Battery cable size rating

[US1Fountain]

Adding a fuse block for my exisitng and newly added DC loads and sizing the wiring to upgrade from the existing 2ga battery cable.

According to the wire calculators, 2ga is rated for up to an allowable 178A in engine compartment. (I am installing 1/0 using both the wire carrying capacity and calculators for my loads, length and voltage drop)

This is my confusion:

Out of curiousity, if I were to input 2ga and the 3' (1 way) wire lenght @ 220A load, this results in only a 1.77% voltage drop. Wouldn't that go to say that 2ga is capable of carrying this load, even though it is considerably higher than the recommended carrying load? Using 350A as the load only results in a under 3% loss.

What am I overlooking?

[btrayfors]

Yeah, and that's why auto makers use such woefully undersized battery cables.

IF everything's working right the engine will start OK.

In a marine environment, though, you can be sure that there will be some deterioration in the circuit over time: surface corrosion, loose or improperly terminated cables, poor ground connection, much longer runs, etc., etc.

Since you want to maximize the chance of getting your engine going fast, you want every bit of amperage the starter needs actually delivered to the solenoid. Larger is better!

Bill

[Artif]

You don't just take amp ratings/voltage drop into account, ambient temperature is an important factor as well, i.e. engine compartments have a much higher ambient temp to start with, then add the temp of the wire at the rated load and you end up with the cable insulation getting too hot, bundling of cables is another factor to consider also.

[djmarchand]

US1:

There are two issues in deciding what cable size to use: ampacity and voltage drop.

Ampacity refers to the wire's amp carrying ability. It is limited by the insulation getting too hot and beginning to melt. That is why engine temperature downrates the ampacity or bundling multiple conductors together. OTOH some super insulation can increase it.

Voltage drop affects the load at the other end. Too much voltage drop and it won't run right or light enough. For sensitive items, like electronics you want to stay below 3%. Starters and windlasses can go higher.

Think about this: as long as you stay within a wire's ampacity you will be safe in so far as the insulation is concerned with infinite length. It just won't have any voltage at the other end. It has all dissapated as heat along the way. But the heat per foot is low enough that it won't harm the insulation.

David

[GordMay]

Your confusion is justified, as you’ve discovered an anomaly in the ratings.

[CharlieJ]

At the chance of being a redundant responder, let me restate the answers already provided:
Your calculation is correct, AWG 2 is capable of handling a 220A load in a 6' circuit and will yield a voltage drop of 1.7%.
However, in the engine room the insulation is not capable of handling over 178A.

This is why the design of DC circuits is a two step process, although maintaining a low voltage drop usually ensures a large enough conductor so that the ampacity is not an issue.

Hope this helps.
Charlie

[barnakiel]

Use the thickest cables you can fit / afford and you will be fine.

b.

[US1Fountain]

Thanks for the replies. That does ease my mind knowing I have chosen my wire size now with a clearer understanding. As I stated, I am using 1/0 from using the calculators as it meets my requirements. So I am good to go there.

I was looking for clarification of my confusion of the 2ga usage. I would have assumed since I have to input an amp load for a given wire size and length in the wire calculators, it would have rejected the wire ga. input since I can put in a 350a load which is over the wires capability for use even as a single conductor out of the engine compartment use and it 'appears' to be acceptable. I did find 1 calculator that when given the load, in/out engine space, wire length, and voltage drop allowance, does list both a wire size that will meet the amp carrying capability and also a wire size to meet the voltage drop separately, but then it list the wire size needed to meet both requirements. For someone only referring to the load calculator alone, they could be mislead into using an undersized wire.

[djmarchand]

A good wire sizing program can be downloaded from Alden Trull Yacht Electrical Design

It checks for ampacity as well as voltage drop. It lets you specify the voltage drop percentage allowed as well as ambient temperature and number of wires bundled.

David

[wolfenzee]

From an electrician/live aboard friend of mine who has recently rewired his boat, 2ga is just the ticket. You don't want to try and see how cheap you can get away with, this is your home....do it right. I have seen a 200watt inverter (with 2ga) work where a 1000watt inverter with light weight wiring wouldn't.

[CharlieJ]

Here is the formula from ABYC E-11:

11.14.2.7.1 To use TABLE IX and TABLE X , measure the length of the conductor from the positive power source connection to the electrical device and back to the negative power source connection. Use the conductor length, the system voltage, and the ampacity as specified in E-11.14.2.5, in conjunction with
the appropriate volt drop table, i.e., three percent or 10 percent – TABLE IX or TABLE X, to determine conductor size.
NOTES:
1. The power source connection may be the battery, or a panelboard or switchboard, if used.
2. If the ampacity as specified in E- 11.14.2.5 exceeds the ampacities in TABLE IX and TABLE X, the conductor size necessary to keep voltage drop below the maximum permitted level may be calculated by means of the following formula:

CM = (K x I x L)/E


Where:
CM = Circular mil area of conductor.
K = 10.75 (constant representing the resistivity of copper)
I = Load current in amperes
L = Length of conductor from the positive power source connection to the electrical
device and back to the negative power source connection, measured in feet.
E = Maximum allowable voltage drop at load in volts (e.g., for a three percent voltage drop at nominal 12V, E= 0.03 x 12 = 0.36; for a 10 percent voltage drop at nominal 12V, E = 1.2).
3. Use TABLE XI or TABLE XII to convert circular mils (cm) to conductor gauge. If the cm area falls between two gauge sizes, the larger conductor shall be used.

This is the formula that all of the wire sizer programs are based on.

Charlie