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Is there something wrong with my calculation?
#128847
02/22/04 10:59 AM

Joined: Sep 2002
Posts: 1,497
OP
Member

Hi, I've been scratching my head over something. I have a scenario (theoretical) where you can either use 2 wires @ 230V or 3 wires @ 200V 3ph. The size of the wires is limited by voltage drop, which is the same percentage wise for both cases. According to textbooks and common knowledge, 3phase is more efficient than just using two wires. But I fail to arrive at this result in this case. If I put 1A load the 3ph case I need 1.73/1.15 = 1.5A single phase to get the same power. This means I have to increase the cross sectional area of the conductors by 50% compared to the 3phase case. But I only need 2 wires instead of 3 and am back at the same amount of copper. Is there something wrong with my calculations? Wouldn't be the first time...



Re: Is there something wrong with my calculation?
#128848
02/22/04 02:55 PM

Joined: Apr 2002
Posts: 2,527
Moderator

CH, that is a great riddle/brainteaser. The “trick” is 3ø versus 1ø power must be compared at the same voltage.
1.5 amperes at 230V 1ø is 345VA, where 1 ampere at 200V 3ø is 346.4VA, BUT 1 ampere at 230V 3ø is 398.4VA. Ratioing, 345/398.4 yields 0.866, exactly as expected.
Does that help?



Re: Is there something wrong with my calculation?
#128849
02/23/04 07:47 AM

Joined: Sep 2002
Posts: 1,497
OP
Member

Thanks! I needed to check my calculation with someone. I wanted the same potential to ground for both cases (wye or center tapped source).
By the way, are you and Busbar one and the same person?



Re: Is there something wrong with my calculation?
#128850
02/23/04 10:16 AM

Joined: Sep 2003
Posts: 650
Member

CH, You've found an identity (In the mathematical sense.) For the same potential to neutral and the same 'aggregate current', you get the same VA delivered to the load, no matter 1 phase or 3 phase or 17 phases Jon



Re: Is there something wrong with my calculation?
#128851
02/23/04 10:25 AM

Joined: Apr 2002
Posts: 2,527
Moderator

CH — Yes, but only on alternate Tuesdays. ;)




