.001491-Ohm

In the recommended cable size and length, the total resistance is limited to 0.002478-Ohm. If the 4-AWG wire already contributes 0.001491-Ohm, then the remaining cable must not have a resistance greater than

0.002478-Ohm - 0.001491 = 0.000987-Ohm in a 14-foot cable
Scaling this for Ohms/1000-feet we get

(0.000987-Ohm/14-feet) x (1000) = 0.0705-Ohm/1000-feet

Looking for a size size with a resistance of 0.0705-Ohm/1000-feet, we see that wire of 00-AWG has a resistance of

0.079-Ohm/1000-feet (00-AWG)

and this is greater than the necessary maximum resistance (0.0705-Ohm/1000-feet). The extension cables would need to be made from 000-AWG. 000-AWG has a resistance of 0.0618-Ohm/1000-feet, and a 14-foot length would then have a resistance of 0.0008652-Ohm. We see that the total cable resistance in a combination of 4-AWG and 000-AWG would be as follows:

4-AWG cable for 6-feet    = 0.001491-Ohm
000-AWG cable for 14-feet = 0.000865-Ohm
TOTAL CABLE RESISTANCE = 0.002356-Ohm


We next compare the resistance of this combination-size cable with the recommended 1-AWG cable:

20-feet of 1-AWG    = 0.002478-Ohm
20-feet combo-cable = 0.002356-Ohm


We see that our combination cable has a lower resistance than the recommended cable. That is good because there will be some added resistance in the connection between the two pieces of cable, such as a terminal post and connectors.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Recommended Battery Cable Size

Postby jimh » Sun May 14, 2017 3:51 pm

Here is a table of wire conductor resistance for various American Wire Gauge (AWG) size conductors; the resistance is given in Ohms-per-1000-feet of copper conductor.

AWG   OHMS per 1000-feet
0000 0.049
000 0.0618
00 0.0779
0 0.0983
1 0.1239
2 0.1563
3 0.197
4 0.2485
5 0.3133
6 0.3951
7 0.4982
8 0.6282
9 0.7921
10 0.9989
11 1.26
12 1.588
13 2.003
14 2.525
15 3.184
16 4.016


To work with these values, you have to divide the value in the table by 1000 to get the resistance per foot, then multiply by the length of the conductor to get the total resistance. Here is an example:

Q: What is the resistance of 12-feet of 8-AWG wire?

A: 8-AWG has 0.6282-Ohm per 1000-foot, or 0.0006282-Ohm/1-foot. Thus in 12-feet the resistance would be

(0.0006282-Ohm/1-foot) x 12-feet = 0.0075384-Ohm

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Recommended Battery Cable Size

Postby jimh » Sun May 14, 2017 5:47 pm

Looking at the recommended wire size and lengths from Evinrude, we can also find the total resistance of the other combinations. I show this below without the details:

10-feet of 4-AWG
Resistance = 0.002485-Ohm

15-feet of 2-AWG
Resistance = 0.0023445-Ohm

20-feet of 1-AWG
Resistance = 0.002478

The highest resistance is actually for the case of 4-AWG and 10-feet. The lowest is for 2-AWG and 15-feet. The average resistance of the three cases is 0.0024358-Ohm. That is a good value to use if calculating the wire size to be used in any situation where a larger-than-30-HP outboard engine is connected to its cranking battery. You can use two-paired-conductors as long as the resistance in each conductor of the two conductors is not more than 0.00244-Ohm. If you mix two conductors length of different size for one circuit in the cable, the total resistance of the two segments should be well below 0.00244-Ohm in order to account for some added resistance at the point of connection between the two wire segments.

In this analysis of resistance, we assume that both conductors in the circuit are the same size. We are only calculating half the total resistance because we only calculate the resistance of one conductor; the second conductor would have the same resistance, making the total circuit resistance twice the values shown above.