First cable calc that is non-theoretical!

[Echo]

Hi, just wondering if anyone would be kind enough to take a look over my cable calc as I haven't done one really since I finished college as it's all been house bashing.

The customer wants a supply feed up to the top of his garden (cable run will be 70m) The cable is to be run along the top of a fence so I have used reference method C. And I have also used the ambient temperature of 30 degrees (but im not too sure if this is right)

The cable is to feed a jacuzzi from homebase (guessing no more than 4kw, it cost £300) a summerhouse which consists of a socket for general use (probably a tv) and a light fitting. It is also to feed some garden lighting.
A pond with a water pump may also be added at a later date.

I've estimated the power consumption to be around 9kw.

9kw@230v = 39A

The cable does not run through any thermal insulation nor is it grouped with any other cables. Is there any other derating factors I should be looking at?

10mm - 1 two-core cable, single phase ac - 67A (table 4D4A - pg280)

Voltage Drop
4.4 mv/a/m = 4.4x39Ax70m/1000 = 12v

Max permissible VD for 230v is 6.9v (3%)???
So I think I need to either use a larger cable or restrict the usage? Just 10mm seems an awful lot for a few lights and a jacuzzi. I think the next step would be to find the exact rating of the jacuzzi.

I know this is really basic for alot of you but it's been a long day, and what feels like a really long time since i've done this!

Any advice would be greatly appreciated!

 

[JUD]

Voltage Drop
4.4 mv/a/m = 4.4x39Ax70m/1000 = 12v

The voltage drop for 70m of 10mm² cable carrying 39A is 10.93V when you take into account the reduced operating temp of the cable (Ct = 0.91)(See calculation).

(4.4/1000) x 39 x 70 x 0.91

This is acceptable for your jacuzzi etc. as it is under 5% (11.5V) but it is no good for your lights (i.e. above 3% (6.9V)) .



16mm² will bring it down to 6.8V (Ct = 0.89) but then you need to take into account the voltage drop on the cable run to your lights (See calculation).

(2.8/1000) x 39 x 70 x 0.89

The calculation for Ct is on page 258 of BRB.

 

[GT1]

I know this is really basic for alot of you but it's been a long day, and what feels like a really long time since i've done this!

Dont be so sure..!!
There are plenty out there that wouldnt bother spending as much time on the theory, so bravo to you for doing so..!!

Just one though on your calcs, and ive no books on me at the mo, so i dont know the answer, but are you able to apply any diversity to the demand..?

 

[pushrod]

The voltage drop for 70m of 10mm² cable carrying 39A is 10.93V when you take into account the reduced operating temp of the cable (Ct = 0.91)(See calculation).

(4.4/1000) x 39 x 70 x 0.91



The calculation for Ct is on page 258 of BRB.

You've got me confused on your correction factor of 0.91, I thought correction for operating temp on volt drop only applied to ambient temps over 30 C




Echo i think you must get that jacuzzi power rating so you can work out Ib exactly - could be 2 kw could be 4kw . whichever it is makes a big difference to your calcs.

Re air ambient temperature even if you went for a cooler temp you only gain a fraction (2 or 3%) and it is possible that the jacuzzi would be in use on those three very hot days we call summer so nothing wrong with ignoring it and sticking to 30C, IMO.


edit : also think your estimate of 5kW for the other bits and bobs is rather generous - your sockets could be be on a 16 A breaker and the lighting is probs never going to be more than 1A. That is going to bring it in at more like 4kW max. Pond pumps have quite a small demand most are under a 100W so not a lot to worry about there.

 

[JUD]

Hi, just wondering if anyone would be kind enough to take a look over my cable calc as I haven't done one really since I finished college as it's all been house bashing.

The customer wants a supply feed up to the top of his garden (cable run will be 70m) The cable is to be run along the top of a fence so I have used reference method C. And I have also used the ambient temperature of 30 degrees (but im not too sure if this is right)

You've got me confused on your correction factor of 0.91, I thought correction for operating temp on volt drop only applied to ambient temps over 30 C

It is >= 30°C. In the OP he said he had ambient temp of 30°C although I can't see the ambient temp getting that high in a garden very often (not in this country anyway)

 

[pushrod]

Hi JUD, you would use the ambient temp when calculating It for working out out cable size as one of the cable rating factors, not for the voltage drop calc (see pages 123 +124 of the OSG) .

Agree about the chances of seeing 30C in this country

 

[JUD]

Hi JUD, you would use the ambient temp when calculating It for working out out cable size as one of the cable rating factors, not for the voltage drop calc (see pages 123 +124 of the OSG) .

Agree about the chances of seeing 30C in this country

Ct is applied for an ambient temperature of 30°C and above if the cable you are using can carry more than the design current.

For example, if your design current is say 39A as in the OP but for a long run of 70m you have to use a 16mm² cable that can carry 89A then the cable will never reach it's maximum operating temperature (70°C) as it's only carrying less than half it's CCC.

