Cable Calcs - Help!

[Essex1]

Am I missing something here?  My cable calculation software is advising a 50mm earth for a 20A single phase radial circuit - 150 meters in length.  This is to get the touch voltage below 50 volts.  If I put 2 parrell earth cores I can get it down to 25mm but this still seems stupid.  The lives need to be 16mm in order to meet VD requirements.

Any advice/help would be appreciated.

 

[Andy™]

go back to the old method of looking at the relevant tables in 7671 and doing the appropriate calculations yourself?

 

[Sidewinder]

Post your full circuit parameters up.

 

[Essex1]

I have left the office now. Here goes:

20A single phase radial. Assume full load of 20A.

150M singles in 50x50 trunking on a wall shared with 5 other circuits.

Ambient temp - 20 degrees

OCPD - 20A b type

 

[Zeespark]

What sized cables are the other circuits? ambient/max temp?

Andy Guinness

 

[Essex1]

At a guess between 2.5mm and 4.0mm.

Ambient 20 degrees. Max 25.

 

[Zeespark]

Barring the conversion from amps to watts, you pretty much have everything you need.

Andy Guinness

 

[Essex1]

I know that. I am wondering why I am getting such a huge cpc? If someone else dos the same calc do they get the same. Am I missing something?

 

[binky]

cpc of cable should be rated at a suitable size as standard.

 

[poni]

• 16mm2
• This circuit design load is 20 AmpsChosen Circuit Breaker OK
  
  Chosen cable capacity in this circuit is 76 Amps (Iz)
  
  Your chosen cable size for this load is OK
  
  Volt drop is 8.4 Volts
  
  Cable size OK for volt drop
  
  Maximum Volt drop is 9.2 Volts ie 4%
  
  Maximum length of cable for this circuit is 164.29 Mtrs you have selected 150Mtrs
  
   this is not my work but retrieved off a free site
  

 

[Essex1]

• 16mm2
• This circuit design load is 20 AmpsChosen Circuit Breaker OK
  
  Chosen cable capacity in this circuit is 76 Amps (Iz)
  
  Your chosen cable size for this load is OK
  
  Volt drop is 8.4 Volts
  
  Cable size OK for volt drop
  
  Maximum Volt drop is 9.2 Volts ie 4%
  
  Maximum length of cable for this circuit is 164.29 Mtrs you have selected 150Mtrs
  
   this is not my work but retrieved off a free site
  

Thanks. I have the same as you. What do you get for touch voltage using a 16mm cpc?

 

[steptoe]

Thanks. I have the same as you. What do you get for touch voltage using a 16mm cpc?

is touch voltage still a permissable method?

I thought touch voltage nowadays was merely a RoT , and thats why MFTs no longer have it inbuilt.