Maximum zs calculation

[bristolsparky]

Hi All,

Wondering if any of you could help...

I am currently pricing up a Nursing home extension and the easiest method to get a supply including the cheapest is for me to run a 16mm 3 core SWA from the existing care home to the extension which is approx 85 metres. The supply will be fed from 3 phase DB via a EATON memshield 2 C63 amp MCB. I am worried that this rather long run will give me a higher ZS reading than what is permitable by the regs?

If this can't be done which my calculations suggest it is too far of a run then i will need to get western power to install a new supply.

Please give me your feedback lads!

 

[Guest77]

When you design a circuit you calculate the load first, why did you decide on a 16mm2? what is the load?

 

[Andy78]

What about the normal design methods of achieving a lower Zs ? What will be the Zs of your final circuits from this extension DB be as they will be a deciding factor along with volt drop.

 

[davesparks]

1- don't feed a submain from an mcb
2- go up a cable size or two to lower the Zs

 

[bristolsparky]

i wanted to put 16mm 3 core SWA to aid in the loss of the zs as the length is 85 metres was maybe thinking switch fuse 63amp? bs 1361 type?

 

[davesparks]

What calculations have you done for this?
Off the top of my head I'd expect it to need something more like 25mm or 35mm to meet VD and Zs requirements on the final circuits

 

[bristolsparky]

i used a calculator that comes with am tech but does not make sense to me but i completely get what your saying davesparks but i just think 85metres is ridiculous for any cable length... would you recommend a switch fuse with 63amp BS1361 or similar and possible 25mm 3 core due the length and potential voltage drop?

 

[Andy78]

I'd recommend doing your own calcs if you are not happy with the software.

 

[Farmelectrics]

Get your gn3 and bs7671 books out and do the calculations yourself

 

[HappyHippyDad]

Without the max demand it's a bit difficult to say. If you take your max demand as 63A (given the fuse you suggest) then your voltage drop will be:

(85x63x2.9) / 1000 = 15.5V

so far too high.

Without looking at Zs tables I couldn't say if it would meet the regulations, but if you're using one of the cores as the earth then you have a resistance of 2.3mΩ p/metre for your R1+R2 which equals 0.196Ω for the 85m run, that gives you an indication of whether you'll meet Zs figures or not, but without any final circuit information (or Ze) it's impossible to say.

I'm not sure why you are contemplating 16mm Bristolsparky? Your max demand would have to be <28A in order to stay within voltage drop levels and thats without taking account of the volt drop of the final circuits.

 

[Simonslimline]

Whats the deal with all this using software malarkey?

We are taught how to properly design circuits at college from scratch. Whether they are distribution/final circuits makes no difference.

Its all in the book. Sorry for the rant.

 

[Flanders]

So you are taking a 63 amp single phase load of a 3 phase db you may well course a large inbalance on load over the phases it may be beter to take a 3 phase supply over to balance out load and help with voltage drop do you know what is in the exstention ? Showers, cooker, laundry, or just a few lights and sockets

 

[bigspark17]

1- don't feed a submain from an mcb
2- go up a cable size or two to lower the Zs

Why have you got a problem serving a sub board off an mcb this is very common.?!

 

[Leesparkykent]

Why have you got a problem serving a sub board off an mcb this is very common.?!

discrimination.

 

[uksparks]

I was thinking that... What's the problem?