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Topic Title: Volt drop
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Created On: 30 November 2012 12:31 PM
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 30 November 2012 12:31 PM
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marclambert

Posts: 312
Joined: 23 June 2010

I wonder if someone could help me with this question.
When would we actually use 400v as the nominal voltage when calculating volt drop. I've always tended to use 230v as the nominal voltage no matter what the question. Probably wrong, but can anyone give me a practical example I can use to illustrate the point.
Thanks in advance
Marc
 30 November 2012 12:43 PM
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OMS

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Well, for any unbalanced load I would suggest Marc

I'm not sure I understand the question though ?

Regards

OMS

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Failure is always an option
 30 November 2012 01:17 PM
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marclambert

Posts: 312
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Sorry, I'll try to be clearer. When calculating vd we use the nominal voltage.
What circuits would have a nominal voltage of 400v?
Would it only be 3 wire (eg motors)?
Or is nominal voltage the voltage to earth?
In which case is always 230v.
I'd like an example of where we would use 5% of 400v as opposed to 5% of 230v if possible.
Thanks for your reply OMS, I'm sure you can help if I make myself understood
 30 November 2012 03:13 PM
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Westonelectrical

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Nominal voltage is line to earth, so always 230v... The 5% would be for motors, commercial ovens etc..
 30 November 2012 03:16 PM
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Westonelectrical

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Sorry the 5% of 400 would be for motors etc***
 30 November 2012 03:50 PM
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OMS

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What circuits would have a nominal voltage of 400v?


Two phase loads, three phase 3 wire delta connected loads.

Are you missing the fact that BS 7671 gives us mV/Am for 2 cores at single phase and the vector additive balanced load for 3 phase- with or without neutral.

Forget the published data for a moment, volt drop is basically ohms law ie V = I x R

So, if you know the resistance per meter of a core the voltage drop (ignoring reactance for now) is:

For single phase

V = I x R

Then multiply by 2 to account for the line conductor and the neutral conductor

For 3 phase

V= I x R (ie the same)

Then multiply by Root 3 to get the line to line drop

In a balanced three-phase system the current in the neutral is zero. If it is unbalanced, the current in the neutral will be the phasor sum of the phases. The total cable drop for any phase would be the algebraic sum of the voltage drop in the line plus the voltage drop in the neutral (keeping in mind this is vector addition).

As an exercise take a 9kW load (ignore PF, efficiency etc for now and select say a 4.0mm cable 30m long - use the single phase volt drop value divided by two to determine R

Supply it first as a single phase load and then as a 3 phase load - compare the answers and look for the relationship between them

Regards

OMS

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Failure is always an option
 30 November 2012 04:08 PM
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marclambert

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Thanks both, given me something to work with there.
Regards
Marc
It's Friday and it's beer o'clock
 30 November 2012 04:24 PM
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OMS

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It's Friday and it's beer o'clock


and you bloody educationalists moan about your working conditions -

I got back to the hotel at 05.00 this morning, was back behind my desk at 10.00, won't finish tonight until at least 11.00 and back in tomorrow by 07.00 with some vague hope of getting from London to home "sometime" tomorrow evening if I'm lucky.

Beer o'clock indeed - aren't you supposed to be working our way out of reccesssion -

Blow the froth off a cold one for me Marc

Regards

OMS

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Failure is always an option
 30 November 2012 04:30 PM
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mawry

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You'll miss the rugby!! That's Commitment!;-)
 30 November 2012 04:31 PM
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rocknroll

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The only difference between the two cables is the constant 1.73 for 3 phase cable and 2 for single phase cable not a lot of difference between the two.

Vd = (2R x L x I) / csa (for single phase)

Vd = (1.73R x L x I) / csa (for three phase)

R = Resistance per metre

L = The length of one conductor

I = Current

csa = csa of conductor

regards

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"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
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"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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 30 November 2012 04:41 PM
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OMS

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Originally posted by: mawry

You'll miss the rugby!! That's Commitment!;-)


When there's a cliff edge deadline and a seven figure sum at stake watching the game will be a long way down the "To Do" list I'm afraid.

The trouble is I'm not sticking to my "To Don't" list

Regards

OMS

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Failure is always an option
 30 November 2012 05:23 PM
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mawry

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ahhh the old monkey list! Things that stop you doing what you should be!

Note to self: Must stop reading management bumf!!

"enjoy" your weekend!
 30 November 2012 05:33 PM
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OMS

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LoL - Tidy darts, butt

OMS

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Failure is always an option
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