208/120V system 2-pole load a 2 phase or single phase?

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
L1-N = 120v 1ph
L2-N = 120v 1ph
L1-L2 = 208v 1ph

There is no equipment that will care about the phase angle, which only manifests itself as a voltage difference.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
L1-N = 120v 1ph
L2-N = 120v 1ph
L1-L2 = 208v 1ph

There is no equipment that will care about the phase angle, which only manifests itself as a voltage difference.

I would suggest that there is no generally used equipment that will care about the phase angle, but one most certainly can build such equipment.

-Jon
 

gar

Senior Member
210805-1723 EDT

A single phase input to a transformer with a center tapped secondary ( 2 phases on the secondary ) feeding two diodes to provide full wave rectification is an extremely common application.

.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
A single phase input to a transformer with a center tapped secondary ( 2 phases on the secondary ) feeding two diodes to provide full wave rectification is an extremely common application.
Yes, within equipment using an internal isolation transformer.

However, we've agreed that a center-tapped secondary provides opposing polarities, not phase angles.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
L1-N = 120v 1ph
L2-N = 120v 1ph
L1-L2 = 208v 1ph

There is no equipment that will care about the phase angle, which only manifests itself as a voltage difference.
One corner case that has been seen to cause problems in the real world is in some cooking equipmen that uses electrical heat from a sealed element, such as coffee makers. The design of the product often allows for a certain amount of capacitive (and maybe even resistive?) current leakage from each portion along the length of the heating element to grounded metal.
When the unit is driven from 120-0-120, even with no neutral connection to the element, the capacitive leakage current from L1 is roughly equal to and opposite the leakage current from L2. The result is that the apparent ground leakage current is reduced by an order of magnitude or so and the unit does not trip a GFCI, either receptacle or breaker type.
But when L1 and L2 are two-out-of-three from a three phase source, producing 208 volts across the element, the reduction in heating is tolerable. But the visible ground leakage current is now the vector sum of two components which are at a 120 degree phase angle instead of a 180 degree phase angle. And the GFCI will invariably trip.
If the metal shell of the appliance is not grounded, the GFCI may not trip, but a person touching the metal and a ground will get a noticeable shock.

Similar problems in residential use are less common because most kltchen applicances are powered from 120V and so their leakage in use will correspond to the leakage during testing.
 
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