Voltage drop Calculation 1

[Hakim Eng]

Hello guys,
i have to connect a generator of 1125 KVA of 400 V line to line 50 Hz frequency with an Automatic Transfer switch.
the ATS is located 15 meters away from the generator, the company that made the conception of the project implies that we have to use a cable of 12 x (1x300 ) + 4x (1x150) + 4x(1xG150).
the problem is that i have only three terminals on the generator's circuit breaker wich means i can only use three cable for each phase ( 900 mm2 cable section instead of 1200 mm2 )

- the whole power consumption is 869.1 Kw.

- how can i calculate the voltage drop in case of using only 9x(1x300 mm2 ) instead of 12x (1x300 mm2).

thank you in advance.

 

[cuky2000]

SUGGESTION: Sarch for Voltage Drop Calculator.
There are several application available many of them free.

 

[xnuke]

I don't see voltage drop as the issue. If you only have 3/4 the recommended area of the conductors, you're limited to 3/4 of the rated generator output.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RRaghunath]

I agree with xnuke. With only 15m distance, voltage drop is unlikely to be an issue. Did you check the cable sizing calculation done by the consultant?

 

[waross]

Step one: Calculate the current per phase.
Step two: Divide the current per phase by the number of phase conductors to find the current per conductor.
Step three: Multiply the current per conductor by the impedance of one conductor to find the voltage drop per conductor.
Step four: Multiply the voltage drop per conductor by root three (1.73) to find the voltage drop per phase.
Step five: Compare the current per conductor with the rated maximum allowable current of the conductor.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

Do you have to do the 1.25% thing here?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

For sizing but not for voltage drop.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

Okay thanks.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[7anoter4]

The voltage drop for 15 m it is not an issue since the impedance of 3 parallel conductors in trefoil of 300 mm^2 copper it is less than 0.001 ohm [approx.1.6 V voltage drop].You have to calculate the current carrying capacity [ampacity] of the system.
If you run the cables on a perforated cable tray [or ladder] according to IEC 60364-5-52 Table A52-21 the reduction factor will be 0.96- if you run the cables in trefoil and keep a distance of two cables from trefoil to trefoil. According to Table A52-12 for three loaded conductors in trefoil the maximum admissible current is 703 A for 300 mm^2 copper conductor XLPE insulated. So, 3 parallel cables could withstand 3*703*0.96=2024.64 A.
What do you need it is the generator rated current which is only 1125*1000/sqrt(3)/400=1623.8 A

 

[7anoter4]

According to NEC 445.13 Ampacity of Conductors. the cable ampacity has to be 1.15 the generator rated current.
Then, in USA you need 1.15*1623.8=1867.37 A

 

[itsmoked]

Good to know 7anoter4!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[7anoter4]

There is still here an issue.
According to BS 7671 rule 523.1 if generator terminals rated temperature it is not 90oC:
Where a conductor operates at a temperature exceeding 70oC it shall be ascertained that the equipment connected to the conductor is suitable for the resulting temperature at the connection.
So, in this case even the conductor maximum temperature of 90oC you have to follow the 70 oC table4D1A [561 A maximum].In this case the current carrying capacity of 3*300 A will be only 1615.7 A.