Can you work this out

[lofty84]

what is the maximum permitted voltage drop of a cable supplied from 400v three phase.

a.11.5v
b.6.9v
c.20v
d.12v

I know the answers c but I dont know what the formula is to get there.

does anyone know

 

[sparky1502]

i am looking forward to the answer on this as it was a question on my mock exam the other day

 

[Guest123]

Max permissable volt drop is the same percentage for single and three phase i.e, 5% for power circuits and 3% for lighting.

So 5% of 400V is 20V.

 

[lofty84]

a circuit wired in 1.5mm 2 thermoplastic (pvc) twin with cpc cable is protected by a 16A device and is used to supply a 230 V 3 KW water heater. If the cable has a rated voltage drop of 29 mv/A/m and the circuit is 24 m long, the actual voltage drop will be


a. 2.08v
b. 9.07v
c. 11.14v
d. 69v

can someone help me to work this out

 

[beaman]

hi iwas taught to work out vd to use this formula;
vd=Ib x mv x L / 1000
so my answer would be C

16 x 29 x 24 / 1000 = 11.136
hope this is of help

 

[Sintra]

3000 W / 230V = 13.04A

29 x 13.04 x 24 / 1000 = 9.07V

 

[beaman]

i messed that one up didnt i? speed reading again, i was told to stop that when i did my 17 th last week, obviously not got out of the habit.

 

[Guest123]

hi iwas taught to work out vd to use this formula;
vd=Ib x mv x L / 1000
so my answer would be C

16 x 29 x 24 / 1000 = 11.136
hope this is of help

Your theory is sound but you have used (In) in your calc i.e the rating of the breaker instead of the actual circuit (Ib) of 13.04A.

 

[lofty84]

thanks guys very nice of you.

 

[m25maintenance]

Max permissable volt drop is the same percentage for single and three phase i.e, 5% for power circuits and 3% for lighting.

So 5% of 400V is 20V.

5 % of 400 is 20 but each line is 240 so its 12 volts

 

[pevvers]

but each line is 240

Not in BS7671:2008 it's not. Line to earth supply voltage is 230 nowadays...

 

[m25maintenance]

Not in BS7671:2008 it's not. Line to earth supply voltage is 230 nowadays...

stick ur meter on it and see what u get

 

[pevvers]

stick ur meter on it and see what u get

Yup I know, anything up to about 256V.

Doesn't matter a jot I am afraid, as calculations are done to the the nominal voltage of 230V, as one day (although they've been saying this for donkeys years) they are going to drop the voltage to the harmonised 230V.

Why do you think all the max Zs values have all dropped in the 17th? Because the recognised voltage is now 230V instead of 240V.

 

[johnboy6083]

i got 251v last night!

 

[pevvers]

Just a quick bit of info I've just found to illustrate my point.

"The nominal European voltage is now 230V 50 Hz (formerly 240V in UK, 220V in the rest of Europe) but this does not mean there has been a real change in the supply.

Instead, the new "harmonised voltage limits" in Europe are now:
230V -10% +6% (i.e. 207.0 V-243.8 V)

in most of Europe (the former 220V nominal countries), and
230V -6% +10% (i.e. 216.2 V - 253.0 V)
in UK (former 240V nominal)

This is really a fudge and means there is no real change of supply voltage, only a change in the "label", with no incentive for electricity supply companies to actually change the supply voltage.

To cope with both sets of limits an equipment will therefore need to cover 230V +/-10% i.e. 207-253V. This will actually become the official limit for the whole of the EU in 2003."

From: - European Voltage Harmonisation

 

[Notsosmart]

Thus nominal voltage for 3 phase which used to be 415V is now 400V

5 % of 400 is 20 but each line is 240 so its 12 volts

(230v) Yes but the implication is that the load is distributed across three phases so the 20Volt stands.

 

[Guest123]

The OP reads "a cable supplied form 400V three phase" i.e a three phase load so I stand by my previous post.


Also 12V is incorrect anyway as ALL calculations are based on the nominal Uo of 230V = 11.5V.

 

[Risteard]

Yup I know, anything up to about 256V.

It shouldn't exceed 253V. (230V +10%/-6%)

 

[pevvers]

It shouldn't exceed 253V. (230V +10%/-6%)

Yeah, but it was just a voltage from the top of my head, and I have seen it that high.

 

[Risteard]

Yeah, but it was just a voltage from the top of my head, and I have seen it that high.

Not disputing that you encountered it - just pointing out that they shouldn't be transmitting that voltage!