IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Volt Drop
Topic Summary: Section Board
Created On: 14 May 2013 04:14 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 14 May 2013 04:14 PM
User is offline View Users Profile Print this message



mrcornbeef

Posts: 245
Joined: 05 December 2005

Hi
After advice on working out volt drop for a new 200 amp section board which is to be used for various equipment and lighting via DB,s , what is a sensible percentage volt drop at this new 3 phase section board i don,t know the loads at this stage as it is for future use ,it is a 100 metres from the main panel board which has substation just behind it.
Thanks
 14 May 2013 04:52 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 18919
Joined: 23 March 2004

Approx 0.5% for the supply to the distributor board, say 1.5 to 2.5% for the supply to the new section board and approx 5% total for the final circuits.

You may not know the individual loads, but you can approximate for the section boad feeder from a w/m2 power density taken from an existing part of the plant if need be - or just use the fuse size as Ib

Regards

OMS

-------------------------
Failure is always an option
 14 May 2013 04:53 PM
User is offline View Users Profile Print this message



broadgage

Posts: 1244
Joined: 07 August 2007

Depends on what the customer is willing to pay for.
It could be argued that as little as 1% voltage drop should be allowed, if it is to feed DBs.
1% on cable from substation to section board
1% on sub mains from section board to DBs
1% on final subcircuits.
Would add up to the often recomended figure of 3% for lighting.

On the other hand it could be argued that this gives rise to unduly large cables that are not only expensive to buy but that might not fit standard equipment.

It might be a reasonable assumption that the voltage at the substation will be (or can be set) at close to the upper limit, say 250 volts and that this gives far more leeway.
After all, if this was a street of houses, then the cables in the street would belong to the DNO and no one would worry about voltage drop therein.

Consider a DNO substation with 250 volts at the transformer and 15 volts drop to the far end of a street main, and another 10 volts drop in a long service cable. That is about 10% voltage drop before we even consider the consumers installation.
No one worries about that, yet if it was all privately owned, some would argue that the voltage drop was excesive.

IMHO the suggested figures for voltage drop of 3% for lighting and 5% for power are mainly applicable to domestic or similar size installations when the voltage at the meter could be as low as 217 volts.

In the case of very large installations it would in my view be reasonable to at least partialy follow DNO practice.
Start with at 250 volts at the transformer
Allow for a worst case drop of 5% in the street mains (or that part of the consumers installation that most closely resembles DNO street mains)
Allow for a worst case voltage drop of another 5% in service cables (or that part of the consumers installation that most closely resembles service cables)
Then allow another about 3% drop on final subcircuits, in line with domestic practice.
That would give a worst case total voltage drop of 13% and a lowest voltage at the point of use of 217 volts, which is still a bit better than a house with 217 volts at the meter and perhaps 207 at the far end of the worst subcircuit.
And remember that this is under worst case conditions at the far end of the most distant subcircuit, the average across a large site or facility would be more favourable.
 14 May 2013 05:32 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4479
Joined: 07 April 2004

Originally posted by: mrcornbeef
Hi
what is a sensible percentage volt drop at this new 3 phase section board i don,t know the loads at this stage as it is for future use ,it is a 100 metres from the main panel board which has substation just behind it.

For unknown loads say 1% which may also allow some spare capacity, but get agreement with the client for CYA time.

Regards
 14 May 2013 05:36 PM
User is offline View Users Profile Print this message


Avatar for Parsley.
Parsley

Posts: 884
Joined: 04 November 2004

The IET's electrical design guide is worth a look.

Basically large installations that are fed from their own TX can use the values in ESQCR i.e +10% = 253V at the origin instead of 230V.

Regards
 14 May 2013 07:18 PM
User is offline View Users Profile Print this message



mrcornbeef

Posts: 245
Joined: 05 December 2005

Thanks for advice gents much appreciated
 15 May 2013 10:44 AM
User is offline View Users Profile Print this message



burn

Posts: 115
Joined: 06 June 2003

No matter what the various regs & guides say, it has to work.

If it is an old substation transformer you may well have 250v, but if it is new you are more likely to get 230v.
Suggest you measure it or assume the lower.

burn
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.