VOLTAGE DROP CALCULATIONS

The voltage drop of any insulated cable is dependent upon

1. the route length under consideration (in meters),
2. the required current rating (in amperes) and
3. the relevant total impedance per unit length of the cable.

The maximum impedance and voltage drop applicable to each cable at maximum conductor temperature and under a.c. conditions is given in the tables. For cables operating under dc conditions, the appropriate voltage drops may be calculated using the formula:

2 x route length x current x resistance x 10¯³.

The values detailed in the tables are given in m/V/Am, (volts/100 per ampere per metre), and the nominal maximum acceptable volt drop specified by the IEE Regulations is 2.5% of the system voltage, i.e. 0.025 x 415= 10.5 volts for 3 phase working or 0.025 x 240 = 6.0 volts for single phase working.

Consider a 3 phase system. The requirement may be for a load of 1000A to be transmitted over a route length of 150m, the cable to be clipped to the wall and close protection provided. The rating tables in the IEE Regulations indicate that a 35mm copper conductor PVC/ SWA/ PVC cable would be suitable for the loading required, but the voltage drop must be checked.

Volt drop = Y x current x length
= 1.1 x 100 x 150 millivolts
= 1.1 x 100 x 150 volts/1000
= 16.5 volts
where Y = value from tables in mV/A/m Unless a particular value of voltage drop, acceptable to the user, is specified, the IEE Regulations figure of 10.5 volts must be adhered to.

Thus:
Total volt drop = 10.5 volts = Y x 100 x 150
Therefore Y = 10.5/100 x 150= 0.7/1000 volts/ampere/meters

Reference to the voltage drop tables indicates that the cable size with a voltage drop of 0.7/1000 V/A/m(0.7mV/A/m) OR LESS is a 70mm copper conductor.

Therefore, in order to transmit a 3 phase current of 100A per phase over a route length of 150m, with a total voltage drop equal to or less than the statutory maximum 10.5 volts, the use would require a 70mm (cu.) multicore PVC.

Conversely, The user may have 150m of 35mm (Cu.) multicore PVC cable and require to know what maximum current rating can be applied without exceeding the allowable voltage drop. The method is exactly the same as above,viz: total drop = 16.6

= YxAxM
= 1.1 x A x 150/1000from the tables Y
= 1.1mV/A/m=1.1/1000V/A/m
therefore
A = 10.5 x 1000/1.1.x 150
=64 amperes

From the foregoing, it is apparent that knowing any two values of Y, A or m, the remaining, unknown value canreadily be calculated.

The advice is always available to check, clarify or suggest the most suitable size and type of cable for any particular, specified requirements.

Useful three phase formulae:
1. kW = kVA x power factor

2. kW =
Line amps x Line volts x 1.73 x p.f.
1000

3. kVA =
kW
p.f.

4. Line amps =
kW x 1000
Line volts x 1.73 x p.f.

5. Line amps =
kVA x 1000
Line volts x 1.73

6. Line amps =
h.p. x 746
Line volts x 1.73 x Efficiency x p.f.

7. kVA =
Line amps x Line volts x 1.73
1000

8. kW =
h.p. x 746
1000 x Efficiency

9. kVA =
Line amps x Line volts x 1.73 x Efficiency x p.f.
746

10. h.p. =
kW x 1000 x Efficiency
746

11. h.p. =
kVA x 1000 x Efficiency
746

Resourced from : http://www.csedistributors.co.uk/cable/voltage-drop-pf.htm

ELECTRICAL ENGINEER IN A GLANCE...

1. Being an electrical engineer for over 8 years now, it tells me that working as a technical person is very much indeed demanding.

2. You have to be the subject matter expert or else other people will refer you as an office engineer, carrying the title without any mean.

3. The easiest and fastest way to earn your knowledge is to get down to earth, do it, log it and you'd remember. In fact skills are obtained only from doing it.

4. You need to find a good colleague and mentor in order to counter check your knowledge levels.

5. Apart from that, Engineers must not neglect the economic aspect of the business. You must know what the operations are doing and how is the overall performance of the company.

6. Ability to present and vocalized your opinion are equally important. Remain in silence will show incompetence and you may be regarded as less-knowledgable engineer.

7. When problems arised with regard to electrical, you are the brain to solve it. Technicians will do whatever instructed & recommended with responsibilities mainly on the engineer's shoulder.

8. Most electrical engineers are not competent in doing electrical switching and hence requires the presence and authorisation from a competent person.

9. To further enhance our credibility, we need to either continue our study to the Masters level or pursue the Professional & Competent Engineer status from the regulating body. (ie BEM and IEM for Malaysia)

10. All in all, most local Engineers are quite underpaid unless you are working with a well known overseas project or consultant companies.

11. Being a good Engineer is one thing, but appearing good is more important. Having both is the best.

12. Since technology evolves without stop, technical competency needs to be continually improve from time to time. Learning process is endless indeed.

13. The best way to retain your knowledge is to do it, apart from continuous reading and giving trainings to the younger engineers.

14. If you are an electrical engineering student, you made a good decision as electrical offers plenty of job opportunities around the globe.

15. Once you work, have a vision and complete all the missions. Sometimes you just have to go with the flow.

In summary:

I do believe that financial freedom is still the ultimate vision. Whatever job that I am doing now is simply a platform to bring me closer to that vision. Perhaps I could retire by 35 (if not 40).