Rob Gray :: ontheroad :: wothahellizat :: wot1 :: diaries :: issue-029


2 Sep 2000

The system I described in diary entry #26 worked well but the was some noticable fraying of the cables at the spot where they ran over the pulleys. When one actually broke I decided to revert to plan A.

In the beginning I intended to use a winch to raise and lower the deck but decided to go for a lower-tech method. I could have looked into the derating factor of the cables when run over a pulley and gone for a stronger cable but I do like my toys so the die was cast. I bough a winch.

Below we see the winch installed and temporarily connected to a test the system.

Fig 1. Deck winch installed.

And here we see the wiring of the four relays required to give foward/reverse control from some small pushbuttons inside the lounge room.

Fig 2. Control wiring for deck winch.

It's not as bad as it looks. The two thick 12v feed wires on the right are the power supply for both the winch and the relays. This is 8-guage wire which is a little thin as I get a three volt drop when the winch is lifting the deck but it still does the job so I'll probably leave it at that.

15 Sep 2000

And speaking of fuses (well we were in entry #28) Figure 3 shows a thin gauge wire tap on the main 12v circuit. Note that every time you reduce the size of a wire you must re-fuse to protect the circuit downstream from the reduction (assuming that the existing upstream fuse was sized for the thicker wire).

In my system have a main 12v circuit of 8-guage wire which should be good for about 40 amps. The heaviest current draw I have on the circuit are some winches (at about 20 amps) so I protect the circuit with a 25A circuit breaker. So if a short occurs in the main circuit the wires will try to draw more than 25A and the breaker will trip.

HOWEVER, if you have a smaller wire connected to the main circuit (such as a feed to a light) and a short occurs in the smaller wire then there is a good chance that the wire's resistance will be such that less than 25A is drawn from the circuit.

So what's the problem?

The problem is that the small wire will still draw heaps of current, not enough to trip the breaker but easily enough to heat the wire to a point that the insulation melts, the wire becomes red hot etc etc...

Try it for yourself, pick any two 12v wires in your motorhome and short them together. Go right to the end of a wire, for example remove a light bulb and stick a piece of wire into the terminals. Does a fuse or breaker trip or do the wires just get hot?

If the wires get hot you have a problem.

At each location I take a low-current feed from the main circuit I place a fuse in series with the positive wire as shown in the following photo.

Fig 3. Fuse used when tapping from high to low current wires.

Note that this fuse must be placed as close as possible to the high-current source, ie. the thick wires. There is no point placing at the end of two metres of thin wire, if you do then there's nothing protecting the two metres of wire between the souce and the fuse.

While on the subject of fuses here's a shot of the main circuit breakers.

Fig 4. The motorhome's circuit breakers.