Sunday, January 30, 2011

12V DC Calculation.....

Driving myself nuts with cable size calculation. The main consideration is the possible addition of a bow thruster at some time in the future. It will be powered by a dedicated battery bank in the bow, but I want to install cables to cater for the max amperage possible. With a possible draw of 350Amps from a bow thruster and a total run (there and back) of 40 feet, allowing a 10% drop (as it is an intermittent load, and possibly only for a minute or so) I come up with 2/0 cable. Its possibly overkill but the cost difference is only a buck + change a foot. As I am not building 200 Hunters, I can live with it.

This obeyed ABYC Standards;

"Conductors used for panelboard or switchboard main feeders, bilge blowers, electronic equipment, navigation lights, and other circuits where voltage drop must be kept to a minimum, shall be sized for a voltage drop not to exceed three percent”. (See Table B).

Conductors used for lighting, other than navigation lights, and other circuits where voltage drop is not critical, shall be sized for a voltage drop not to exceed 10 percent.” (See Table C).

All input appreciated!!!!


robert s said...

I would not factor in high load devices (like a thruster) for the primary cable for the distributed system. Afterall, the distributed system cannot switch these high load devices directly anyway.

Despite the modest increase in cost, working with bulky 2/0 wire is a PITA.

I would just plan on a dedicated wiring for those high loads. And, at best, have the distributed system switch it via a relay.

Ocean Dove said...

Here is my 2 cents
If you were going to put a dedicated battery bank in the bow for the thruster you would only need 0/2 cable from the battery to the bow thruster.
Here is why.
The max amps need for a 4 kw (6hp) 12v thruster motor is 400 amps.
The min 150 Ah battery is need.
If you install the batteries within 7‘ of the thruster motor 15’ of 2/0 cable will be OK.
The duty cycle for most bow thruster motors is 2 to 3 minutes max in 60 minutes period.

(3 minutes) X (400 Amp) / 60 minutes = 20 Ah used

If you install a 15 amp Xantrex Echo charger you will be able to maintain the charge in the bow thruster batteries and only need to run 14 gauge wires to echo charger.

The Incredible Hull said...

Thanks for the info, this is an area that still had many gray areas for me. OD, if you can explain (in words of one syllable)the following. The BT takes 400 Amps, just for a minute or so, from the dedicated bank. Why doesn't 400 Amps then run along the cable from the house bank / alternator to fill up the void. Casey's water tank metaphor is stuck in my mind. Thanks all.


Capt'n Larry said...

To use the water theme, the battery bank in the forward locker is the source of "water" for the thruster and windlass when they need the large drink under load. The batteries satisfies the thurst at that moment so heavy guage wire is needed from the battery to the load. The engine/recharge source can then refill the glass slowly using the charger so the wire to the forward charger can be smaller {read cheaper}, it will be safe as it does not see the large loads.

Ocean Dove said...

Here is a short answer

2/0 cable 20’ away from 400 amp BT the voltage drops 10.72 % the voltage at the BT is 10.68 Volts
As the voltage drops the amps go up to supply the same power or
watts = V x Amps
12v x 400amp = 4800 watt no voltage drop (BT at 100% power)
10.72v x 400amp = 4288 watt at 10.68 voltage drop (89.28% power)

The amps needed for 100% power at 10.72 volts is
4800w / 10.72v = 447amps

447 amp needed for 100% power of the BT will cause additional voltage drop drop of 11.93 % the voltage or at the BT is now 10.56 volts

With the lower voltage and the higher amps the BT motor will burn out sooner.
= Sad BT Motor

2/0 cable 7.5’ away from 400 amp BT the voltage drops 4.01 % the voltage at the BT is 11.519 volts
= Happy BT Motor

Westmarine 2/0 cable $15 per foot X 40’ = $600

Westmarine 2/0 cable $15 per foot X 15’ = $225
AGM 12V Battery, Group 31, $280
Xantrex Echo charger $165
Total $670
You judge
Sad or Happy BT for $70

All calculation with

The Incredible Hull said...

Thanks again guys, the penny has finally dropped.

The BT and Windlass will be powered from the dedicated batteries in the bow, using the size of cables dictated by the shorter cable run. What I didn't understand was the current flow from the house bank to the bow bank to equalize both. I now understand that this is much less than the draw from the BT.

Robert, I was not aware of the Xantrex Echo charger, but it looks like a good option.

Consultancy cheques are in the post. Thanks all.

Ocean Dove said...

On Ocean Dove the Main Engine alternator charges the House battery bank Only and the Engine starting battery is changed with a 15 amp Echo charger. The starting battery is isolated from the house batteries but charged by every charging source solar, wind, alternator, or AC charger,

Silverheels III said...

First of all we're down in the southern Caribbean and we've seen a lot in 2-1/2 years. If you cruise extensively (and don't have dogs to go ashore 3x daily) you will hopefully visit docks very infrequently. Anchoring out is most always free unless in a marine park where balls are provided. Learn how to use your large diesel when maneuvering. You'll save big $$ and maintenance hassle by keeping your technology SIMPLE! You wouldn't believe how many medium sized vessels with single AND twin screws needlessly Hoovering the bow around this way and that, coming up to a mooring ball or dock. The guy next to us sucked in his dinghy painter yesterday when coming up to a ball here in St Lucia.
Keep your technology simple eg 6V golf cart batteries in one large house bank rather than having batteries which get little use waiting for the "thrust" command.

Ocean Dove said...

I do agree with Silverheels if you are not making a lot of docks you don't need a BT.
The Admiral would love a BT but it is very low on my list.
My windlass is setup by the P.O. with 0/2 cables from the single house battery bank to the windlass. The Voltage drop is only .35 volts.
I also like 6v golf batteries. Cheap (Thrifty), light 68 lbs and available every where.

The Incredible Hull said...

Silver Heels and OD, no argument for me. The Bow Thruster is just an option I am planning for later. It too is low on my list at the moment, but may move up as my rapidly advancing old age progresses. Anything that keeps you cruising longer is a good thing IMHO. I work in the boat driving business in Miami and one of the biggest problem is with owners who have always had a BT and never learned to maneuver a boat in the first place. Silverheels, are you in Marigot Bay? I'm trying to think where else has mooring balls in St. Lucia, one of my favorite islands. Fair Winds, Gerry

Silverheels III said...

There are mooring balls at Soufriere and between the pitons where we are now.
We took 5 years in the water in Toronto to prepare our Niagara 35.
Get off the dock soonest dude. Keep the technology simple. Just GO!
BTW two of our 6V flooded batteries are forward under the setee. The other four are aft beside the engine. All six are wired together in one house bank 3x 225aH=675aH
Power your windlass from the main bank rather than a dedicated windlass battery. Do you really need a heavy 12v battery in the pointy end? Increases your pitch potential for sure.You'll need 250ft of chain and two anchors up there which are enuff weight in the bow IMHO
We're friends of Alchemy back in Toronto. ;-)

The Incredible Hull said...

Could not agree more;