Measuring current requirements of Landy accessories

[mickeyw]

I add electrical accessories to my Landy on a regular basis without really knowing what their power needs are. Wire sizes are chosen generously and fuses, well, a bit finger in the air really, if they pop too easily - put a bigger one in. Not really the correct way I know.

Take for example electric cooling fans, heated mirrors etc. The fans are from the scrappy so have no markings as to their wattage.

Now I know my multimeter and most others I have seen can measure upto 10A, but I suspect the fans take a good deal more than that, judging by the size of their original wiring. The pair currently have a 30A fuse fitted, which has been known to pop occassionally.

So, what other devices are there to measure the amps drawn to correctly size the fuses? My clamp on ammeter goes upto a few hundred amps AC but not DC

Ideas?

[Ruuman]

how about bodging up something like this as a temporary tester? http://cgi.ebay.co.uk/AMMETER-60-0-60-meter-dial-gauge-amp-52mm-NEW-CHROME-/330486250015?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4cf283fa1f#ht_1651wt_906

[FridgeFreezer]

These are a nifty idea:

http://cpc.farnell.com/_/cf-02/current-tester-automotive/dp/IN04874

Although DC clamp meters are cheap enough nowadays:

http://cpc.farnell.com/tenma/72-7224/clamp-meter-with-frequency/dp/IN05110

[mickeyw]

Although DC clamp meters are cheap enough nowadays:

http://cpc.farnell.com/tenma/72-7224/clamp-meter-with-frequency/dp/IN05110

Ruu, great idea, but I was hoping for a more accurate indication of DC amps, which is what Fridge has pointed to. Thanks Fridge.

My own searches for DC clamp meter were less successful, sometime it pays to look further than eBay

[nickwilliams]

The nifty little meter which plugs into a fuse holder is hard to beat on price - I think I'll have one one of them. However, if you want to go above 20A and need more resolution than the clamp meter will provide then buy yourself a shunt resistor for your existing multimeter. Here's an example. You put it in series with the load you are trying to measure and then measure the voltage across it with your multimeter, and read millivolts as amps to get the current.

Nick.

[elbekko]

I've always thought you should calculate fuse strength based on the amperage your wire is rated for. After all, their point is to burn before the wire does, no?

[integerspin]

I have a few old ammeters, a Mini ammeter with wires and tags the size of the fuse legs is

a good test tool, just wack it in the fuse holder and watch the ammeter when you switch the

item your wondering about on. The Mini ammeters are ±30A.

[simonr]

These are a nifty idea:

http://cpc.farnell.c...tive/dp/IN04874

Nick (RogueVogue) bought me one of these for my Birthday - and it's been really useful! Thanks Nick!

Although DC clamp meters are cheap enough nowadays:

http://cpc.farnell.c...ency/dp/IN05110

I have a DC clamp meter, also from eBay which has proven pretty good. It measures up to 1000A and down to 0.1A with acceptable accuracy. It was invaluable building the electric Freelander!

Si

[dirtydiesel]

I have a DC clamp meter, also from eBay which has proven pretty good. It measures up to 1000A and down to 0.1A with acceptable accuracy. It was invaluable building the electric Freelander!

Si

Any chance you could post a link to the one you have Si? i cannot seem to find any that measure below 40A.

Even the one in the cpc catalogue that Ju linked to only says it goes down to 40A, or am i missing somthing?

[Dave W]

For most DC devices you can simply work out the likely current draw by measuring the resistance across them which can be done with any multimeter.

I = V/R

Assume voltage to be 14v, I is the current in Amps, R is the resistance in Ohms. A device that measures 2 ohms will draw 7 Amps, a device that measure 0.5 of an ohm will draw 28 Amps.

[mickeyw]

For most DC devices you can simply work out the likely current draw by measuring the resistance across them which can be done with any multimeter.

