what is the maximum distance you can run 240v.

[stuartR1963]

Hi, now first of all I can understand its bit like asking how long is a piece of string,but curious question how far can you run 240v before it is useless, Lets just say a large 300mm 4 core 3 phase 240/415v cable coming out a typical 11kv transformer. How many meters can it travel generally to supply the houses? say 50 houses for example, thanks.

 

[bernardgreen]

You can take 240 Volts along a cable to infinity and maybe beyond. But if you then try and take any current at that far end then you will have no volts left due to voltage drop along the cable. Voltage drop = current along cable X the resistance of the cable.

In practise you can have several miles of cable if all you have at the end to take power is a very low power lamp.

For your example the sensible load of all the houses would have to be estimated. This would give the current the cable would be carrying. The voltage drop along the cable can then be calculated for the various cable sizes avialable to use. Using the optimum cable and its voltage drop the required voltage at the transformer can be determined. This may be several volts higher than the nominal 230 ( actual 240 ) if the voltage drop along the cable is high.

It gets complicated as the load varies from hour to hour while the voltage at the transformer remains constant, the skill is deciding which size cable to use so that varying loads do not create excessive voltage changes at the houses ( large diameter cables, low resistance but expensive ) without making the cost of the cable excessive..

 

[ericmark]

Permitted volt drop has to be the start so if we take 230 vac +10% -6% assuming starting at top volts permitted we have 36.8 volts to play with. Next is current, but with your example not quite so easy as although we could assume 50 houses using 100 amp each what we don't know is if even spread on the cable or if all at the end of the cable. What we do is always an estimate as we never know how much draw with houses. However with cables our tables give us figures like for example 16mm² 70°C thermoplastic insulated and sheathed flat cable is 2.8 mV/A/m so if supplying 50 amp then 36.8/0.0028/50 = 262.8 meters although we don't use that cable to supply houses I have the figures for that cable to give an example.

In theroy we also should apply correction figures for temperature so same cable supplying 25 amp will be slightly more than double the distance.

So with that example working on 16% the answer for house to shed would be very different as lighting only permitted a 3% volt drop so figure drops to around 50 meters. Today I do feel one has to consider the lighting used, I have just ordered a LED unit to replace a quartz tube rated 150 ~ 250 volt so in real terms one has to consider type of lighting used.

For really long runs for example air port runway lighting we use current to voltage transformers at every lamp so we control current rather than voltage and all lamps have same brightness rather than each lamp getting a little dimmer the further away from source.

I am sure you have something in mind? As to houses today with micro generation it gets rather complex to calculate and will vary with time or year and day and weather conditions. I would expect it's more rule of thumb than full calculations. i.e. housing estate last year around same size we had volt drop problems so this one we will use thicker cable or vice versa.

 

[PBC_1966]

Lets just say a large 300mm 4 core 3 phase 240/415v cable

Another point you need to keep in mind is that with 3-phase distribution such as this you're not talking about a "straight" 240V supply. There are many 3-phase transformers around the U.K. which are feeding nothing but single-phase 240V residential loads, but the 3-phase arrangement is used to get the benefit of the 415V distribution voltage to reduce losses. Even though it requires a 4-core cable, the higher voltage results in greater economy than if a 2-core cable distributing 240V single-phase were used, since the cores of the latter would need to be much larger for a given load over a given distance. A similar economy is afforded by the single-phase 3-wire 240/480V distribution found in some rural areas (and the equivalent 3-wire d.c. systems which were adopted in the early days in some cities).

By way of comparison, here in the U.S. transformers feeding residential streets are almost always single-phase providing 240V on the secondary, and the runs at this lower voltage are kept considerably shorter. It's common to find one transformer feeding only half a dozen houses, so we tend to have a greater number of smaller transformers for a given number of homes compared to Britain.