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Electric Cable Rating Help!


soapstar

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Hey Folks,

 

A question for the sparkies out there! We are needing to get the correct rating for an electricity supply from our main meter which will feed 17 separate units which will draw 16amps each. The distance to cover is approximately 250m. The cable will be 3 core, armoured and laid underground.

 

I have used various calculators online but cannot get a positive answer to the size of cable required - some say 30mm and other 90mm, the cost difference is massive obviously!

 

This is a little off topic and not related to the house build but its the only place I get sensible answers! ?

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17 times 16 is 272 Amps.

 

No 1:  That lot will not be powered from a typical domestic 100A single phase supply.

 

No 2: It will be voltage drop that determines the cable size over that distance, and it will be a BIG cable.

 

No 3 you would not normally export an earth over that distance so it would be 2 core cable and probably a local earth.

 

This is a serious design issue and as a very minimum it sounds like you want a new DNO installed 3 phase supply as close tothese 17 units as possible.

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2 hours ago, soapstar said:

Hey Folks,

 

A question for the sparkies out there! We are needing to get the correct rating for an electricity supply from our main meter which will feed 17 separate units which will draw 16amps each. The distance to cover is approximately 250m. The cable will be 3 core, armoured and laid underground.

 

I have used various calculators online but cannot get a positive answer to the size of cable required - some say 30mm and other 90mm, the cost difference is massive obviously!

 

This is a little off topic and not related to the house build but its the only place I get sensible answers! ?

 so 17 x 16amp ==thats a lot 

 

get an electrician to work it out -- single phase it will be bigger than 90mm  and cost a alot -

as suggested 3 phase by DNO to the place where all these u8nits take power from 

 

 

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1 hour ago, Gav_P said:

Let’s have a guess... could be fun! ?

 

campsite I reckon! 

 

1 hour ago, ProDave said:

I was thinking a collection of camping pods or glamping or whatever todays trendy work is.

 Good guess ?

 

Sorry I miss typed in my original post, the distance is 200m not 250m. We never assume we will have a constant 16amp load from every unit however it would be a safer option encase we ever came across this situation. We currently have a 3 phase supply to this land.

 

I will admit I am a very limited on knowledge when it comes to electrical projects, I would agree to get an electrician to do the math however (apologies to the tradesmen on here) I just don't trust to get an honest quote, soon as they know you aren't up to scratch on the subject you will be taken for a ride 9 times out of 10, sad fact! I just want to get some facts together to help me understand the various factors to take into consideration when rating the cable.

 

Would a 35mm 2 core cable be sufficient (even if only 50% of all 17 units will be pulling 16 amps)? You are correct @ProDave, the units will be individually earthed.

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9 minutes ago, soapstar said:

Would a 35mm 2 core cable be sufficient (even if only 50% of all 17 units will be pulling 16 amps)?

you cannot do your calculation on that -must for total possible load  

suppose you should be starting from the beginning

How do you know that a 16amp supply is enough ?

 

 ring main usually has a 30 amp fuse -

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So you have a 3 phase supply.  Good.  Use a 4 core cable (Neutral, L1 , L2 and L3)  And feed a 3 phase 6 way distribution board to split it to the 17 units 6 each on 2 phases and 5 on the third phase.

 

35mm will do for current carrying capacity but will still not be big enough to keep voltage drop within allowable limits.

 

At full load of 16A per unit I am coming up with 85 square mm cable to keep voltage drop to no more than 5%.  It would be even bigger if you were trying to keep voltage drop to the lower limit for lighting circuits.

 

Your ONLY hope of getting it smaller is to do some diversity calculations to get the total load down to a LOT lower than 16A per unit.

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This is an interesting problem as it does depend on the layout of the loads.

 

If the 200m cable is devided by the number of primary loads, the 16A, evenly i.e. at 12m intervals, then I calculate that a 42mm2 cable is needed to keep the voltage at the very end above the minimum of 216.2V (230V - 6%).

One problem is that if the initial voltage is already at the lower limit of 216.2V, or close to it, then you are stuffed.

 

I may try and add in some random 'diversity' to see what happens.

 

 

(I am using 19 ÷ by C.S.A [mm²] to give ohms per km. ÷ by 1000 for resistance per meter., then Ohms Law)

 

 

Edited by SteamyTea
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I would recommend engaging a  qualified electrical engineer/electrician  that can review this installation with all the actual detailed data. Given the range of cable sizes and potential costs, it is worth spending a little money to reach the optimum size, underwritten by a qualified electrical engineer/electrician.

 

As usual, everybody is trying to help and giving an honest opinion but none of these opinions may be relied on because they do not have the actual data to work with. 

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