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!

 

[plugsandsparks]

True... I asked him what size, but he hadn't bought them yet. The little bit of research I did this evening showed me that they are usually around 1.5 - 2.8kW!!

http://www.amazon.co.uk/Hylite-Elec...376341961&sr=8-15&keywords=greenhouse+heaters

Seen them before like this. These low watt ones keep frost at bay, the bigger ones try and maintain a climate, a bit like keeping tropical fish i suppose.

If he wants a really big one - going to be spinning the meter a tad..

 

[plugsandsparks]

Absolutely, because we all know that the electricity distributors are going to invest billions of pounds making sure that all households get 230v real soon....

Well all i have seen in last 5 years is volts going up and up, now regularly seeing 250V. Its bizarre, anyway its only a heater, not like it wont work, just takes longer to heat the greenhouse

 

[Brightspark2]

So what is drawing 1000W at the moment?...

What I'm getting at is diversity, are these loads all going to be on at the same time?...

 

[HappyHippyDad]

Hylite 4Ft Green Electric Tubular Greenhouse Heater - 180W: Amazon.co.uk: Garden & Outdoors

Seen them before like this. These low watt ones keep frost at bay, the bigger ones try and maintain a climate, a bit like keeping tropical fish i suppose.

If he wants a really big one - going to be spinning the meter a tad..

Thanks Plugs, I saw the lower watt tubular ones but he specifically said 'fan'!

So what is drawing 1000W at the moment?...

What I'm getting at is diversity, are these loads all going to be on at the same time?...

Cheers Brightspark, did think about diversity but the 1000w would probably be on at the same time.. they consist of about 5 lights on sensors as you walk up the garden and 3 pumps which were all on when I was there.

 

[Richard Burns]

Voltage drop will mean that things like heaters will run less hot and lights will be a bit dim.
remember that section 525 says that if the manufacturer says it will work at lower levels of voltage then it is OK.
or the voltage must not impair the safe functioning of the equipment.
Then says the limits.
The MCB will not trip, the circuit will not be unsafe. Though the accessories may be.
Electronic equipment that may need the full voltage might fail.

If you are feeding a supply to the fan heaters can you up the csa of that part so that you have more leeway?
Just googling this heater for instance has a 1kW energy saving mode, like bathroom fan heaters. Could be a thought.
Have you tried splitting down the VD if the circuit splits up or is it all at the end, this could give you more potential!

 

[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.