32A single phase output from 3 x 16A three phase supply. Is there a way?

[pc1966]

A quick check of the cable resistance values (Prysmain BS5467 data sheet) has R1 for 16mm as 1.15 mOhm/m and R2 as 3.1 mOhm/m, paralleling the conductors leads to R1+R2 of about 3.68 mOhm/m so for the 185m run around 0.68 Ohms.

For a 16A MCB that is usable to D-curve.

But for a 32A MCB that already (even before looking at the supply DB Zs) puts the supply MCB limit at a B-curve, and that might be too trip-happy on the machine's start up current.

So you might be looking at a 100mA delay RCD to meet sub-main disconnection times (unless you look at a 32A or 40A switched-fuse, etc)

 

[Mike Johnson]

We seem to have lost our way on this thread, the OP wants 32amps single phase from a 16amp/P three phase supply, or is it me that's lost the plot?

 

[pc1966]

We seem to have lost our way on this thread, the OP wants 32amps single phase from a 16amp/P three phase supply, or is it me that's lost the plot?

Well, the issue was feeding a 32A single-phase load from the end of a long 3-phase cable that only meets VD, etc, for 16A.

• Converting 3P => 1P via transformer(s) of VFD is one route, but not cheap,
• Reconfiguring 4C 16mm to avoid a new sub-main is another possibility, but loss of 3P options
• Or the simple, obvious, but potentially expensive iv civil work needed to put in a dedicated 1P 31A feed.
• Possible, but maybe not feasible for cost, difficultly & warranty reasons, etc, would be to convert the load to use a 3P motor, etc.

Any other options?

 

[Mike Johnson]

Post 10?

 

[pc1966]

Post 10?

Was first in list, 3P->1P conversion.

I did look at the site, but it sucks in terms of finding stuff. They did list a 1P -> 1P VFD but I did not see 3P -> 1P mentioned. Do you know of a part?

I know some "3 phase" UPS are really 3P charger, 1P inverter (you can tell as they tell you on bypass the L1 needs 63A instead of the 25A MCB for the L2/L3), but they are hardly a cheep option.

 

[Mike Johnson]

When I bought from Invertek the site was much easier to navigate they seem to have retro graded the site somewhat, used to have a list down the left side of all the drives they had available, I can't find a link to even the one I have let alone a 3P->1P conversion.

 

[Mark42]

Hi, OP here - sorry for the disappearing act, I've been on 16 Hr days building even more sheds for an upcoming job. All great ideas, will digest and respond over the weekend. Thanks all.

 

[Mark42]

Thanks again for all the advice.

The machine is some kind of funky super-powerful x-ray machine for scanning vehicles. I don't have the full specs but I have to assume it does need constant current, not just inrush. However, I doubt it would be energised and generating radiation for very long periods.

The rental is only for a couple of weeks for a research project (that’s what I offer here), so there’s no question of digging trenches or otherwise messing with the direct-buried underground infrastructure.

The solution is so obvious I missed it! As several on here said, just temporarily swap a few cables and breakers at each end to single-phase the DB on 2 x 16mm paralleled conductors.

Physically lock off any three-phase breakers in the building’s DB (supplying only a few 5P sockets, and two small roller shutters which can be opened manually.)

Sorted in an hour at zero expense and at least another hundred quid a day on the rental for bespoke 'supply upgrade' ?

 

[Lucien Nunes]

Completely OT but curiously relevant. After WW2 there was an upsurge in cases of TB and the (then new) NHS launched a 'mass radiography' programme, sending dozens of mobile chest X-Ray screening vans out to hotspots (e.g. industrial towns) to locate cases before people developed symptoms. The X-Ray machines needed a 30A single-phase supply for which they re-purposed diesel generators from redundant Type 14 radar sets. These were actually 3-phase generators but rated to deliver full load on any one phase. They decommissioned the 3-phase outlets and partially rewired the control panels, and off they went around Britain.