deck lights are tripping?

[Ash]

I have rewired my deck lights in one complete circuit via the switch panel.
the total draw is 2 x 25watt bulbs and 1 x 10 watt the cable run is a 70 feet loop.

I am 90% sure the conections are good and there are no shorts, the light on the switch board is much dimmer than the rest and the trip goes after a short period on.

the lights are recieving about 8volts each when tested.

could the cable be insuficient,will this cause these symtoms? any ideas?

 

[BarryH]

I'm surprised that your even getting 8 volts at the lights. 70 feet on what i am guessing is a 12 volt system, is quiet a long run. Beefing up the cable may well improve matters. With the loading that you have, I would personally look at feeding it through a relay rather than through the switch panel. There is a good book published called Small Boat Electris or something like that. Had a copy a while ago and its quiet a "bible" in its own right.

OK, to hell with it. Unbolt it and we'll use it as an anchor!

 

[peterb]

With a total power of 60 watts you should be drawing about 5 amps on a 12 volt system. Lamp bulbs tend to be 'constant current' devices, so that even with only 8 volts the current draw will still be close to 5 amps. What current is the circuit breaker rated at? If it is rated at 5 amps or higher then it certainly should not be tripping. My first reaction would be to take out the bulbs, so that there should be no current, then remeasure the voltage at the end of the cable. If it is lower than the battery volts then you have a short somewhere.

 

[Joe_Cole]

Nigel Calders book has tables on wire sizes.

For a 70foot run (there and back) on a 12 volt system using 5 amps he recommends the following :

To get just a 3% voltage drop he recommends 8 SAE cable (about 3.26 mm diameter)
To get 10% voltage drop he recommends 14 SAE cable (1.63 mm diameter)

He reckons that a 10% drop is OK for lighting circuits, but suggests that its better to go for 3%

I'm no electrician so I hope that I've interpreted his tables properly. I hope this helps rather than confuses!

Unless you've got pretty thin wiring it does suggest a poor connection or short somewhere in the system.

Joe

 

[petery]

If I remember my physics correctly, a long cable run and/or too thin cable would tend to increase the circuit resistance and reduce the current - thus reducing the tendency for the circuit breaker to trip. On that basis, a short circuit seems the most likely cause, as others have suggested.

 

[VMALLOWS]

Sorry, but since when have inandescent (ie normal) lamp bulbs been 'constant current devices'?? They are (nearly) 100 percent resistive. The voltage drop will be proportional to the resistance of the cable according to the time honoured formula V=I*R.

 

[Joe_Cole]

Don't know what happened there but "96" should read "percent" (The symbol isn't showing properly). Does that make it clearer?

Joe

 

[bedouin]

The resistence of a filament is not constant but increases with increasing temperature. Increasing the power-dissipation (i.e. by increasing the voltage) in the filament increases the temperature and so decreases the current flow.

Try measuring the resistance of a 100W light bulb with a DMM and see how low it is at room temperature!

 

[VMALLOWS]

I was not suggesting the filament resistance is constant....as you say it increases dramatically when hot.

However peterb was suggesting the CURRENT would be almost the same at 8v as at 12v. Although the voltage/current relationship is not linear, the current will certainly be less the lower the voltage.

 

[bedouin]

But it doesn't have to be. I'm not an expert in the behaviour of filaments, but as a rough approximation:

As a first order approximation I assume the resistance is proportional to the temperature, which is itself proportional to the power dissipated in the filament. If those assumptions hold then the filament would indeed be a constant current device.

 

[alex_rogers]

Whilst the resistance of a light bulb filament is nearly proportional to the temperature, the power emitted by the filament is proportional to the fourth power of the temperature. This means that the resistance change isn't enough to keep the current constant.

It does have some effect however and difference in current flowing when a lamp has 10V across it compared to 12V is smaller than otherwise expected.

 

[aztec]

now't to do with boats but 70 feet! my pond lights are/were only 50 ft and the voltage drop made them dim.. so i shortened the cable.. it works.

either that or check you're using the correct cable.

can you ring main DC?

ahh say what you like.... what's done is done!

http://homepage.ntlworld.com/s.amos/index.html

 

[peterb]

You are right when you say that lamp bulbs are purely resistive, but the resistance changes with temperature. As the temperature goes up the resistance increases, so that the current is not directly proportional to voltage. Bulbs are not true "constant current" devices, but the current change is much smaller than the voltage change.

In the distant past, when wireless sets (sorry, radios) depended on thermionic valves with heater elements, AC/DC sets often put the heaters in series and fed them from the mains with a series resistance to control the current. On some sets, instead of providing a tapped resistor to accommodate different voltages, a device called a "barretter" was used. This was effectively a filament light bulb (except that the filament was usually iron rather than tungsten, and ran at a lower temperature). Looing back at my old references, a 0.3A barretter would change its current from 0.29A to 0.31A for a voltage change from 110V to 197V. Not quite constant current, but not far off it. Lamp bulbs aren't quite that good, but they certainly don't behave like a fixed resistance.