generator cable size query

[grahamb]

I am currently in India helping a friend to wire his house to IEE standards (UK).

The design current (before diversity) is approximately 170 amps across the three phases, with only one intermittent three-phase load of 5hp for the water purification system and pumps.

The real life load is likely to be less than 15 amps per phase (single phase loads) in total.

We ordered a 30kva 230v 3-phase generator (24kw) for standby purposes, which is only for the house supply, excluding water treatment, all pumps and A/C units.

The suppliers originally stated that we required a 100kva DG set for this small house, but we established that they were wrong ... the actual size that would suffice is 20kva, but we uprated to 30kva to provide overhead.

They now demand that we supply 3.5 core 50mm SWA for connection to and from the 30kva DG set, which appears to me to be completely wrong.

I know very little about generators and my research on the web has indicated that 16mm cross section is more than sufficient, however, we were going to supply 25mm four-core SWA anyway.

Some advice on the real cable requirement would be appreciated.

My thanks

Grahamb

 

[broadgage]

To correctly size the cable you need to know the maximum current per phase.
Given a 30KVA generator, and presuming a 3 phase 230/400 volt supply, this will be about 45 amps.
Select cable rated for at least this current.
16mm would be ample, and the 25 mm that you propose sounds plenty.
If the cable is very long, then it needs to be upsized to reduce voltage drop, however even the 16mm should be good for all but improbably long runs.

I would suggest 4 core cable, not 3.5 core, on account of the chance of one phase being fully loaded with no load on the others.
If however the phase cores are twice the size actually required, then no harm can result from 3.5 core cable.

If the generator is for standby service, then a suitable transfer switch is required to prevent any possibilty of connecting the generator to the utility service.
Brittish practice may be a good guide, but there may be additional local rules in addition.

 

[grahamb]

Re: Generator cable size query

Re: Generator cable size query

Thank you 'broadgage' for your prompt response. Will take your advice.

The maximum load on the genset will be no more than 25 amps per phase (single phase loads) and most of that intermittent loads, like kitchen equipment and water heaters. The majority of the continuous load is going to be lamps, fans and and some minor recreational equipment (TV, computers, etc.).

The A/C units and water system are on separate DB's supplied only from the EB incomer. We do have an emergency changeover switch incorporated in the design to permit the water purifier system to be run from the genset on the odd day that the power cuts here are more than the regular 3 hours, but that load is only 3hp three-phase. So the entire potential load is well within the 45 amps capability of the genset.