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Topic Title: New distribution circuits (formerly known as sub mains)
Topic Summary: Volt drop calculations
Created On: 13 June 2018 03:44 PM
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 13 June 2018 03:44 PM
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LACarbon

Posts: 22
Joined: 15 March 2018

When calculating volt drop for new distribution circuits and without knowing the design current of the properties being supplied. Would you a) Use the rating of the fuse supplying said properties? or b) Insert your recommendation?

The properties to be supplied are 1 bed flats but I have no information on the CCU or final circuits.

Carbon
 13 June 2018 03:55 PM
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psychicwarrior

Posts: 618
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I'm curious , why do you want to know the volt drop at a stage where you have no other required information ?

To answer your question as put, I'd have to take option b) with a recommendation to wait until the other pertinent info becomes available to be able to do it.
 13 June 2018 04:00 PM
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LACarbon

Posts: 22
Joined: 15 March 2018

For working out cable size requirements for quoting works to be carried out, don't always get a full picture and all the information when putting a quote together.
 13 June 2018 04:14 PM
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mapj1

Posts: 11223
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For flats, you may like to assume the main fuse as an upper limit per flat, and taper towards a figure from the DNOs ADMD tables, when aggregating a great number of flats together.
Page 19 of UKPN's LV network planning guide suggests 1-2kW long term average depending if storage heaters or not.

-------------------------
regards Mike
 13 June 2018 04:17 PM
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psychicwarrior

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well if you want to take the risk without having clarity on design before quoting, you'd have to guess at some sort of typical maximum [current] demand after diversity for a flat and then use that to size a cable... and in doing such an assessment, consider if there is to be electric based heating in each flat (then it becomes interesting) or not. If not, the demand will be pretty low, so smaller csa could be used. There are other considerations that may affect chosen cable size too, such as if the cable is to provide main protective bonding. etc etc.
 13 June 2018 04:22 PM
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AJJewsbury

Posts: 17169
Joined: 13 August 2003

You presumably have some idea of maximum demand for each flat in order to size the submain cables & protective devices.

More of an issue I would have thought was how to divide up the allowable overall voltage drop allowance between your submains and each flat's circuits - e.g. to comply with the usual requirement of 3% for lighting circuits it might be decided to allow maximum 1% for the submain and 2% for the final circuits. If no-one else is willing to come up with some figures then it might be up to you to decide and then forward that as a design constraint to the designers of the individual flat's installations.

Alternatively if this is a BNO type of situation you might decide that your voltage drop is allowable as part of what was traditionally DNO's side of things - i.e. as long as the L-N loop impedance suggested no more than say 16% v.d. in the overall supply up to each flat then you're good.

- Andy.
 13 June 2018 04:32 PM
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LACarbon

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Thanks Mike, that is very helpful.

Personal experience tells me that there wont be a single flat that draws 60 amps at any time (unless something is being grown that shouldn't) but sometimes you have to go on worse case scenario. If drop down to 40 amps Ib on possible 85 metre runs it would be within the parameters with 16mm singles but the residents wouldn't be able to have any lights :-)
 13 June 2018 04:35 PM
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LACarbon

Posts: 22
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Also Mike,

if I have read that right, the average 1-2 bedroom property with gas central heating would be 5.2 amps?

Regards

Carbon
 13 June 2018 04:43 PM
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mapj1

Posts: 11223
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yes - this is how you can have 400A fuses at the substation, feeding 50-70 houses per phase, even though each house has a 100A or maybe 80A supply and main fuse.
But, beware, if you only have a small number of users, you cannot rely on this load averaging to the same degree - any one user may have the cooker on, and decide to take a quick shower while their dinner guests are warmed by an electric heater...
The way the DNOs use diversity is quite aggressive compared to the OSG approach for final circuits, but it is backed up by many years of it actually working. (and the occasional half hour 100% overload of the transformer is accepted, as it takes hours to heat all that metal up)

-------------------------
regards Mike
 13 June 2018 04:46 PM
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AJJewsbury

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if I have read that right, the average 1-2 bedroom property with gas central heating would be 5.2 amps?

Yes, but bare in mind that's after diversity across a large number of consumers - good for sizing substations, but not individual service cables.

I've seen a more complete description somewhere which takes account of the number of consumers - I'll see if I can find it....

- Andy.
 13 June 2018 04:53 PM
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AJJewsbury

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Some bedtime reading here: http://www.northernpowergrid.c...set/0/document/109.pdf (the ADMD bit starts around 3.4.2.1)
- Andy.
 13 June 2018 05:00 PM
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LACarbon

Posts: 22
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Thanks Mike, one of the blocks has 80 flats so its quite a large install.

I'm just happy to have documentation to justify the quote I put forward.

I must say, i have never put maximum demand so low on a condition report and I do remember an 'old timer' I use to work with simply said "add up all your breakers and divide by 4" very primitive i know
 13 June 2018 05:07 PM
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mapj1

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That Northern power doc has an elegant way of doing it with a power law fit.
4.6 *n^(-0.22)
where
n represents the number of customers supplied from the system assets being designed
Looks very nice on a log/lin plot. (just done it)
Wonder how much charging cars will require a change to these sort of calculations.

-------------------------
regards Mike
 13 June 2018 05:17 PM
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mapj1

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If each flat has a 60A fuse, (13kW max )then allowing 1/4 of it, 15a per flat, would be about right, in the sense that the old boys rule of thumb is more or less the same, for a small no of flats between 5 and 10 or so, if that Northern Power formula for small nos of users is to be trusted.
Obviously a line feeding one flat has to be sized for full load, but a line feeding a whole floor full can assume some diversity. However, the DNOs do cut it much finer than BS7671, so I'd be looking at rounding up rather than down.

You may be volt drop limited rather than current rating anyway, but you can still make similar diversity assumptions.

-------------------------
regards Mike
 13 June 2018 06:39 PM
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broadgage

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Joined: 07 August 2007

Originally posted by: LACarbon

Also Mike,
if I have read that right, the average 1-2 bedroom property with gas central heating would be 5.2 amps?
Regards
Carbon


Yes, but only as an average over a large number of homes, and also the result may not comply with the wiring regulations, remembering that DNOs work to different rules.
 14 June 2018 11:40 AM
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LACarbon

Posts: 22
Joined: 15 March 2018

Volt drop was my biggest issue just for the sheer length of some of the runs.

thanks guys, helpful as ever
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