HT Cable Calculation

No! 185mm for the primary might be closer.

You are not sizing for 100A on voltdrop. The cable has to be able to withstand mechanical forces in the event of a short and these are massive. I take it you want to use the cable more than once?

Cable size will greatly influenced by the protection and you make no mention of this.

Suggest you get help before making a massive mistake.

The few important factors to be considered while selecting size of HT cables are:

load current, fault current, fault clearing time and allowable voltage drop.

For small size transformer and motor, load current and allowable voltage drop are not critical issues and more predominant factors will be fault current and fault clearing time. The cable has to withstand the effect of short circuit current ( both thermal and electromagnetic) till the fault is cleared by the SCPD i.e. fuse or circuit breaker. We may consider fault clearing time typically as 10 msec (half cycle for 50 Hz system) for fuse and 200 msec for circuit breaker ( considering adequate margin for relay sensing, relay operation and circuit breaker tripping time). The cable size should not be less than the value of A which is given by the formula:

A = (Isc x √ t)/ K

where Isc = value of short circuit current

t = fault clearing time

K = 0.094 for aluminium cable

Then for 13 AMP we can use the 16mm2

With no de-rating factors.

At the transformers FLC the volt drop on the MV side would be 0.19%. If we take 150A as being on the longest LV cable the cumulative volt drop (MV+LV) becomes 0.47%.

I think it will do. But as Peter has said there's more to it than that.

Tony

I am using 150 amps with 250 kcm which is very close to 120 mm sq and getting about 6% just on the secondary. Why not move the transformer closer to the 100 and 150 amp loads?

Yes you're right. I've just found a bit of a scew up in a cable calculation spreadsheet I put together.

Just gone through it again and come up with 7.1% cumulative volt drop. So 120mm isn't good enough, even at 185mm the VD is 4.7%. Something is going to have to be moved

God knows what I was playing at the first time

Then the HT Cable length will become more which is costly.if you have any conflict with my answer please reply

Cable sizing tool.

Works real well.

These are tiny loads for 250kVA trafo??

600m LV cable? If you have to then sure. I put your figures through that tool and 120mm armoured Al is OK with 7.85% voltage drop.

Wal

I used 150 amps and 600 meters 4% drop. It showed 2-150 mm Cu parallel.

Using AL conductor 8% VD gave 250 mm.

Tony, Wareagle...exactly why we have peer review!!!

I used 100A loading before. Don't ask why.

Faffed around some more and it really depends on what voltage drop the OP can tolerate.

What has OP decided?

Wal

No peer review intended. I was questioning my use of the calculator. I thought I might had used it incorrectly. However, I will question the use of 100 amps instead of the 150 amps.

Senior moment.....

In instances such as these volt drop on the HV cable is not an issue. So you lose 50V on an 11kV cable - so what. You size the cable to stand the fault available from upstream supply. The 185mm² I threw in was not sized on volt drop. Look to the tables/graphs. Your time trip comes from your HV protection settings.

You tap the transformers if things are a little lower than you would expect at the LV end.

LV side - ok now its time to get your cable calculators out. You should now be sizing on volt drop, ability to safely carry the load and of such an impedance that Zs is lower than Zs for your protective device.

Why you not used 16 mm2 for the current.

Ar