Sub mains / distribution circuits

[Devil7]

Hi all, new here, just looking for a few pointers.
I'm not doing this, just keeping my brain active in these weird times with things i've not learnt yet.
How do you calculate cables etc for a single phase sub main (to a garage for instance) - is it the same as a cooker? Ib> In> etc etc??
TIA

 

[Andy78]

Hi all, new here, just looking for a few pointers.
I'm not doing this, just keeping my brain active in these weird times with things i've not learnt yet.
How do you calculate cables etc for a single phase sub main (to a garage for instance) - is it the same as a cooker? Ib> In> etc etc??
TIA

Yes the process is essentially the same as any cable design with a few considerations.

The VD will have to take into account final circuits connected to the distribution circuit so either calc them in too and/or leave plenty of room for final circuit VD. This will also apply to the Zs value of final circuits.

Discrimination/selectivity of protective devices should be thought out and this may even alter the installation method of the distribution circuit.

Others might add some points I've not come up with.

 

[Devil7]

Yes the process is essentially the same as any cable design with a few considerations.

The VD will have to take into account final circuits connected to the distribution circuit so either calc them in too and/or leave plenty of room for final circuit VD. This will also apply to the Zs value of final circuits.

Discrimination/selectivity of protective devices should be thought out and this may even alter the installation method of the distribution circuit.

Others might add some points I've not come up with.

Thanks for the reply Andy
Based on a cooker or shower you know the kW of the appliance to calculate the amps, on a sub main are you combining the total MCB's to get this?

 

[Andy78]

Thanks for the reply Andy
Based on a cooker or shower you know the kW of the appliance to calculate the amps, on a sub main are you combining the total MCB's to get this?

You would assess the load taking into account diversity and base your calcs on that.

I can recommend the IET design guide as a good resource for learning.

 

[Devil7]

You would assess the load taking into account diversity and base your calcs on that.

I can recommend the IET design guide as a good resource for learning.

Excellent Andy thanks, i'll get it ordered - thanks for your time, much appreciated !

 

[pc1966]

Discrimination/selectivity of protective devices should be thought out and this may even alter the installation method of the distribution circuit.

If you are looking at a low-ish power sub-main for a garage, etc, you have the trade off between:

• The easy option of taking the feed cable from a MCB in the CU
• The bigger cost/effort of splitting the tails and adding a fused switch to feed it

Both are safe, but achieving discrimination between two MCB is series is much harder to do as the main one's instantaneous magnetic trip is quite likely to be reached before the downstream breaker has cleared the fault.

A few MCB suppliers publish tables to show the sort of fault current where they are able to discriminate to, but it is not that dependable. Also Schneider have an on-line tool to help compute for their MCB (as well as and BS88 fuses, etc) which I will try to link to but they have gibberish style addresses and it seems not to work with Firefox so you might have to sell your soul to Google and use Chrome web browser:

Electrical Calculation Tool - https://hto.power.schneider-electric.com/cbt/app/index.html?code=34a79e11-548d-4e49-8a1e-10f9fa18492b&client_id=cbt#/Dashboard

To get any hope of a selective response for MCB in cascade try to keep the upstream one as high as the feed cable is safe for and go for a 'D' curve if you can at all meet the disconnection times (for a sub-main I guess that is 5s), and then try to keep the local breakers as small as you sensibly can (say 20A 'B' curve for any ring in the garage).
[automerge]1586706677[/automerge]
Also to add if this garage is being used as a workshop instead of junk store (after all nobody actually parks their car in one, do they?) you might want to consider some emergency lights in case the power does fail. Only another £20 or so to the cost for a simple non-maintained bulkhead light to avoid sudden darkness in a workshop with sharp tools!

 

[Devil7]

If you are looking at a low-ish power sub-main for a garage, etc, you have the trade off between:

• The easy option of taking the feed cable from a MCB in the CU
• The bigger cost/effort of splitting the tails and adding a fused switch to feed it

Both are safe, but achieving discrimination between two MCB is series is much harder to do as the main one's instantaneous magnetic trip is quite likely to be reached before the downstream breaker has cleared the fault.

A few MCB suppliers publish tables to show the sort of fault current where they are able to discriminate to, but it is not that dependable. Also Schneider have an on-line tool to help compute for their MCB (as well as and BS88 fuses, etc) which I will try to link to but they have gibberish style addresses and it seems not to work with Firfox so you might have to seel your soul to Google and use Chrome web browser:

Electrical Calculation Tool - https://hto.power.schneider-electric.com/cbt/app/index.html?code=34a79e11-548d-4e49-8a1e-10f9fa18492b&client_id=cbt#/Dashboard

To get any hope of a selective response for MCB in cascade try to keep the upstream one as high as the feed cable is safe for and go for a 'D' curve if you can at all meet the disconnection times (for a sub-main I guess that is 5s), and then try to keep the local breakers as small as you sensibly can (say 20A 'B' curve for any ring in the garage).

Thank you, thanks for the response, i'll look at that link, really good information mate it's very much appreciated !!

 

[Devil7]

Also to add if this garage is being used as a workshop instead of junk store (after all nobody actually parks their car in one, do they?) you might want to consider some emergency lights in case the power does fail. Only another £20 or so to the cost for a simple non-maintained bulkhead light to avoid sudden darkness in a workshop with sharp tools!

It's all imaginary at the moment, just wanting to understand the theory behind it all, but a very good point and something i probably won't have thought of