Fear of voltage drop!

[HappyHippyDad]

Its been along day, I've just finished off with looking around a big garden which has all sorts of electrical things going and would appreciate a bit of advice on VD.

Garden is supplied by 2.5mm SWA (buried), at present taking about 1000W from a 20A MCB in the garage CU. The chap wants 2 greenhouse fan heaters installed. The run is about 40m's in total.

Max allowable VD = 11.5
VD (mV/A/m) for 2.5mm = 18
length = 40m

so available power = (max allowable VD x 1000)/length x VD

= 16A (approx) = 3680W

Garden is already taking 1000W so I'm left with 2680W for 2 greenhouse fan heaters.

I have 2 questions:

1. Can anyone see a half decent greenhouse fan heater that is only 1.3kW?
2. Really, Really tired and just cant figure out if VD means the cable will be unsafe or the MCB more likely to trip.... ridiculous question I know but I've had a glass of port (yeah I'm ready for the jip :rolleyes2, I'm tired and I just want to figure it out. Quote doesn't even need to be done for a week!

 

[Knobhead]

stick 'em in, switch on, measure the VD. if it's under 220V at point of utilisation, then start worrying.

Ye gods, common sense?

No, I must be on the wrong site.

 

[Welchyboy1]

stick 'em in, switch on, measure the VD. if it's under 220V at point of utilisation, then start worrying.

Absolutely!

if it was a sub main to a workshop then i would be considering things more carefully, but a few out side lights and a heater in a green house??

just do it and see what happens, just try and keep the heater as low KW as poss

worse case would be dimmer lights and a not as efficient heater, explain this beforehand to customer, and if this happens and he cant put up with it tell him its a new supply cable


or you could talk him into changing the lamps to LED to lower the load

bit more work for you too!

 

[MDJ]

surely he only needs frost protection? if so a couple of 100 watt tubular heaters in each controlled via a frost stat set at 2 degrees will do, whats he trying to grow out there Bananas? LOL

 

[snowhead]

Is this a Greenhouse or Sauna or for growing "Something else"?


Make up an extension lead of 2.5 t&E, borrow a 3kw fan heater, and do some tests.

I'll guess the greenhouse will get warm and light and the cable won't melt and the volt drop Police won't turn up.

 

[daverogers]

stick 'em in, switch on, measure the VD. if it's under 220V at point of utilisation, then start worrying.

Agreed.

 

[Guitarist]

Is this a Greenhouse or Sauna or for growing "Something else"?


Make up an extension lead of 2.5 t&E, borrow a 3kw fan heater, and do some tests.

I'll guess the greenhouse will get warm and light and the cable won't melt and the volt drop Police won't turn up.

I can picture them now, forensic suits and MFT's...."Sergeant, we have a code 1 in this greenhouse!"

 

[Guest111]

As others have said check what voltage you have to start with,voltage can vary at times of peak demand in the same way as water pressure does,but I would be surprised if you have a straight 230v coming in mine varies from around 245 up to 254,if you are worrying about your 230 you'd need to check anyway just to make sure you actually have it,worst case scenario would be to base your calcs on 230 and then find out you have 225,just remember to check your voltage while the circuit is under load.

 

[GLENNSPARK]

just to give the man an idea :

if designing...say from the comfort of an office then you use nominal voltages (230V/400V)...
if you have direct access to measure a voltage...say with a loop tester for instance...then go with that..
What Phil has said is quite correct Hippy....
usually a measured phase voltage on star connected is around the 244V-248V mark...
use the square route calc for line voltage ...

 

[Hawkmoon]

If the volt drop is excessive would that not have an impact on fault current and disconnection times?

Ze is normally tested on an unloaded circuit with full voltage available at test point - however if there is a fault to earth under load (eg metal case of heater) the voltage to earth will be at the reduced level - surely voltage level too low will have the same effect on fault current as earth resistance too high? (more of an issue with circuits not protected by RCD I know).

Or am I talking borroks - I often do lol :crazy:

 

[telectrix]

the fault current would be proportional to the voltage, so if the volts were down by 10%, then so would the fault current be 10% less. 900A as opposed to 1000A. don't think OCPD would be too bothered by that.

 

[MDJ]

the fault current would be proportional to the voltage, so if the volts were down by 10%, then so would the fault current be 10% less. 900A as opposed to 1000A. don't think OCPD would be too bothered by that.

Yep ohms law, your right tel

 

[Hawkmoon]

the fault current would be proportional to the voltage, so if the volts were down by 10%, then so would the fault current be 10% less. 900A as opposed to 1000A. don't think OCPD would be too bothered by that.

Agreed - but I was talking about volt drop generally and perhaps more excessive than 10% - Just making the point that it may not only be the operation of equipment that you have to consider in these situations, especially if you are intending to 'non-comply'.