200% neutral vs. 208V load

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Eclair

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Hi

I design a 120/208V UPS system for a computer room. Someone told me to size the neutral to 200% (square root of 3 actually)because of the 3n harmonics coming for a typical 120V computer load.

But!
Almost all of the loads are 2 phase 208V (I will balance them of course). The 3n harmonics don't go trough the neutral; it is not used... I just can't figure out how to do my calculation to size my cables in such a case.

Someone here knows?
 
SINCE most of your loads are not 120 Volt, then your neutral conductor won't be handling those. How much of the loads will be connected to 120?
 
Actually, the big question is: On a 208V computer load, where the 3n harmonics go in a 120/208V system?
 
Eclair said:
Almost all of the loads are 2 phase 208V
Click to expand...
No. They are single phase 208 volt, but they use two poles to derive that source.

Since you are not using the neutral for most of your loads, you don't have to upsize the panel feeder's neutral to 200%.
 
Sorry, but I still don't have my answer. Where goes the 3rd, 6th and 9th harmonic in a phase to phase non-linear load? Does it go by the neutral anyway or...?
 
They (as in the harmonics) go via the phase conductors. The big difference from the 120V position is you dont get three of them piling up on one conductor, that any old school sparky will tell you only carries the phase difference current.
 
Triplen harmonics in line to line connected loads are in phase across the various phases. In other words, the triplen harmonic flowing between phase A and phase B is in phase with that flowing from B to C and C to A. But instead of all being connected to a single neutral point, this 'in phase' current is connected in a closed circuit incorporating all of the loads.

Triplen harmonics _circulate_ on delta loads; if you had perfectly balanced loads then all of the triplen harmonic current would circulate and none would return to the transformer.

-Jon
 
Eclair said:
Sorry, but I still don't have my answer. Where goes the 3rd, 6th and 9th harmonic in a phase to phase non-linear load?
Click to expand...
I agree with the last two responses, except in one minor aspect. The 6th harmonic does not go where they said it goes. Indeed, it doesn't go anywhere at all. In a power system, all the even harmonics (i.e., 2nd, 4th, 6th, 8th, etc.) "vanish." Harmonic analysis is a mathematical model of a real phenomenon. When you do the math, you get an "amount" of each harmonic (i.e., 1st or fundamental, 2nd, 3rd, 4th, 5th, etc.). It just happens to turn out that the "amount" of the 2nd is zero, as is the "amount" of the 4th, the 6th, and all other even harmonics.

That, at least, is how I remember the math working out. But please don't ask me to prove it, as I long since succeeded in forgetting all that math. :grin:
 
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