Cable Calculations

[ajdeath]

Hi everyone, Ive got to install a sub main for a large bugalow. The supply is in the garage and the 2 nd mains board is approx 40 meters away. I was going to use 16mm t + e, 1 meter in trunking then the rest will be in the loft clipped high up away from the insualtion. ( so no RCD) It will be connected to a 60 amp switch fuse. I could use armoured but the fuse board is plastic making it a expensive jobs. I've done calculations and i think its ok however ive not long been self employed and at my old place my boss did all the calcs so I want tio make sure its ok; any help most gratful thanks Andy

 

[Sparky Ninja]

Havn't got the book in front of me but believe the formula is

Vd= (mv/a/m) x Ib x length x (any correction factors)

Divided by 1000



If no-one else can help out I will check tonight.

 

[JUD]

Looks OK to me....

16mm² Current-carrying capacity (Reference Method B) = 69A from Table 6E1 OSG

VD = 2.8 x 60 x 40 ÷ 1000 = 6.72V (2.9%)

As long as Zs etc. checks out you should be fine.

 

[Sav]

Hi,
Looks ok to me as well.
Also volt drop max is 3% as must be based on the lighting factor ?? Is that correct folks??

I did a similar job in a brickbuilt outhouse.
I had a 60amp isolator in the main house, this fed a 16mm 3 core swa which was buried underground, approx 40 meters into the outhouse,
I too had a plastic consumer unit and what i did was to put a length of 3"x3" plastic trunking under the fuseoard, make of the swa into the trunking (using glands and a 20mm hole which was cut into the trunking) and then straight into the board.

It looked neat and was cheaper than a metal clad board.
Hope this helps,
Best regards,
Sav

 

[unemployed]

firstly, I want to say you need clearify your loads which is going to be drived by sub main, only if you know how many loads in your bugalow then you can use very basic calculation formular:
Loads( watts ) divided by voltage(230v,if you dont want to 3-phase power in your bugalow ) = designing current,
next step pop in B/Q find bigger rating calble.
that's it, job done!

 

[Spudnik]

Split con is your friend

 

[JUD]

Hi,
Looks ok to me as well.
Also volt drop max is 3% as must be based on the lighting factor ?? Is that correct folks??

I did a similar job in a brickbuilt outhouse.
I had a 60amp isolator in the main house, this fed a 16mm 3 core swa which was buried underground, approx 40 meters into the outhouse,
I too had a plastic consumer unit and what i did was to put a length of 3"x3" plastic trunking under the fuseoard, make of the swa into the trunking (using glands and a 20mm hole which was cut into the trunking) and then straight into the board.

It looked neat and was cheaper than a metal clad board.
Hope this helps,
Best regards,
Sav

That's an interesting point and has got me thinking.

What is the Current demand of the Sub-main (after diversity) as voltage drop calculations for final circuits fed from the Sub-main will have to include the voltage drop of the 16mm² feed.

Example

VD1 = Voltage Drop of Sub-main
VD2 = Voltage Drop of Final Circuit
VDTOTAL = VD1 + VD2 (Total Voltage Drop at End of Final Circuit)

 

[ajdeath]

Thanks for all of your comments, Load wise there will be a cooker, underfloor heating, immersion heater ( back up only) a couple of ring mains and a couple of lighting circuits. There is a shower but this is near the supply so can wire this from the main supply via a small D/B. can i get away with no RCD? as the board it is feeding is 17th edtion split load.the cable will not be concealed in wall ect

 

[JUD]

Thanks for all of your comments, Load wise there will be a cooker, underfloor heating, immersion heater ( back up only) a couple of ring mains and a couple of lighting circuits. There is a shower but this is near the supply so can wire this from the main supply via a small D/B. can i get away with no RCD? as the board it is feeding is 17th edtion split load.the cable will not be concealed in wall ect

You need to work out what the total Maximum Demand will be for the Sub-main by applying diversity to each circuit (see Table 1B on page 97 of OSG)

Cooker = 10A + 30% of remaining demand (e.g. for a 40A cooker the calculation would be 10 + (40 - 10) x 0.3 = 19A)

Underfloor Heating = No diversity allowed

Immersion Heater = No diversity allowed

Socket Outlets = 100% of demand of largest circuit + 40% of demand of every other circuit (e.g. calculation for 2 ring circuits would be 32 + (32 x 0.4) = 44.8A)

Lighting = 66% of total demand (e.g. calculation for a 6A lighting circuit with 12 lights would be ((12 x 100) ÷ 230) * 0.66 = 3.44A)

 

[Guest123]

You are indeed restricted to a max of 3% volt drop due to the lighting circuits in the bungalow.

What a lot of people forget is that volt drop is calculated from the origin of an installation to the end of the final circuit in question, not just the sub-main cable.