Cable calculation

[Simon Burton]

Il give you the scenario, stables in a field, e-on have been out and put a supply in a hut with a meter on and a one shot, main fuse is 80amps, the customer need a fuseboard putting in the barn, 75metres away, what size cable would i be looking at, ive done a calculation, 1 x swa cable 3 core, burried, ive use 80amp as the demand as this is the maximum at the origin and the cable size is coming back at 25mm. Is the right or is there some sort of diversity applied which would allow you todrop to 16mm. All that will be going in are sockets and light, quite a few to be fair but altimatley just wanted to get the armoures cable to the fuseboard and just questioning the size. Any help is appreciated

 

[Sintra]

Work it out allowing for 3% voltage drop on the lighting.

 

[westward10]

Your calculations are incorrect.

 

[westward10]

Appendix 4 Reg 6.4 gives permitted vd limits. These limits apply from the origin of the installation to any load points and for lighting this is 3% therefore, if you have a 3% allowance on your submain any lighting circuit is going to exceed this if the submain is operating at full load, not likely. It is often quoted submains feeding lighting and power has a vd limit of 1% allowing a small allowance for the lighting.

 

[Strima]

What’s you design load and max demand? Earthing arrangement? Any utilities that require bonding?

 

[Pete999]

Il give you the scenario, stables in a field, e-on have been out and put a supply in a hut with a meter on and a one shot, main fuse is 80amps, the customer need a fuseboard putting in the barn, 75metres away, what size cable would i be looking at, ive done a calculation, 1 x swa cable 3 core, burried, ive use 80amp as the demand as this is the maximum at the origin and the cable size is coming back at 25mm. Is the right or is there some sort of diversity applied which would allow you todrop to 16mm. All that will be going in are sockets and light, quite a few to be fair but altimatley just wanted to get the armoures cable to the fuseboard and just questioning the size. Any help is appreciated

The hut you talk of does it require any bonding?

 

[Spoon]

1 x swa cable 3 core, burried, ive use 80amp as the demand as this is the maximum at the origin and the cable size is coming back at 25mm.

When you say "is coming back" is this your own cals or have you inputted the info into a web site?
As above. Your calcs are incorrect.

 

[westward10]

The hut you talk of does it require any bonding?

He is quoting a three core and hence with an 80A head the third core should be adequate for bonding purposes.

 

[westward10]

If you allow for a 1% drop that is a hefty cable.

 

[Pete999]

He is quoting a three core and hence with an 80A head the third core should be adequate for bonding purposes.

Maybe , he may not know that he can use the armour as a cpc, after all he is asking questions that an electrician should know the answer to, don't you think, probably not though.

 

[bill01803]

At 80m and 80A I get 50mm

 

[The Ghost]

25mm at 63a

 

[Simon Burton]

Your calculations are incorrect.

In what way.

 

[westward10]

In what way.

Reconsider your volt drop you have used 5%

 

[Simon Burton]

When you say "is coming back" is this your own cals or have you inputted the info into a web site?
As above. Your calcs are incorrect.

I dont know the design current so im taking it as 80amps, as thats is the fuse rating on the head.

 

[Deleted member 26818]

Are you using 70 degree or 90 degree armour?

 

[Spoon]

I dont know the design current so im taking it as 80amps, as thats is the fuse rating on the head.

Fare enough mate. Hard to do cable calcs without it though.
If this is an actual job then don't you need to have a fused insolator fitted? Won't the insolator fuse be lower than 80A?
Please correct me if I'm wrong in this.

 

[Nigel]

I dont know the design current so im taking it as 80amps, as thats is the fuse rating on the head.

If you do not know the design current then you are looking at worst case so that will push costs up. Are you not installing the final circuits?

 

[Simon Burton]

Are you using 70 degree or 90 degree armour?

70 degree mate,

 

[Simon Burton]

If you do not know the design current then you are looking at worst case so that will push costs up. Are you not installing the final circuits?

Yeah however he may be adding at later dates i know all the lighgs will be led tho and its literally gonna be 13 amp sockets, so in terms of circuits, 4 lighying and 2 or 3 radials maybe, the circuits again will be a good run in armoured so il also be cable calculating these but this wont be a problem as i will have design current as and when the cistomer knows what hes having in. Just this main cable with the run like