I = V/R

Assume voltage to be 14v, I is the current in Amps, R is the resistance in Ohms. A device that measures 2 ohms will draw 7 Amps, a device that measure 0.5 of an ohm will draw 28 Amps.

Ooooh - a technical answer, me likey - I can measure R already

I seem to remember something about Vs, Is and Rs at school. Of course at the time I never thought I'd find an application for it,

Thanks Dave.

[TheRecklessEngineer]

The above is true for anything resistive (i.e. extra bulbs). If you start looking at motors/mobile phone chargers/amplifiers then it works a little differently....

[pony262]

All your items will have power in watts I=P/V

[mickeyw]

All your items will have power in watts I=P/V

P ???????

[FridgeFreezer]

P ???????

Power in Watts.

Ohm's Law on Wikipedia

It's worth re-itertaing that the steady-state power of a load is not the same as the current spike it may take at startup or the wire/fuse size you should use, even for basics like light bulbs.

As an example, if you calculate a 55W headlight bulb running on ~14v you get ~4A, but LR fit 10A fuses per light and use 1mm cable (10-15A rated). Likewise, an electric fan may blow a 25A fuse at startup but draw half that when running.

[mickeyw]

Brilliant info Fridge, many thanks.

Is it therefore a good rule of thumb to fit a fuse rated at twice the steady current draw?

[Bowie69]

Probably not a bad rule to have

[redneck]

You should use cable to suit the application and the length of cable being used, longer runs require larger cable to minimise voltage drop, I always aim for 5% voltage drop max. Once you know the cable size then you can use a suitable fuse to protect the wiring and or component. Sizing a cable solely by its amp rating is misleading to say the least.

[redneck]

Brilliant info Fridge, many thanks.

Is it therefore a good rule of thumb to fit a fuse rated at twice the steady current draw?

Definitely not, the fuse should be less then the maximum current capacity of the cable, regardless of the load.

[landmannnn]

Too complicated.

1mm wiring for small power,eg sidelights 10A fuse

2mm wiring for bigger power, eg spotlights 20A fuse

[redneck]

A quick example

20 amp load with 3% voltage drop with 5m cable run requires 16mm cable

20amp load with 3% voltage drop with 2m cable run requires 6mm cable

while the 2 mm cable will cope with 20 amp loads the voltage drop means you lose much of the power, which in the case of lights means they will be much dimmer than they could be.

[FridgeFreezer]

I tend to work on rounding up to the next size of whatever:

So a 5A load I'd stick a 7.5A fuse in, if that blew I may go to 10A but no further.

And since the cable should be more than capable of taking the rated fuse current, I use 1mm for most stuff and 2mm+ for anything that's likely to draw over 10A for prolonged periods.

The only stuff I use 0.5mm cable for is little bits like sidelights, interior lights, and sensors/senders/gauges (almost zero current).

[stumonkey17]

Although it's been a while since the original post I'm in the process of designing a loom for my series 3 with a 200tdi conversion. Old wiring had been messed around with by previous owners so I've decided to start from scratch. Whilst re-learning my secondary school physics I came across this calculator. Thought it might be of help to others so here's the link.

http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

[SORNagain]

You should use cable to suit the application and the length of cable being used, longer runs require larger cable to minimise voltage drop, I always aim for 5% voltage drop max. Once you know the cable size then you can use a suitable fuse to protect the wiring and or component. Sizing a cable solely by its amp rating is misleading to say the least.

No kidding. I have found the ratings for different kinds of cable very misleading:

If you buy Tri-rated swichgear cable the stated current rating is based on the maximum dissipation of the cable. If you got 240 volts to play with, losing a few in the wiring is no big deal. A bit different when you have only got 12 though!

Automotive current ratings seem to be based on a standard volt drop of 0.15 volts per m. More conservative, but far more sensible for the application. It does suggest though that there is probably far more lee-way in fuse selection though, because under fault conditions cable dissipation is what you worry about.

[muddy]

Where do scotchblocks fit into all this?