Installing a Sub main

[Justinsane]

Hi, Can anyone offer me some advice?

I am installing a sub main to a barn/workshop. There will also be a classroom attached to this and a supply to a shower block. The following circuits are required...

2 lighting circuits (one for indoor and one for outdoor)
1 Ring circuit
A supply to a garage unit in a small office for lights and sockets
A supply to a shower block on a campsite also to a garage unit to supply lights and control to two boilers.

The classroom is also used as a breakfast bar for the campsite and may have an electric oven connected in the future which is why i have used the 16mm cable.

I have run a 3 core 16mm armoured cable from the main fuse board part clipped to the outside of the house and part on a catenary wire. The overall length is 20metres.

This supplies a dual RCD board which will supply all circuits.

I intend to split the tails coming from the main fuse using Henleys and install a 60amp main switch from which the armoured cable will be fed.

From the new consumer unit in the barn/workshop I will be running a 25metre cable to the shower block for lights and control units on two gas boilers.

I'm recently qualified and doubting my choices halfway thorugh the job and could really do with any advice before i connect up the supply...

The dual rcd board i am installing has a 100amp main switch but this will be fed from a 60 amp main switch at the board so as to allow for smaller cable size.

The earthing arrangment for the instalation is TT. Do i need to install an RCD at my 60 amp main switch from the board to protect my armoured cable?
Should i put an earth rod in at the barn/workshop end or will the main earthing system suffice?

any help or advice welcome....

cheers,
Justin

 

[Guest123]

Hey.

Got to be honest mate from what you've mentioned there I would be running a 25mm3c SWA as this sub-main is feeding another 3 sub-mains??

I would have a 100mA TD RCD at the origin then into your switchfuse to protect the sub.

I would install an earth rod at every DB not just your first one.

You need to think about protecting your other sub-mains as if you come directly from one of the RCD's of your 17th board then any fault in any of the other buildings would cause you yo loose everthing else that is protected by that RCD.

How will these other sub's be wired? You may not need RCD protection if other criteria can be met.

RCBO's are an option.

Food for thought.

 

[electro]

Hi
Just read you post, I think you should identify and quantify your loads first (maximum demand). once you are clear on your load you can size your submain. Is the origin of the supply SPN 100amp?. You also need the lengths of your final circuits to split your volt drop between your submain and final circuits. I recommend you use between 2% and 3% for your submain which leaves you with 3% to 4% for your final circuits.

Due to the nature of the installation it is not recommended you supply this type with of installation with one RCD due to the loss of supply in shower blocks ect. have you thought about a main board and supply the blocks separately? you also need to identify earth leakage and take this in account.

A 16mm SWA cable is approx 100amps max @ 26m @ 3% VD to give you a rough guide.

Regards Electro

 

[Justinsane]

Hi,
I have been back at the job today and the main fuse at the distribution board is 60 amp not 100 amp as i thought.

With the house load already on this I'm not sure if i will overload the supply.
Also the main consumer unit in the house has no RCD....

With regards to the final circuits I was going to put the ring main on the RCD and use RCBOs for both lighting circuits. I couldn't decide whether to have the two garage consumer units run off the other RCD or to use a split load box and run them off the main switch as each unit will have its own rcd.

I calculated my maximum demand by taking 100 % of the largest circuit and half of all the others and came up with 58 amps. (is this the correct way to work it out as if you apply the same method to the consumer unit in the house you come up with 135 amps? )

The 16mm cable is just over 20metres long and i worked out that with ambient temp and voltage drop it could take 79amps.

Would it be better to use a 63 amp main switch at the barn/workshop end instead of a 100 amp board although the sub main is supplied from a 63 amp main switch? Also the 16mm sub is attached along the side of the house and then suspended across on a catenary, would this still need to be protected by an RCD?

I was going to test earth fault loop impedance's before installing earth rods
but do you think i should install them anyway?

thanks for the replies...

Justin

 

[Spudnik]

Personally, i think you need to address the supply first as 63A sounds way to little for the additional load you are about to install.

 

[electro]

Hi
The way forward would be to get a new supply to the barn/Workshop.. it will be expensive, but the supply you have is a small domestic supply. If they decide to have a shower and cook a meal in the house goodbye barn/Workshop power and all of those poor people taking showers in the shower block....

Regards

 

[old dog]

Hi
The way forward would be to get a new supply to the barn/Workshop.. it will be expensive, but the supply you have is a small domestic supply. If they decide to have a shower and cook a meal in the house goodbye barn/Workshop power and all of those poor people taking showers in the shower block....

Regards

just about to suggest the same you beat me to it

 

[Justinsane]

would it be possible to get the fuse rating at the board uprated to 100amp and continue with plan? the 16mm cable is already installed........

If i have to get a new supply to the barn/workshop then I will need to remove the cable i have already installed...

Are there any other ways around this?

 

[Guest123]

Hey there.

I dont mean to dampen your spirits but the design process comes first, before putting any cables in.

The first thing you should have done is made sure there was scope to expand the existing installation before installing your armoured.

Even if the main fuse is uprated to 100A you need to work out the maximum demand for the whole installation (as previously stated) apply diversity and go from there.

Should it exceed 100A what will you do then??

Will your 16mm be big enough anyway??

As the lads have said a new supply for the workshop may end up being your only option, but really you should already know this.

Regards.

 

[Justinsane]

I've had a sinking feeling about this all day!

I calculated the maximum demand for the existing installation and it came to 130 amps, this is on a 60 amp main fuse and i want to add another 60....

That would be overloading an already overloaded supply.....

I will contact my customer tomorrow, eat some humble pie and tell him that the price is about to spiral....if i dont charge for the days i have already done then he may be accommodating.....

First big job on my own and what a balls up....nevermind its all a learning curve...

thanks for all the advice...

 

[Spudnik]

One of the very first things you should ascertain on a job is whether the supply is adequate for additional load.

Still, as you say, its your first major job and these things can catch you out sometimes!

 

[electro]

Hi


The key to electrical design in the the early stages of your career is to over design. I have worked with many different engineers and over design is very common. Its not until you get a lot more experience you can really use the the more difficult design methods which take you away from the rule of thumb methods and percentages which I recommended earlier. You seem very honest in your approach and we have all been there. Good luck


Regards

 

[Justinsane]

Thanks!
I made a mistake which I'll rectify....

I'm meeting the client on wednesday morning to discuss the situation and remove the cable!

I'll let you know how I get on.......

cheers