# Cable calculation

Discuss Cable calculation in the Commercial Electrical Advice area at ElectriciansForums.net

#### Simon Burton

##### Active EF Member
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

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

#### westward10

##### In echoed steps I walk across an empty dream.
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#### westward10

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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

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

#### Pete999

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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

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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

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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

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

#### Pete999

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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

##### Regular EF Member
At 80m and 80A I get 50mm

T

25mm at 63a

OP
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#### westward10

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OP
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#### Simon Burton

##### Active EF Member
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.

D

#### Deleted member 26818

Are you using 70 degree or 90 degree armour?

#### Spoon

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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.

N

#### 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?

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#### Simon Burton

##### Active EF Member
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

OP
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#### Simon Burton

##### Active EF Member
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

OP
S

#### Simon Burton

##### Active EF Member
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.
Yeah im now thinking degrade the fuse rating in my isolator, going off topic how do this work in domestic when the heas is a 60amp but the mainswitch on a db is 100amp

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#### Nigel

Yeah im now thinking degrade the fuse rating in my isolator, going off topic how do this work in domestic when the heas is a 60amp but the mainswitch on a db is 100amp
The main switch is not a fuse. Are you a qualified electrician?

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#### Simon Burton

##### Active EF Member
What’s you design load and max demand? Earthing arrangement? Any utilities that require bonding?
I dont have one as the customer doesnt exactly know what he is having, i was going with the worst case being the fuse rating on the main head. No utilities as its pvc water main. And main earth will be tt as its in a field, western power have put a tncs in but we will be putting a earth rod in along with rcd ect ect

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#### Nigel

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
I would design to 63A if this was me.

N

#### Nigel

I dont have one as the customer doesnt exactly know what he is having, i was going with the worst case being the fuse rating on the main head. No utilities as its pvc water main. And main earth will be tt as its in a field, western power have put a tncs in but we will be putting a earth rod in along with rcd ect ect
How comes you are not using the TN-C-S?

OP
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#### Simon Burton

##### Active EF Member
The main switch is not a fuse. Are you a qualified electrician?
No but in terms of over current in a domestic dwelling ive seen a db have 100amp main switchbwith and the main head bein 60amp fuse, the fuse in the main head would blow before rhe main switch would trip. Which is incorrect but why is this done.

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#### Nigel

No but in terms of over current in a domestic dwelling ive seen a db have 100amp main switchbwith and the main head bein 60amp fuse, the fuse in the main head would blow before rhe main switch would trip. Which is incorrect but why is this done.
So you are a DIYer?

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#### Simon Burton

##### Active EF Member
How comes you are not using the TN-C-S?
Not allowed to export the earth. Think about it, if you loose your neutral on the main, youve also lost your earth, so anybout building ect will not be earthed

##### Still simmering Ken, not boiled yet buddy x
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The main switch wouldn’t trip mate......

OP
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#### Andy78

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No but in terms of over current in a domestic dwelling ive seen a db have 100amp main switchbwith and the main head bein 60amp fuse, the fuse in the main head would blow before rhe main switch would trip. Which is incorrect but why is this done.
I think this is the clincher. Get someone competent on this job mate.

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#### Simon Burton

##### Active EF Member
I think this is the clincher. Get someone competent on this job mate.
Typo- main switch wouldnt trip,

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#### Nigel

What makes you say that
The fact that you don’t know the function of a CU main switch.

## Should the 'Respected Member' Tag be removed? it's 2019, surely we're all respected.

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