In alt.engineering.electrical John Woodgate wrote: | I read in sci.engr.electrical.compliance that snipped-for-privacy@ipal.net | wrote (in ) about 'Wire Size/Load | Question', on Wed, 25 Feb 2004: | |>I see a lot of equipment (transfomers, switch panels) rated for 200% |>neutral current. But I can see scenarios where it could reach as high |>as 300%. Take, for example, an AC to DC power supply that uses three |>phases and a full wave rectification. Wouldn't these rectifiers only be |>conducting when the respective phase is the highest voltage? If this |>were happening and it were the only (very large) load, you'd have to |>make sure everything is well overrated for it (3x on neutral for wye, |>and 2x on each leg for delta). Does anyone even use stuff like that? | | | Yes. You have done well to work out these scenarios independently. But | the situation isn't quite as bad a pure arithmetic addition of the | current pulses in the neutral. The upper limit is about 200 %, not 300%. |>
|>I understand the common switching power supply for computers switches on |>the current when the voltage is high enough to replenish the tank it |>keeps. So it would see the smaller the computer load relative to the |>power supply capacity, the narrower the current pulses would be. Once |>they are smaller than 50%, you could be going past the 200% mark on the |>neutral, | | But there is a balancing effect; these short pulses are associated with | less than maximum loads on the power supplies. In other words, a 300 W | supply would do this if loaded to say 100 W, but the currents involved | would be related to 100 W, not 300 W.
But circuit sizing is not usually done for computers based on the power supply capacity, but rather, the actual DC load itself. So a computer with a 300 watt p/s and a 100 watt load could be considered a 100 watt computer. Now put 100 of those computers on one leg of a 208Y/120, and
100 more on the 2nd leg, and 100 more on the 3rd leg. If those computers are drawing power with a narrow enough pulse, you could see 250 amps on the neutral, but only 83.34 amps on each leg.
Keep in mind I'm only describing worst case scenarios which include both very narrow current pulses (and I'm not sure that happens very often) and the bulk of the load being this type (generally not an issue even in an office environment). Real life probably doesn't even see 150% very often.