Cables and Volt Drop

What's your allowable volt drop over the 650m cable run? Which country is this in?

This is a cool little shareware program that has a voltage drop calculator in it.

http://www.lmphotonics.com/busbarexe.htm

this is in the uk, I looked in the electricians guide to the iee regs and found volt drop maximum should not be more than 4%, or 9.6volts per phase so i worked back from that.

My table only goes up to 600m but it looks like about 185mm2 minimum for that cable run and volt drop. I don't think it would be cheaper to use a transformer and run a higher voltage out to your equipment. That is a serious distance for that current at 400V.

I think this is a matter of economics, power loss and voltage variabilty at the end of a long cable may cause some financial loss but for some applications may be of little concequence.

For instence if you are powering a saw bench or air compressor at the end of an inadequate cable there may be some loss of perfomance but it may well be tolerable, if you are powering a radio transmitter it is a matter of the design of the equiptment to work at a slightly lower voltage albeit at a higher current to mainttain the same power, no big problem.

thanks for your help guys, just had a price for 185mm 4c cable @ 37.50p/m so i suspect we will end up with a generator! my question was really down to lack of faith in my electrical knowledge (not an electrician), i was expecting replies of 'no you dont do it like that' etc, I also thought i had way overcooked the size of cable required by not understanding volt drop calculations.

You might investigate having your power company install a line to that location - they'll do the appropriate thing (transformer, or whatever) and you'll have good power there.

This was actually a good question, I was working on industrial plant electrical design and every time we are calculating based on the allowable voltage drop based on the code book, but our system voltage is 600V and the motor is rated for 575V so right there we have 25V extra, plus almost all motor manufacturers allow 10%V tolerance still guaranting a 100% performance, means if the voltage is above 520V on the motor terminals it is okay. So from supply voltage total we will have a 14% tolerance. Then why we restrict ourselves with 3% to 5% tolerance? except the economic aspect of it?

Because that is what the local standards say.

I want to see the logic for standards.

You say 63A is that shared between phases or each phase? Is it continuous current (not dc) or intermitent loading. What is your load? Motor or small furnace? Could you put in busbars? Will the cable be surface mounted or underground? 63A is not a very big load if split across phases even at 650m a 25mm underground cable would suffice.

16mm solid copper busbar will give 1000A load.

No professional is going to run 650m of busbar for this type of application. Ignoring the price (have you seen the cost of copper lately) it is not practical.

25mm2 3 phase cable is just not acceptable due to the volt drop.

What is the cross sectional area of your 16mm (wide) busbar with a 1000A rating. It is certainly not 16mm2 (unless its magic).

Still you need help and give an e-mail address, you may use a formula that I am using all my projects. I wanted to attach this message, unfortunately, I could not.

You will enter the values as distance, load and cable cross section in green cells, than you will see the voltage drop percentage in red cell. By changing the cable cross section you may find allowable voltage drop. That cross section is need to be used.

The machine to be installed is a waste recycling station consisting of 6 electric motors, all motors will be running together but start sequentially, 2 of the motors have soft starts, full load current taken from the motor plates added together is 65amps, none of the motors are likely to run at full load but the manufacturer is advising a 63a supply (understandbly not wanting to be responsible for an expensive mistake) if your spending serious money putting in a supply i think its probably best to allow for near enough full load. its my understanding that motor plates show the full load current per phase.

once again thanks for the comments. - cass

UL = 400V, I = 63A, L=630m

Ud = 400 x 4%

Ud = I x R

Ud = 63 x R

Ud = UL x 4%

R = (L x rho) / a

UL x 0,04 = I x (L x rho) / a

a x UL x 0,04 = I x L x rho

a = I x (L x rho) / UL x 4%

a = 63 x 630 x 0,0175 / 400 x 0,04

a = 63 x 11,O25 / 16

a = 43 mm² > in reality : 50 mm²

you need a cable of 4 x 50 mm² to run your motor(s)

Ud = voltage drop
Ul = line voltage (400V)
I = current
R = total resistance of cable
L = lenght cable
rho = resistance per meter per square milimeter of copper wire (0,0175)
a = diameter (in mm²) of cable.

right ?

paulvandenbossche@monti.be

A) Please double check the supply votage. Regulations apply to the supply company in that they must maintain a voltage within strict limits but you could be at the tail 'end' of their mains cable, so you might find your voltage already low. It would a good idea to measure the actual voltage as well.

B) The current rating on the motor plate means a total of 65 amps will flow (in each phase) when all six motors are under full load (60hp?). However, full-load is governed by the machine and the job you are actually doing. Usually the two are matched but it is worth checking. Get the machine manufacturer to tell you the current (of each machine) and add these together. That will give you the total working current the cable must carry. Also try to find out the lowest permissible voltage for your machines in your working conditions.

C) You now need to know the cable resistance. This will be given in Ohms per unit length. (you convert it to metres).

From supply voltage of A) minus the min motor voltage of B) you have your max permissible volts drop.

Volts drop is a simple calculation. You multiply the Amps of B) x Ohms of C) - the length cable.

From which you can select the right size cable(s) - ie, a choice - from whcih you take your pick.

Regulations for installations are mainly concerned with 'safety'. Thus cable ratings are usually based on heat dissipation. For your situation you should be nowhere near the max current rating.

You are right not to undersize things. The cost of rectifying a mistake would be out of all proportion to the cost of doing it right in the first place. If the cable turns out to be too large, you have little choice but the look at alternatives mentioned elsewhere. Transformers, generators, etc.

Just a sanity check for you:

Using Canadian standards, 5% drop for your distance we would suggest minimum 4/0 cable (206.4mm^2).

I would be sure the supply transformer is tapped high.

I believe you have it right, 240mm^2 would probably be closest metric size.

I am assuming this is a permanent installation and will be used for some years.

First as already suggested, consult your power company. They may tackle the job for you, again as previously stated. Also, since the power consumption is not trivial is your exisiting incoming service adequate?

Otherwise the best solution would involve using a properly sized step down transformer at the location of your equipment, or additionally combined with a step up transformer at the service entrance end, depending on the nature of your service entrance: incoming voltage, whether a step down transformer was already present to drop a higher incoming voltage down to 400 V, etc. This will allow for much smaller conductors.

Generally, large conductors are only economic for short distances and you are only talking about the motor needs- you will likely need additonal power as well at lower voltages for lights, and various other equipment, both temporary and permanent so a 100 amp feed would not be unreasonable. While motors will tolerate lower voltages, they are less efficient and remember you pay at the meter, so any power wasted by a voltage drop between the meter and the load only serves to heat the wires and is therefore wasted money, so you want to minimize losses consistent with the overall economics of the project over time.

Finally, depending on what your power provider can offer, consult with and engage properly licensed electrical contractors for detailed quotes. Then if you have to, or decide to, do all or part of it yourself, make sure local electrical codes are followed for safety and liability reasons.

Other than safety and liablility, it boils down to the economics so you have to run through the numbers.

Good luck.

Following my previous comment and those of others, it looks as though volt drop is only part of your problem. The logistics of installing a cable over your distance in itself could be a major task. Will it be buried, laid on the surface or suspended.

This is a problem that falls in the lap of the supply people. Get quotes from them.

Then compare prices for a local generator.