Volt drop calculation and cable size

[Paul12v]

Hi

A good friend of mine is having a large electrical installation carried out in his garden.Hes had 3 prices from various electrical contractors regrading the install.

He is having a summer house built that will house
9kw 3 phase sauna
5kw hot tub
27kw 3 phase electric boiler
A 32 amp ring circuit for 2 x 3kw fan heaters
A 40amp circuit for 2 x 3 kw ( possibly more) electrical patio heaters
12 mains volt led down lights

The length of run is 25-30m the cable will be in 100mm ducting 600mm down installed by ground work contractors belonging to the landscape company

He has 3 phase in his house I was with 1 of the electricians as he walked round the job,the original 3 phase dist board has room for the correct breakers for a sub main ? cable to be run out to the new building.All the electricians have then specified 3 phase dist board with the appropriate breakers etc

2 of the quotes inc a 25mm 5 core cable
1 of the quotes inc a 16mm 5 core cable

The spec is quite involved and says about permissible volt drop and earth rods being installed which is common to all 3

1 of the quotes list a triple pole 100ma RCD to give greater protection to the sub main but the other 2 do not

They all specify 30ma RCBO's for the summer house distribution board

If any one help can I get clarification what size cable he does need please and woukld it be advisablke to fit the 100ma rcd ??

thanks
P

 

[westward10]

Have they applied diversity to the circuits and what % volt drop did they apply. 16mm is too small and the 100ma rcd is probably not necessary but factors such as the earthing system may deem it necessary.

 

[telectrix]

at first glance 25mm seems appropriate, allowing for diversity. 16mm is way too small. as for VD, i'd be looking at a Ib of 100A, so work that out over 30m. and get your mate to buy shares in his leccy supplier.

 

[DPG]

No wonder our planet is on its uppers. Although at least he's using LED lighting to cut down on usage a bit

 

[Paul12v]

Thanks for your replies Il pass it on and I will found out about the volt drop they allowed not sure about diversity but I think he will now go for 1 of the 2 that shows 25mm cable

Large recent inherited wealth and over controlling architect ..There not sauna hot tub people but have been influenced unfortunately ..large family home with a view of making 1 more move ..........see it as an investment .. im quite happy in my 3 bed semi...on candle power here !

 

[SJT Contacts]

You may also need to pay particular attention to the installation method as you are planning to bury the cable and install it in ducting,
I would think you will need a 100A 3 phase MCCB with and overload to be able to reduce it to about 90A to protect the cable and there is no way 16mm would be sufficient for the project,
The best thing would be to ask them for a cable calculation for the main cable feeding the new summer house

 

[Pete999]

Hi

A good friend of mine is having a large electrical installation carried out in his garden.Hes had 3 prices from various electrical contractors regrading the install.

He is having a summer house built that will house
9kw 3 phase sauna
5kw hot tub
27kw 3 phase electric boiler
A 32 amp ring circuit for 2 x 3kw fan heaters
A 40amp circuit for 2 x 3 kw ( possibly more) electrical patio heaters
12 mains volt led down lights

The length of run is 25-30m the cable will be in 100mm ducting 600mm down installed by ground work contractors belonging to the landscape company

He has 3 phase in his house I was with 1 of the electricians as he walked round the job,the original 3 phase dist board has room for the correct breakers for a sub main ? cable to be run out to the new building.All the electricians have then specified 3 phase dist board with the appropriate breakers etc

2 of the quotes inc a 25mm 5 core cable
1 of the quotes inc a 16mm 5 core cable

The spec is quite involved and says about permissible volt drop and earth rods being installed which is common to all 3

1 of the quotes list a triple pole 100ma RCD to give greater protection to the sub main but the other 2 do not

They all specify 30ma RCBO's for the summer house distribution board

If any one help can I get clarification what size cable he does need please and woukld it be advisablke to fit the 100ma rcd ??

thanks
P

Any chance you could post the original calculations?

 

[Paul12v]

Hi

I will try and get them I dont think it showed calculations just recommended cable sizes and certain factors ie the volt drop etc

 

[Pete999]

Hi

I will try and get them I dont think it showed calculations just recommended cable sizes and certain factors ie the volt drop etc

Thanks Paul, but to recommend a cable size for an installation some calculations will be required.

 

[MJPD29]

I would not like to be paying the quarterly bill on that lot. Then again if they have the money to do it im sure they can afford it.