my brother's shower

[ricicle]

Or grouped or run in MT...

And those

 

[Softus]

The problem is that for example if the shower heater element (or whatever the load is) is 10kW then at 240V it will use 41.7A, but if the cable run is too long the and the volts drop to say 220V then the current rises to 45.5A.

Well you've just blown all of my understanding of basic electrics completely out of the water.

So, in your heater element scenario, if the volts drop to an infinitely low measurement, then the current will rise to infinite?

 

[davy_owen_88]

No, in a purely resistive load (shower) the current will decrease with voltage as the resistance is fixed. (Ohms law VIR etc)

A 6 ohm resistance with 240V going through it = 40A
A 6 ohm resistance with 220V going through it = 36.7A

This isn't the case for all loads though.

 

[Softus]

No, in a purely resistive load (shower) the wattage will decrease with voltage as the resistance is fixed.

So why does the current rise in RF Lighting's scenario?

 

[ricicle]

RF's forgot his ohms law

 

[davy_owen_88]

So why does the current rise in RF Lighting's scenario?

Because RF is thinking of non-purely resistive loads Or he forgot Ohms law, one or the other.

 

[RF Lighting]

Because RF's had a long day and needs to go to the pub

 

[Adam_151]

Well its not very efficent use of the energy as you are wasteing more in the cable, but the only other side effect is having a lower terminal voltage which differs in its effects depending on type of load

A heating load like a shower will just run at lower power, a lighting load will become less efficent (more efficent heat/light ratio at higher temperatures), halogen lighting loads will fail quicker (need the tenperature there for the halogen cycle to work) motors as said might not start, loads containing a switchmode powersupply will just suck more current to maintain a constant wattage (and if the volt drop is caused by loads of switchmode psus you can get harmonic issues where the currents in the neutral add rather than cancel... but thats a topic of its own!)

I wouldn't worry about the odd volt or two over the allowed 4% in general, the suppliers tolerances make a bit of a mockery of it anyway (although if your measured volatage is 230V and your p-n loop is 0.3, you might think twice before installing a heavy load on a cable thats going to drop 9.2v...)

 

[Softus]

...(although if your measured volatage is 230V and your p-n loop is 0.3, you might think twice before installing a heavy load on a cable thats going to drop 9.2v...)

I understood all that you wrote Adam, except for the above.

For example, "p-n"?

 

[davy_owen_88]

Phase to neutral.

 

[ricicle]

If your Phase/Neutral impedance was 0.3 ohms then adding a load of say 45A would cause a VD of 13.5V

 

[Adam_151]

I understood all that you wrote Adam, except for the above.

For example, "p-n"?

sorry, shorthand

"p-n loop" = phase-neutral loop, same as the ELFI test but done between phase and neutral instead of phase and earth

its normally used to work out the PSCC (prospective short circuit current) by dividing the nominal voltage by it, for example if you got 0.3, the PSCC would be 240/0.3=800A

 

[davelx]

The problem is that for example if the shower heater element (or whatever the load is) is 10kW then at 240V it will use 41.7A, but if the cable run is too long the and the volts drop to say 220V then the current rises to 45.5A.

No it doesn't - the shower is an almost fixed resitive load of 5.75 Ohms and the supply at source (CU) can be considered fixed at 240V. Current flows according to total resistance of shower and cable. If the voltage drop across the cable is 20V, then the voltage across the shower will be 220V, the current will be approx. 220/5.75= 38.3A (slightly more due to the element being a little cooler, but not 45.5A). The power output of the shower will be 220*38.3=8.43kW and 20*38.3= 766W will be dissipated as heat in the cable.

BAH - sound of multiple colliding posts!