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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #11  
Old 03-29-2010
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Keep in mind that you are charging via AC and that needs to be changed to DC. I would assume that there will be loss there just from the conversion via your charger. A multimeter is a low cost item that will be needed later on, and will give you a more accurate reading on your real 12v use.
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Old 03-29-2010
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Mick,

OK, let's talk about DC systems (battery powered systems), NOT about shore power. No spinning dials. DC meters don't work that way.

There are several ways to measure DC amperage draw. These include:

1. a simple multimeter placed in series with the power lead to a particular piece of gear; however, these can only be used up to about 10A maximum, i.e., for gear which doesn't draw much power.

2. a DC ammeter in-line with the power lead -- much like the old DC ammeters in cars and boats. Not a good solution.

3. a modern DC ammeter connected to a shunt, usually mounted in line with the negative or ground lead. These come in assorted sizes and can measure up to hundreds of amperes. So-called "battery monitors" fall into this category, but they have some additional capabilities which many boaters find useful.

4. a good clamp-on AC/DC ammeter. This is an excellent tool for most boat owners. A good one, like the Mastech 2138R, can be bought for about $100 and will pay for itself quickly in avoiding lost time, blind alleys, and big mistakes.

You can also draw up an energy budget using a pencil and paper, or a spreadsheet if you're computer-inclined. Most equipment is labeled with amperage draw, or it's wattage (which is easily converted to amperage). Once you have inventoried every electrical thing on board, you can figure out pretty easily how much power you draw in typical circumstances, and how much battery power you need (a minimum of twice the daily amp-hour draw). If, for example, you find that you use about 100AH per day, then you need a house battery bank of at least 200AH capacity. 300-400 would be better.

That then leads to the size of the battery charger. Typically, for flooded batteries you'd want a battery charger with about 20-25% of the capacity of your house battery bank. Say, a 25-30A charger for a 100AH battery bank.

The wiring size depends on the load to be put on each circuit, and how much voltage drop is permissible. For lights and other non-critical gear, a 10% voltage drop is acceptable. For more critical systems (like bilge pumps, frigs, radios, etc.) you'd want wiring which would have not more than a 3% drop. For charging wires from the battery charger and/or alternator, I'd go with even less voltage drop, i.e., larger wire sizes.

If all this sounds like Greek to you, it would be good to buy a book on basic marine electrical wiring (Charlie Wing's book is very good), and read it carefully. Then, before you get started with the rewiring project, get a qualified marine electrician -- preferably ABYC certified -- to have a look at your boat and/or your plan and finalize it. The $100-200 spent for such a professional look will save you lots of $$$, heartaches, and worse as you go forward with the rewiring project.

Bill

Last edited by btrayfors; 03-29-2010 at 02:46 PM.
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  #13  
Old 03-29-2010
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Edit-Bill beat me to it with a much better explanation
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Last edited by funjohnson; 03-29-2010 at 03:17 PM.
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Old 03-29-2010
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Quote:
Originally Posted by QuickMick View Post
I need to rewire the boat, and it has been widely (and wisely) advised that the first place to start is to figure out my consumption pattern. Is there an easy way to do this by using the demised electrical box on the dock? take a reading on a friday at 5, sunday at five, divide result by two (i thought a two day sample would be more accurate). anyone know 1)if this makes sense and 2)how the heck do i read an electric meter?

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Please do not take this the wrong way as I mean it in sincerity and with the safety of you, your crew and your vessel in mind.

If you are asking this question, the way you are, you really should do a LOT more reading on the subject of re-wiring your boat.

We also don't know which consumption you are trying to ascertain? DC consumption? This is the wrong way to go about trying to figure it and one that is not likely to be accurate even if you do all the math including charge efficiency factors, inverter efficiency factors, charger / acceptance factors etc. etc. on and on.

Figuring "consumption" for 110v shore power is rather non-sensical as you size the wiring, over current protection and circuits based on loads. A 30 amp shore power system should be more than adequate for a G-30.

Charlie Wing and Nigel Calder's books are about the best going. I always recommend Charlie's book first as it is more simple and to the point and includes multiple wiring diagrams.

If you don't know how to figure basic consumption, re-wiring a whole boat could be a scary & unsafe process in the end result. Please, please, please educate yourself on marine AC/DC wiring before undertaking this project. Burn down a marina due to non-accepted wiring practices and your insurance company may not stand behind you.. Please be careful!

There are many ways to ascertain DC consumption but the most accurate, because many items are variable consumers, is a battery monitor.

You can also take a guess at hours/use/amps, based on rated amperage of an item, but in the case of a stereo, VHF, refridge, copmputers or other variable consuming items you will still likely have about a 10-15% variance from actual..
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Old 03-29-2010
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That was my intent in another thread on solar power... as sizing a battery bank really requires you have some idea of what your daily usage is...

Then, if you are long term cruising, you multiply the daily usage by the number of days you want to go between recharging and then multiply that by THREE to get a rough bank size. This is based on the idea that you will generally not be recharging to 100% but to 85%, since it takes a lot more energy to go from 85% to 100%...

Then you try and size your passive recharging capacity to equal or be a bit more than your average daily usage.

Quote:
Originally Posted by btrayfors View Post
Uhhh....I think we have a disconnect here (pun intended).

Don't know what amp measurement of shorepower really has to do with rewiring the boat.

I suspect that whoever made the suggestion to "figure out my consumption pattern" was talking mostly about 12V DC power, i.e., power drawn from the batteries. That is very much related to boat wiring.

Am I right?

Bill
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I assumed that the OP knew about DC amp measurements and was asking about shore power for a reason, am I wrong ?
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Originally Posted by wind_magic View Post
I assumed that the OP knew about DC amp measurements and was asking about shore power for a reason, am I wrong ?
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Old 03-31-2010
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I've been in marinas where I've had to pay for my electricity, and my estimate of liveaboard usage is 1-1.5 kwh/day. That is equivalent to 80-120 amp-hours at 12v, but it is hard to get an accurate estimate. The battery chargers are notoriously inefficient, I leave my solar panels connected, and we tend to leave things like the TV plugged in at the dock.
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