The values of voltage drop in BS 7671 are given at a conductor operating temperature of 70°C so you apply Ct to the voltage drop calculation to take into account the lower operating temperature of the cable.

But like you said in your previous post this is only if the cable is in an ambient temperature of 30°C and above.

Another example would be Table 7.1 in the OSG (page 44). Take a look at the maximum length for a 32A ring circuit wired in 2.5/1.5mm² T&E. It's 106m.

This is worked out using voltage drop as the limiting factor.

Working out without applying Ct gives you.....4 x (11.5 x 1000) / 26 / 18 = 98m
Now working it out with Ct applied gives you.....4 x (11.5 x 1000) / 26 / 18 / 0.923 = 106m

Hope that helps.

 

[pushrod]

Hi JUD - i get the idea that if Ib is well below the Iz for the cable it will be no where near its max operating temp and so there can be a correction factor for volt drop - thanks ( that has never been mentioned at college, but then again plenty just struggled with the ordinary calcs for volt drop!) The calc on P258 is pretty daunting but i did at least manage to get the same value as you.

With your example from the OSG page 44

"Working out without applying Ct gives you.....4 x (11.5 x 1000) / 26 / 18 = 98m"


where did you get the 4 from and the (i assume) Ib of 26A - is the 4 because it is a ring with parallel paths?

If you take another example eg the radial circuit 16A on 2.5mm cable(on page 47 of the OSG) you get their answer of 40M without using any correction factor and the full value of 16 amps as Ib - how come?


cheers

 

[JUD]

Hi JUD - i get the idea that if Ib is well below the Iz for the cable it will be no where near its max operating temp and so there can be a correction factor for volt drop - thanks ( that has never been mentioned at college, but then again plenty just struggled with the ordinary calcs for volt drop!) The calc on P258 is pretty daunting but i did at least manage to get the same value as you.

With your example from the OSG page 44

"Working out without applying Ct gives you.....4 x (11.5 x 1000) / 26 / 18 = 98m"


where did you get the 4 from and the (i assume) Ib of 26A - is the 4 because it is a ring with parallel paths?

If you take another example eg the radial circuit 16A on 2.5mm cable(on page 47 of the OSG) you get their answer of 40M without using any correction factor and the full value of 16 amps as Ib - how come?


cheers

Yes the 4 is because it is a ring circuit. A ring circuit is assumed to have a load of 20A at the furthest point and the balance of the rating of the protective device evenly distributed. (For a 32A device this equates to a load of 26A at the furthest point.) <--Page 54 OSG

 

[Echo]

Thanks for all your input, this really is a learning curve for me as like i've said I don't really have a great deal of experience, having been made redundant as soon as I completed my apprenticeship!

The other question i have to ask is, the customer now wants a "junction box" halfway down the cable run for future use. I'm planning on using a weatherproof box with a din rail and din rail connectors. However I'm now wondering how I go about downsizing the cable for general use.

FUSED SPUR SUNGLE IP66 - TGV103 - TIMEGUARD

Would the terminals on the fcu take a 16mm?

Thanks for all the help lads, it's really appreciated. If i can ever return the favour I will!!

 

[JUD]

Personally I would run a sub-main to the summerhouse into a 3way CU then feed everything off that.

20A circuit for jacuzzi (if it's 4KW as you say in your OP)
16A Radial Socket circuit
6A lighting circuit

You really need to know how much power the jacuzzi is going to use. It may not end up as much as you think.

 

[Echo]

Yeah was going to do that for the summer house but thats 60m up the garden. The customer also wants a feed 10m up the garden.
Do I have to go up to the summerhouse with the submain and then 50m back down the garden?

 

[pushrod]

Agree about the submain. The other thing to decide on is your earthing arrangement. Exporting the earth to a TT system/earth rod is probably the way to go, especially as it sounds as if you have a water supply there.[ Using a 2 core cable compared to a 3 core could be quite a saving on such a long run with a large cable, if you were thinking about that.] You need to earth the armouring at the supply end only.

I would be very surprised if a 13A FCU would take a 16mm cable anyway. I think that you could just bring the cable up to an IP65 adaptable box, as long as that's not buried, but as JUD said it is probably more straight forward to go to a submain and come back.

[thanks for the previous answer JUD ]

 

[JUD]

Yeah was going to do that for the summer house but thats 60m up the garden. The customer also wants a feed 10m up the garden.
Do I have to go up to the summerhouse with the submain and then 50m back down the garden?

What is it they want 10m up the garden? It may be cheaper and easier to run a sub-main for the jacuzzi and summer house etc. and a seperate feed from main CU for the other thing (if possible).

Remember to check that main supply can take all this extra load before you do anything.

 

[Echo]

Yeah everything should of been alright JUD.

HOWEVER after ordering all the materials today I got a phone call from said customer saying they wanted provisions for the future installation of a cooker/oven!

Something makes me think that a new main supply is going to be needed!!!