Sizing Main Battery Cables

[waeshael]

I am an electrical engineer, a power systems designer, a yachtie, and an expert on battery systems for yachts. Let's keep it simple:

The size of the wire you need is determined by whether or not the wires get hot enough to melt the jacket and cause a fire on the boat. The voltage drop is hardly important. Everything on a boat will operate properly between 12 volts and 14.3 volts, so if there is a voltage drop in a wire of say 0.5 of a volt, it doesn't matter. Just so long as the wire doesn't get warm in use. This is a concern for wire harnesses where many wires are strapped together and heat each other.
The battery could melt its wire if the positive and negative are shorted together somewhere in the circuitry. To prevent a fire the positive cable to the battery terminal must be fused within a foot of the battery post, so that a big short anywhere beyond the one foot distance would blow the fuse and stop the current flow, and prevent a fire. It is also good practice to fuse every system with a smaller value to protect the instruments themselves. A HF Radio would need a 20A fuse or so, a refrigerator perhaps 10A, and so on. For most equipment a 10ga wire should be good enough. But for solar panels it should be 6ga. For the engine to starter wire it should be 4 ga or 2 ga, because the starter only runs a few seconds so the wire won't get hot, and the voltage drop of 2 volts or so caused by the smaller wire gauge won't prevent the starter from turning over the engine. The biggest voltage drop is internal to the battery and gets worse as the battery ages. A good battery can start an engine even with small gauge wire as you can see from these car emergency kits which have a good battery and small gauge wires. An old battery may have so much internal voltage drop (measures perhaps 10V at the terminal under load,) that it wouldn't matter how fat the battery wire was, it wouldn't start the engine.

Now, the biggest concern is not the wire gauge but the condition of the terminals and connections at the battery post. The wire must be marine grade tinned copper and the terminals must be swaged on with a press. The terminals and bolt must be coated with corrosion inhibitor.
All wire on the boat must be marine grade tinned copper wire. All terminals the same.

You should have a DC current clamps probe, and a good voltmeter. Measure the load currents in the positive wire at the battery and switch on and off your equipment. Record the measurements. Feel the wires to see if they get warm, which means the wire may be a bit small for the job. Record the voltage at the battery terminal when all the devices are switched on and see how much it drops. If the meter shows only half a volt change then the battery is probably okay (its internal resistance is low,)
Make a chart showing the current draw for each device, and another column to show how many hours it is in use per day. Multiply to find the number of A-H each device uses per day. You must put back in 120% of the amount used per day, to keep the battery from sulfating which creates internal resistance.

Hope that helps.

[john61ct]

Voltage drop can be critical for certain load consumers.

A circuit that is perfectly safe (thermally) at #2 may require 1/0 to prevent excessive voltage drop.

[Terra Nova]

Originally Posted by waeshael

I am...an expert on battery systems for yachts...

For the engine to starter wire it should be 4 ga or 2 ga, because the starter only runs a few seconds so the wire won't get hot, and the voltage drop of 2 volts or so caused by the smaller wire gauge won't prevent the starter from turning over the engine...

Oh brother...

As the voltage drops, cranking amps rises. Sometimes enough to melt the solder on a starter's internal connections. A starter is sometimes required to run for more than a few seconds, allowing internal heat to build.

Even though using too small a wire gauge might not prevent the starter from turning the engine over, it can damage the starter.

[rgleason]

Redsky
What is OCPD please? We need a nomenclature acronym terminology page.

[Zil]

Yes. Any newly introduced acronym should be preceded by the term.

'Over Current Protection Device' then later OCPD.

[john61ct]

CP = fuse or CB

[rgleason]

Thanks for the Term Definitions.

Found EATON's Bussmann Series MRBF-225 Marine Rated Battery Fuse, 225A, 58V at Waytech and
EATON's Bussmann Series MRBF Mounting Bar, 3/8" Mounting Hole, Single Stud, CFBAR1-250SP
at considerably less than BlueSea

In the attached partial schematic, I suppose the following loads & sensors in the Battery Fuse Panel all should be directly attached to the Battery + Post!

