Two three phase machines on one circuit

It's nonsense!

I don't know of any current reg it's contravening either.

A very long time ago regs did have stipulations along the lines that circuits rated 15A or higher could only serve a single "point" but while that's long since gone the collective memory and standard practices do sometimes seem to retain a shadow of such relics (rather like people being afraid of having single phase loads supplied from different phases in the same room).

In terms of "how bad" such an arrangement is, that really depends on the circumstances. In general it's probably less than ideal - lack of discrimination, perhaps a need to oversize OPD & conductors etc. On the other hand if the two machines had a related function - e.g. one put things onto a conveyor and the other took them off - then having them both stop on a single fault might be seen as a definite positive.

Most likely it's a bit of a botch when a new machine arrived and the existing DB was already full (or an extra correct sized MCB wasn't available).

Worth checking the manufacturer's instructions w.r.t. maximum overcurrent protective devices too.

    - Andy.

Absolutely Chris, just as you say. It appears that some seem to have a "three phase" disease that blinds them to reality. Radial circuits with more than one point of utilisation are common, and this applies to single and 3 phase installations. If the machines have very significant consumption them dedicated circuits are more common, but a ring distribution circuit around a room full of machines may well be the most economical solution,  with each having a switch fuse as the isolator (obviously all poles). I don't know why these things come up, probably still back in the 14th or even before!

Fine for relatively small loads, and MIGHT BE OK for larger loads but this depends.

Suppose that you have two machines each 3 phase with a FLC of 4 amps. Both on the same 16 amp circuit should be fine.

Now consider two much larger 3 phase machines each with a FLC of 25 amps. A 32 amp circuit for each machine would be the usual approach. If both machines are to be on the same circuit, then this will probably need to be 63 amps. A 63 amp OCPD may be larger than is sensible for a 25 amp load, or larger than the supplier of equipment allows.

Without more info it is hard to say what the objection is. Certainly there is no reg against a 3 phase radial feeding many machines, though those loads may or may not require fusing locally depending on the overload protection associated with the starter if its a motor, or internal protection if this is something like 3 phase ovens. There may be the 3 phase version of the thin flex short circuit but not overload protected by a large fuse debate, but it is not a general case.

Certainly a no of 16A 3 phase sockets  fed by a 20A breaker is a recognized workshop arrangement where the loads are all a lot less than 20A total and/ or cannot be used together (grind stones, suds pumps, bead blaster etc).

Mike.

Thanks for the clarification - the contractors had already installed it utilising two feeds anyway 

Thanks, yes it was effectively a botch, for some small workshop machines on a jig which didn't come with spare capacity in the distribution board