3phase genset hookup for backup of 240v single phase service

First, you need a building permit to do this. If anything happened your insurance company could easily disclaim your filing based on not having the service inspected. (And of course, if anything is wrong, they would be able to tell you.) As far as you have gotten so far, you will need to identify the transfer equipment as to exactly what it is (fire dept. requires.) Also permanently identify the second panelboard as being used for emergency use. Make certain the transfer equipment and secondary main disconnects are rated as service equipment (prob 6000 amperes should be Ok, but your local AHJ may require larger.) The standby service overcurrent breaker must disconnect all phases if one phase opens.

You should be able to get 120 v between the A and C phase (the smaller centertap is between the A and C phases, B phase is the High Phase and is required to be colored orange.) Between the tap (neutral and either A or C should give you 120 v (your measuring 125 v with no load). What's critical is make certain you bond the neutrals together (neutral from Edison and the tap or smaller connector).

Make absolutely certain you get a release from your local building department for this.

Thanks for the responses. Am a little confused about the neutral situation. I just got through talking to a retired electrician and he told me you have to disconnect the neutrals from the mains and make sure they are not connected to the genset in any way. I figured he was right since when I tried to throw the transfer panel switch for the pumps the particular breaker on the SEP tripped. Right now, all the neutrals are tied together for everything. No equipment was damaged fortunately. I will be checking more wiring and all that. It may be that I might have to use just the one phase for entire backup and forget about the other phase. Yes, the mains are basically two phases 180 degrees out of phase and I neglected to .consider the effect of that on my pumps. Not sure if it will mess them up. I was also concerned that trying to pull loads on two of the phases which are not necessarily balanced would mess up the genset, but the electrician told me it wouldn't. Will be trying to find the manuals for the genset and study up a bit more on multiphase power before trying another smoke test.

you might get better qualified responses in the electrical engineering group (seeing as most guys there are power engineers anyway)..

OK, thanks for the info. I did some more searching and troubleshooting and most of the crisis seems to be over. I did find a wiring error in the pump subpanel which explained why the breaker tripped when I turned on backup power. This has been corrected. There is a voltage regulator adjust control on the 208 tap which all you have to do is turn to get it to advance to 240 volts. I did find out about reconfiguring the 12 leads in what is called a zig zag connection and another connection called double delta which is balanced but the genset is derated about 83 per cent when this is done. Finally, I found I did not have to switch the neutral of the mains out when on backup power. All the neutrals are bonded together and I can switch back and forth with the transfer panels now. I made a mistake saying the mains were two phases 180degs out; they are in fact called split phase and the two 120v to ground legs for each pole are in phase. Thanks to all for the help.