1. Three Solar sources
2. Regulator Sense +
3. ACR Combiner Sense +
4. Smartgauge +
5. Link10 Sense +

Which is pretty much the entire panel except the anchor/nav light and the bilge pumps. Is it possible to remove the Fuse Panel from the Positive BusBar and simply attach directly to the Battery + post without using the 25a fuse?

I am thinking of keeping the main fuse (200a,225a or 250a) in the battery box, using the EUT T105 connection with an MBTF fuse holder (similar to the photo below) and the direct connection of the #6 cable to battery Pos + post from the Battery Fuse Panel. The #6 cable would be unprotected but less than 7" so that all the various sensors and solar get their preferred location.
Would this be an acceptable arrangement? Then I don't have to have multiple inline fuses all over the place!


The positive busbar would be Blue Seas 5196 MRBF Common

[Terra Nova]

Those don't look like approved components.

[rgleason]

Terra Nova, what do you mean? Eaton is a large well known company and Waytek is in Minnesota. Right now what I am more interested in is thoughts about the change to the wiring diagram.

[rgleason]

Changes:

1. Moved the 24hr Battery Fuse Panel AWG 6 to direct to Battery Positive + Post (less than 7" away)
2. Moved the SW#1 AWG 1/0 to be direct to Battery 225a (eliminating two connections).
3. Moved the Alternator AWG 1/0 to be direct to Battery 225a (eliminating two connections).

Is the Emergency Battery switch still meaningful?

[Terra Nova]

Originally Posted by rgleason

Terra Nova, what do you mean?...

I mean they look like cheap components that are not Coast Guard approved and have no place on a yacht.

[rgleason]

Originally Posted by Terra Nova

I mean they look like cheap components that are not Coast Guard approved and have no place on a yacht.

So I assume you would disapprove of the same product sold by Blue Sea.
I believe Eaton is the manufacturer of Blue Sea's line of MRBF fuses and holders.
These are "Marine Rated" https://www.waytekwire.com/item/4556...BoCGYMQAvD_BwE

Sorry you aren't able to comment on the revised drawing.

[stormalong]

Originally Posted by Terra Nova

I mean they look like cheap components that are not Coast Guard approved and have no place on a yacht.

The coast guard does not approve electrical components. They specify certain minimums. Underwriters Laboratories and ABYC set standards. Anything that is rated for 110 volt will pass those specs at 12 volt. Fuses and fuse holders for 110/1220 volts can easily handle 12 volts.

The only issue I see with using these holders is that they are enclosed in bulky steel boxes. That is a USA thing. Look at the household wiring in most of the world and you will see plastic enclosures.


I see nothing wrong with using household wiring slug fuses in a marine environment and I have seen them used often, Even by boat builders.
Look at commercial coast guard approved vessels and you will see a lot of wiring meant for residential and industrial use.

[Maine Sail]

Originally Posted by Terra Nova

As the voltage drops, cranking amps rises.

Place a DC starter on the bench and connect it to a fully charge bank and capture the peak low voltage (during cranking) and peak in-rush current. Now do the exact same thing at 50% SOC... The higher the voltage at a DC starter motor the higher the current generated. The lower the terminal voltage the lower the current generated.


It would however be nice if our starter motors could develop more power with lower battery voltage..


What can happen with low voltage is a longer cranking duration and this can lead to more heat build up in the wire when compared to a very quick start.



On a device such as an inverter it will draw more DC power as voltage drops.

[rgleason]

Originally Posted by Maine Sail

Place a DC starter on the bench and connect it to a fully charge bank and capture the peak low voltage (during cranking) and peak in-rush current. Now do the exact same thing at 50% SOC... What can happen with low voltage is a longer cranking duration and this can lead to more heat build up..

Maine Sail,

Can a person using a DC Clamp/Volt meter capture these values?
Answer --Yes with a Fluke, https://marinehowto.com/voltage-tran...rical-systems/
Mine does not do peak.


For a Yanmar 3YM30 starter (64amp, 400cca) (While searching I found a forum email from you with these values, after your inquiry to Yanmar). Should I use:

1. 200 amp fuse + awg 1 (existing wires) 12 feet full length.
2. 225 amp fuse + awg 1/0 12 feet
3. 250 amp fuse + awg 2/0 12 feet