Glen, one of the biggest problems we have with explaining additive harmonics is our generic terminology of single phase when we are refering to a double pole connection from a wye connected source. This would also be the case in our use of the term multi wire branch circuit which should be multi phase branch circuit. In the original question of a 120/240 single phase service (one winding center grounded) the harmonics would not be additive due to this truly being single phase. In the muti phase panelboards in your question, served from the 120/208 wye system, due to the phase angles the harmonics would be additive even if we were to use single phase to describe the panelboards. You are correct in refering to 220.22 in this case.
Roger
[This message has been edited by Roger (edited 06-09-2002).]
