Linking just the negatives of two batteries in series

[guyrj33]

Originally Posted by Tmacmi

Its a Yanmar 2qm15. The manual says that based on a voltage drop of .2volts/amp with a cable with a cross section of 20 mm squared (approx 4 gauge a circuit length of 2.5 meters is acceptable.

The description of the starter says the no-load current is 60 amps or less.

But then there is this that suggests the in-rush amps are 300!

There is a significant disparity between the sizing recommended in the manual and what would be calculated at a 300 amp inrush

Being very conservative (i.e. assuming that each light bulb is an old fashioned 25 volt incandescent bulb) I figure that if everything was turned on at the same time it would draw approximately 75 amps.

I then estimate the average length of the lines to be 30 feet. I know estimating isn't optimal, but I am not able to measure each line.[/QUOTE]




I think you need to review your units. "a drop of .2v/amp would be a 12 volt drop at 60 amps.

Likewise your bulbs are 12 volt, not 25 volt.
I think you meant 25 watts not volts.

A 25 watt bulb draws 25watts/12volts = 2.08amps.
the equation is
Power (watts) / voltage = current (amps)
also useful is
Voltage = resistance * current

A 25 watt bulb is really bright and really hot. I'd be surprised if any of your bulbs are more than 10 watt bulbs. 10W/12V= .9 A , so just use one amp per bulb in your calculations.
The calculations don't have to be exact you already know that everything works without blowing fuses right?
Battery cables are expensive, but the difference between just ok and really good isn't that much. I strongly suggest making sure your battery cables are big enough. That one day when it's cold and your batteries are low you don't want to waste energy heating up battery cables that are a size too small.

[MerMike]

Originally Posted by Tmacmi

Its a Yanmar 2qm15. The manual says that based on a voltage drop of .2volts/amp with a cable with a cross section of 20 mm squared (approx 4 gauge a circuit length of 2.5 meters is acceptable.

The description of the starter says the no-load current is 60 amps or less.

But then there is this that suggests the in-rush amps are 300!

The inrush is brief and a fuse of the correct size will not blow during a brief inrush event. Using a conservative figure of 150A for your starter and a circuit length of 2.5 meters or 8.2 ft, you can go with 4 AWG and a 150A fuse. I like the MRBF terminal-mounted fuses, as they protect the entire length of cable. ABYC does not require overcurrent protection (OCP) but I prefer to fuse it. There's more in the thread: https://www.cruisersforum.com/forums...on-244921.html

Being very conservative (i.e. assuming that each light bulb is an old fashioned 25 volt incandescent bulb) I figure that if everything was turned on at the same time it would draw approximately 75 amps.

I think you mean 25 watts. A 25 watt draw at 12 volts is a little over 2 amps. If you have 3 of them you're drawing 6.25 amps. Here's where you break out the credit card and get some marine-rated LED bulbs for your fixtures. The total draw would likely drop to under 1A.

I then estimate the average length of the lines to be 30 feet. I know estimating isn't optimal, but I am not able to measure each line.

Close enough. 6.25 amps at 12 VDC over 30 ft with a max 10% voltage drop, using the Blue Sea reference, suggests you use 18 AWG wire and a 10 A fuse. However, ABYC says don't use anything smaller than 16 AWG. There are exceptions, but they're not relevant here.

Before you go farther, please review Maine Sail's excellent web site at http://marinehowto.com.

I also recommend Stu Jackson's writeup at https://c34.org/bbs/index.php/topic,5977.0.html.

There is a LOT of first class information available in these two places alone. It will take some time but if you absorb the information you are far more likely to have a safe and useful installation.

[Tmacmi]

Originally Posted by MerMike

Using a conservative figure of 150A for your starter and a circuit length of 2.5 meters or 8.2 ft, you can go with 4 AWG and a 150A fuse. I like the MRBF terminal-mounted fuses, as they protect the entire length of cable.[/URL]

Thank you I had planned on terminal mounted fuses

Originally Posted by MerMike

I think you mean 25 watts. A 25 watt draw at 12 volts is a little over 2 amps. If you have 3 of them you're drawing 6.25 amps. Here's where you break out the credit card and get some marine-rated LED bulbs for your fixtures. The total draw would likely drop to under 1A.

Sorry, I wasn't clear. I calculated the total load back to the panel for all lights, accessories, etc at 75 amps with an average length of 30 feet.

I'm planning on using 2 awg for both house and starter with 200 amp terminal mount fuses so that either can be used for starting the motor

Originally Posted by MerMike

Before you go farther, please review Maine Sail's excellent web site at http://marinehowto.com.

I also recommend Stu Jackson's writeup at https://c34.org/bbs/index.php/topic,5977.0.html.

There is a LOT of first class information available in these two places alone. It will take some time but if you absorb the information you are far more likely to have a safe and useful installation.

Good stuff in there. Thank you

[MerMike]

Originally Posted by Tmacmi

Sorry, I wasn't clear. I calculated the total load back to the panel for all lights, accessories, etc at 75 amps with an average length of 30 feet.

75 amps seems high for a day/overnight sailing vessel. A back-of-napkin guess would be more like 35-45A.

You should not take an average here. An average does not tell you anything at all. You need to know the load and length for each circuit, verify or install the correct wire, then terminate and fuse it properly. If you can't do that, then hire a competent marine electrician. Anything less and you're putting yourself and your passengers' lives at risk.