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jarcher 04-19-2010 12:03 AM

Locating the master battery switch / DC Panel - Wire cost
 
So in my infinite wisdom, I had planned to relocate the master on/off/combine battery switch aft, near the circuit panel, while the battery banks are just forward of the keel. It seemed like it would make sense, keep everything together.

Then I priced the battery cables. Lets see, I need to go from each battery positive terminal to the switch, then from the switch back to the starter, then a negative back to the batteries... About 80 feet of cable, carrying starter current. Thats only about $700 in cable!

I guess I'll leave the master switch right where it is, about 3 feet from the batteries. Its hard to get to but at least I can afford it!

But then I have to get from the master switch to the panel. The PO used AWG 10. The panel has a 50A breaker. I don't think so. According to the formula in Casey's book, for 40 foot round trip at 50A I'll need AWG2. That's about $75. :eek:

Any way can save on this? The only thing I can think is that I could run the positive to the panel, but then see if I can find something to ground the panel to, since the batteries will be connected to the inboard engine block. If I do that, will I need to worry about electrolysis?

Any other ideas? Did I do my math wrong?

Thanks...

mitiempo 04-19-2010 01:42 AM

2 Attachment(s)
The issue is not the size of the breaker on the panel but the draw in amps of the items served by the panel. For example the Blue Seas 8023, their most popular DC panel, which has 8 positions is rated at 100 amps. They recommend that it be treated as a 100 amp load. I have this panel and the wire run is short but I am using 2 awg cable - probably overkill but bigger never hurts with cable sizes. Estimate the draw on the panel if everything is in use to arrive at a max expected load and use that as the number in your wire size calculations. Since you're using leds extensively the load from lights should be low. If you have a battery monitor make sure the charger is off and you can determine the actual draw of each item one by one. If not add them up from the wattage of lights and published amperage of other items. You might find the max load is well below 50 amps. Below is an exerpt from the Blue Seas instructions on sizing panel feed wires. The key part is sizing the feed wires for the panel rating or maximum usage whichever is less.

Maxboatspeed 04-19-2010 06:02 AM

Yep, boat wire is expensive. If bought off the shelf at the yacht store in small quantities it's really expensive. It's not a good idea to use any part of your boat as a ground conductor. It's hard to get away from using the engine block to ground the starter and some instruments. Unlike a car, other stuff needs to have insulated wire connection to the neg batt terms. Keep in mind a main breaker should be close to your pos batt term, prior to the wire run to your switch.
Fair winds and following seas!

jarcher 04-19-2010 09:33 AM

Thanks guys...

Even if someone goes and switched all the bulbs from LED to halogen, max possible load will probably never even approach 30A. I was thinking I should swap the main breaker in the panel from 50A down to 30A, because I don't want the wire to be too small for the breaker if i size for 30A.

But then you mentioned a fuse in the positive feed wire. So if I fuse the feed at 30A right near the positive battery terminal (on just the house battery) then I guess that issue is resolved.

But in that case, why do I have a 50A breaker on the panel?

mitiempo 04-19-2010 10:00 AM

Most DC panels do not have a main breaker, but just breakers for individual circuits. Blue Seas, probably the most prolific in this category, ships all DC panels with 15 amp breakers for each circuit. This works well in most situations. As posted the feed wire should be fused. What panel do you have?

jarcher 04-19-2010 10:15 AM

I have this one:

http://www.defender.com/product.jsp?path=-1|328|299263|319684&id=838196

And you're right, there is no main breaker. I was confusing it with the AC panel I bought, which is this:

http://www.defender.com/product.jsp?path=-1|328|299263|319684&id=1324755

So I guess I should just add up all the devices, maybe throw in a few extra amps for future expansion, and just size and fuse the feed to the panel accordingly.

My boat uses so little power and has so few devices that the 13 position panel was overkill, but the next one down, with I think 5 or 6 circuits, was pretty much the same price, so I figured what the heck.

When I am sailing along at night, the instruments, VHF, GPS and LED running lights all add up to about 3 amps, and thats a conservative number by far. If I key the VHF rig then that adds 5 amps. The big numbes come from the cabin lights, which I'll size as if they were halogen bulbs, just in case someone changes one out.

My guess is that 30A is plenty.

Thanks, sorry for the confusion about the breaker.

jarcher 04-19-2010 10:23 AM

So with a 30 foot total run (both directions) from the battery to the panel and max load of 30A, I could just squeak by with AWG6, at $1.01 per foot. AWG4 is $1.23 per foot, so for the extra $6.60 I guess I'll do that.

Thanks for the help!

mitiempo 04-19-2010 10:40 AM

Yes, that works, until you add circuits to the panel. I would have gone with a panel without meters though for almost half the price and put the difference into a good battery monitor. The monitor will tell you much more than those meters will.

jarcher 04-19-2010 11:03 AM

Quote:

Originally Posted by mitiempo (Post 594599)
Yes, that works, until you add circuits to the panel. I would have gone with a panel without meters though for almost half the price and put the difference into a good battery monitor. The monitor will tell you much more than those meters will.

Really? I can still return the panel. What does a battery monitor do?

mitiempo 04-19-2010 11:12 AM

A battery monitor will tell you amps in and out of your batteries, state of charge in percentage, and time left at current usage. You don't want to take the batteries below 50 % and a voltmeter will not tell you state of charge in any immediate way so really can't be used for this. Here's a link to Xantrex Linklite. Xantrex Technology Inc. - Boats - LinkLITE Battery Monitor - Product Information
TRhere are others from Xantrex as well as other companies. The Victron BMV 600 is highly rated and $175 at Jamestown.
Victron BMV 600 Battery Monitor


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