Voltage Drop

[s6stu]

Hi all,

I've got a job to do regarding an outside supply to a summer house. The easiest route is to pick power up from a car porch. This small board is fed from the house on 2.5mm, which is approx 15m from the main board. Unfortunately the cable is under a driveway and simply installing a new cable for the intended extra supply is not a simple task.

My question is; When calculating voltage drop, do the calculations still work out correctly, if there are two different cables involved? i.e. from 2.5mm t&e to SWA

The summer house is approx 75m from the car porch consumer unit, and if you were to install a cable from the car porch, assuming approx 1 kW power usage, it is possible to 'get away with' 2.5mm, and although there is not a great deal of expected load, this is still a bit too close.

Thanks for your time in advance,

Stuart

 

[GStueyXR]

What is your design current ?

Assuming approx 1kw is not good enough.

If you are installing sockets and lights, then you have to desidn it to be safe under max load (with diversity applied)

My guess is, the 2.5 supplying the garage is already at it's max. Add to it and you are asking for trouble.

 

[s6stu]

My question is, can you calculate voltage drop using two cable types?

 

[JUD]

My question is, can you calculate voltage drop using two cable types?

As long as the csa of line & neutral of the SWA is the same as the T&E then yes.

If you are installing lights then Max volt drop from the house to the end of the light circuit in the summerhouse is 6.9V. I think you will find this hard, if not impossible to achieve with a 90m run of 2.5mm² especially if it's supplying sockets aswell.

It would also depend on the Ze and what size and type of protective device is protecting the cable as Zs could limit how long your run can be.

 

[GStueyXR]

My question is, can you calculate voltage drop using two cable types?

My question was, do you really need to?

If you cant uprate the cable to the garage, then replacing it when it melts will be just as tiresome... Nearly as tiresome as when you are called out every 5 minutes because the 20 amp breaker that is protecting the garage unit trips out.

Tis your call but the garage will have a few sockets, lights, maybe a tumble dryer etc... I bet it's already at its max.

 

[Guest123]

As a bit of guidance for you.....taking the 75M aditional run and a more concerted 2 KW load into account.

Transposing the VD calc enables us to calculate the maximum length of run allowable for a 2.5mm SWA.

So our max permitted would be 6.9V as you will have lighting in the shed.

This gives us a calculation of - (6.9 x 1000) / (18 x 8.9) = 44.5M max


I would imagine the shock protection to be within limits as I would imagine you to have at max a 20A MCB protecting this circuit giving a max Zs of 2.3ohms.


HTH.

 

[s6stu]

First of all, thank you for your resposnses.

I appreciate that 1kW is a very minimal estimate and that anything above would cause problems and realistically more power would be consumed. And of course I would not take a chance if there was a possibilty of overloading the cable.

I was just curious to see if a larger cable installed after the 2.5 would make any difference to the voltage drop, but as JUD and Lenny have said, only the same csa of live conductors can be used when using the volt drop calculator, thus 2.5 t&e with a 6mm SWA extension would not make a difference, as only 2.5mm SWA can be applied to the formula. Just needed to clear that part of the equation up!

thanks again

 

[Guest123]

If there are different cable sizes in a circuit which is quite common, sub-main and final circuit for example, then a VD calc would need to be done for each cable individualy with all the VD's totalled to give the actual VD form origin to end.

In this instance you would have the 15M run of 2.5 from the main DB to the car port DB. Then a seperate calc for the cable form the car port to the summer house, which IMO will not work out due to the existing loading and length of run. A new cable direct from the main DB to the summer house as has been reccomended by others is the way to go.

 

[s6stu]

Lenny,

I see your point of different cables within the same circuit, but usually the arrangment is from a larger main incoming to smaller final circuit cables, I would need to do the opposite, but it just didn't sound right to me?

Ideally I would not even be considering this option and would run a new cable, but this means digging through an expensive driveway!! As well as disturbing the interior of an oak beamed and flag stoned floor of a 200 year old house!


So just to clarify, theoretical calcs;

2.5 t&e from Main DB to car porch DB, approx 15m and assuming a reasonable load of 2kW

VD = (15 (18 x 8.7) ) / 1000

VD = 2.35 V


then, from car porch DB to summer house, approx 75m, but then upgrading to 10mm SWA;

VD = (75 (4.4 x 8.7) ) / 1000

VD = 2.87 V

Total VD = 5.22 V


I appreciate this is not realistic and would cost a fortune, and to be honest, I've already e-mailed the customer to say this is not a viable option. However, I would still like to clarify for my own benefit, whether the above system, if installed, would give an approx VD of 5.2 V? With the larger cable after the smaller?

 

[Des 56]

Also take into account the volt drop of the installation including the final circuits

 

[GStueyXR]

Also take into account the volt drop of the installation including the final circuits

So high tension overhead lines it is then