Volt Drop Calculation

shane100580

Hi Folks,
I've to run a cable 300 meters in an under ground duct & wondered if it's a 5*25sq SWA or a 4*35sq SWA plus earth that I should run. I want to have a 3 Phase 63Amp switch fuse in the shed that the cable will be feeding.
Any guidance would be appreciated..
Thanks

Risteard81

It is not possible to calculate volt drop without knowing what the design current is.

shane100580

Hi & thanks for your comment, when you talk about "design current" what are you referring to?
The cable is being fed from a a main fuse board with 200Amp capacity 3 phase, the destination of the SWA is a shed that requires no more than 63 Amp 3 phase power for lights, sockets & possibly a couple of 3 phase machines but total load will not exceed 63 Amps 3 phase...

kramer1

Gonna be very big cable for a 300m run at 63a, did a quick Internet cable size test, it reckons you ll need 95 square

Joe1919

My quick calculation is that,Using 35 sq, 5 core
21 amps/phase at 3 phase balanced will loose approx 3 volts on the phases with no loss on the neutral (as no current flowing if balanced.)
If the whole 63 amps is on one phase, then loss will be 9 volts + 9 volts on neutral = 18 volts.
The 25 sq will be 40% higher

http://www.bulkwire.com/wireresistance.asp

kramer1

Joe1919 wrote: »

My quick calculation is that,Using 35 sq, 5 core
21 amps/phase at 3 phase balanced will loose approx 3 volts on the phases with no loss on the neutral (as no current flowing if balanced.)
If the whole 63 amps is on one phase, then loss will be 9 volts + 9 volts on neutral = 18 volts.
The 25 sq will be 40% higher

http://www.bulkwire.com/wireresistance.asp

My mistake, didn't see it was three phase

shane100580

Thanks for the comments folks, so by your estimation will the 5*25sq be ok over 300 meters to deliver/handle a maximum load 63Amps per phase?

BrianDug

Its impossible to answer as Risteard81 said.

If you can post a picture of the motor name plates (on the side of the motor) it will be easier for people to answer your question properly.

Without knowing the design current (load current) you could just base it off 63A which will be worse case.

I had a quick look at a online calculator for SWA cable, 400V, 63A load current at 0.95 power factor, 4% max voltage, cable size = 50mmsq, calculated volt drop is not far from 4%

If load current is 50A, 35mmsq will suffice but again the calculated volt drop is not far from 4%.

Regards,

BD

BrianDug

Joe1919 wrote: »

My quick calculation is that,Using 35 sq, 5 core
21 amps/phase at 3 phase balanced will loose approx 3 volts on the phases with no loss on the neutral (as no current flowing if balanced.)
If the whole 63 amps is on one phase, then loss will be 9 volts + 9 volts on neutral = 18 volts.
The 25 sq will be 40% higher

http://www.bulkwire.com/wireresistance.asp

Hey Joe, if my interpretation is correct when the OP said 63 A Three phase, I'm sure he was thinking 63 A per phase and not 63 A / 3 cores = 21 A per phase.\

Edit - I see you are taking maximum unbalance into consideration but is it likely?

Joe1919

shane100580 wrote: »

Thanks for the comments folks, so by your estimation will the 5*25sq be ok over 300 meters to deliver/handle a maximum load 63Amps per phase?

BrianDug wrote: »

Hey Joe, if my interpretation is correct when the OP said 63 A Three phase, I'm sure he was thinking 63 A per phase and not 63 A / 3 cores = 21 A per phase.\

Edit - I see you are taking maximum unbalance into consideration but is it likely?

By my calculation, 63 amp per phase (on three phase perfectly balanced) with the 25 sq will result in voltage drop of 14 volts on the phases.(Any unbalance will result in some drop on the neutral, which could also affect other single phase equipment). I note that the link suggests 42 sq to give less than 4% drop.

I suppose, as other have mentioned, it depends on the load. For example, some refrigeration/air conditioning compressors & induction motors have huge start up currents in excess of their normal run currents etc. (and can be damaged by low voltages). Also note, an inductive motor will often compensate for lower voltage by increasing its current. Its also the case that some motors purchased from Britain are possibly designed for a slightly higher (240/415) volts, which could make things worst.

Also as noted, power factor has to be taken into account (e.g extra 20% at pf=.8) for inductive loads if not corrected.

So (imo), much depends on what the load is. (its tolerance & necessity for reliability)For example, if you intend using a cold store for valuable foods etc, you would be wise to consult the refrigeration spec. and go for higher spec. On the other hand, if you intend buying say some type of secondhand saw equipment and not work it too hard, then I suppose you could go for a lower spec. if this is safe.

Also I presume you will need to make sure ESB can supply the power you need without additional voltage drops on the mains side etc.

2011

A few points to note:

● The OP is taking this supply from the "main board". There will be a volt drop to this point (in the cable supplying the main board) that needs to be taken into account.

● The volts drop must be calculated to the furthest point on the worst case final circuit (fed from the sub distribution board). In other words the volt drop calculation is only partly made up by the 300m cable supplying the sub distribution board.

● The calculation should not be based on a perfectly balanced load unless that is what it is.

● It must be determined that the final installation has sufficiently low earth fault loop impedance to ensure that the required disconnection times are met.