Volt drop effect on appliances

[Surefire]

Hi Guys

I have a question regarding volt drop.

Some time ago a company I worked for installed wall mounted aircon units around their workshop. They wired them into an existing 2.5mm ring main circuit which was in singles in a truncking. During operation some of the units would cause the protection to trip (MCB's). The reason given for this was that the cable runs were too long causing exessive volt drop and thus more current being drawn from the cct. My question is this: Long able runs will cause exessive volt drops due to the extra resistance of the said long cable runs. If the long cables increase the resistance surely the current delivery capability will be reduced! so how can the aircon units draw more current with this added resistance. Or am I missing something.

 

[telectrix]

it,s more likely that the circuit is overloaded by adding the aircon units to the ring

 

[morph]

simple ohms law here

less volts more current
out goes the mcb
shimples !

 

[Surefire]

The mcb would trip if only one air con unit was running so it wasn't being overloaded.

Normally I would agree: less volts more current. But the less volts is caused by too much resistance in the long cables. More resistance equals less current doesn't it?

 

[morph]

does not matter why the volts reduce the result will be the same

if it is only tripping with one unit on does that mean it is overloaded?
maybe yes maybe no, depends on the loading of the unit and other equipment on same circuit

 

[Surefire]

So how about this: If I have a 230w fillament lamp ohms law says it will draw 1 amp at 230volts right? If I now plug in some long extension leads say 3 x 25mtrs I will have added 75 mtrs worth of resistance and dropped some volts. Will that extra resistance make the the lamp draw more current? my gut instinct says that the current will be reduced not increased.

 

[morph]

depends on the size of the cable does nt it

the fact is the same current is inversly porportional to voltage ...always
thats why 3 phase stuff uses less current and thus smaller cables

 

[Marvo]

simple ohms law here

less volts more current
out goes the mcb
shimples !

With a fixed resistance (say 100 ohms), at 240v current=2.4 Amps. At 200v current=2 Amps, so lower voltage at the load means less current draw.
If you have a constant power output of say 1Kw then to produce this power at 240v you would have a current of 4.166Amps. If you only had a 200v the current required would be 5Amps so in this case you would have a higher current.

In the case of an airconditioner the compressor motor has a current relay which connects the start winding until a certain speed is reached . After that time the relay drops out and the current drops with just the run winding remaining connected. With low voltage the compressor starts much slower which prolongs the time both windings are connected. This prolonged high starting current could be the cause of the tripping breaker. Hence long cabling could cause MCB tripping.