With smaller wire sizes, there is more likely to be an uneven split in current due to termination resistances, etc. It's also unnecessary, adding more possible failure points where they are not needed.

In the US the minimum size is #1/0 AWG, although it normally is not done until maybe #4/0 or so.

Thanks dpc.

I had read something similar.... http://www.ecmag.com/section/systems/conductors-co...

I wouldn't really think at starting to run parallel below 50 or 70mm.sq (which is similar to the 1/0 awg you quote). I could only think maybe manufacturer tolerance in resistance/reactance (proportionality relative to conductor size) would be a contributing factor, but wanted some other view before giving an answer to my colleague other than just quoting a rule that he shouldn't do it.

At the smaller sizes, the economics don't favor multiple conductors per phase anyway. Ampacity increases nearly in proportion to the cross-section in the smaller sizes. As the conductor gets larger, this is not true and the ampacity increases at a lower rate than the cross-section due to heat transfer issues and skin effect. At about 500 kcmil, it makes more sense to double up on a smaller size than run larger and larger conductors. You won't see much 750 kcmil and above installed for that reason - at least for industrial applications. Utilities have other drivers that sometimes push them up to 1000 kcmil (Aluminum, of course), but even then, I don't think it generally is cost-effective.

The Canadian code has an interesting exception to the 1/0 rule:
Ungrounded and grounded circuit conductors of similar conductivity in sizes No. 1/0 AWG and larger,
copper or aluminum, shall be permitted to be installed in parallel sets provided that each parallel phase or
grounded conductor set is individually comprised of conductors that are
(a) free of splices throughout the total length;
(b) the same circular mil area;
(c) the same type of insulation;
(d) terminated in the same manner;
(e) the same conductor material; and
(f) the same length.
......................
(5) Conductors of similar conductivity in sizes smaller than No. 1/0 AWG copper shall be permitted in parallel
to supply control power to indicating instruments and devices, contactors, relays, solenoids, and similar
control devices, provided that
(a) they are contained within one cable;
(b) the ampacity of each individual conductor is sufficient to carry the entire load current shared by the
parallel conductors; and
(c) the overcurrent protection is such that the ampacity of each individual conductor will not be
exceeded if one or more of the parallel conductors becomes inadvertently disconnected.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Thats the same exceptions that NEC has too.. word for word basically