Got a question and don't have a code book....

A 3" P.V.C.conduit is ran from under ground transformers
to electric meter packs with 150 breakers. All meter packs
are main lug, no main breaker. Some meter packs contain six
meters and some five, some four. 120/240 volt residential.
I know I can parallel the Pratt 250-250-4/0 alu. in a 3"
p.v.c. pipe...is that big enough to handle the load for
the five and six meter packs ?
6x 150= 900 total amps for the six banger @ 45% = 405
amps ? can't remember how much to derate...
Five meter pack may work with 250...just not sure about the six ... thanks....

Assuming that you have done calculations on the services and have found that they all are at 150 amps. 5 residential services can be calculated at .45 for a total of 337.5 amps (see 220.84).

6 residential services can be calculated at .44 for a total of 396 amps. Same code reference. Parallel conductor installation of 250 kcmil are good for the 400 amp total (255x2x.8 NEC 310.15(B)(2)(a)) if allowed by your AHJ.

I did not check the nuetral. This also makes the assumption that you meet the multi-residential requirements.