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How will a step-down transformer impact current in this situation?
I'm going to have an 8000 watt, 208V UPS installed. At full load, and rounding up slightly, this will require a 40 amp current from the wall/breaker.
The UPS then incorporates two step-down transformers to provide 120V power. I understand that at 120V, the two transformers (providing 4,000 watts each) will then require a total of 80 amps.
Do I need to plan for a circuit capable of 40 amps or 80 amps from the wall/breaker? Is the 80 amp load isolated within the UPS?
Hopefully this makes sense. I'm obviously not an electrician but am trying to get an idea of what all would be involved.
Thanks!
1 Answer
- 8 years agoFavorite Answer
I'm assuming you have commercial 120/208 3-phase service:
8kW at 208 is @40A
120 is more than half of 208, so it's less than half for each 120v leg.
8kW at 120 is @70A, or 35A for each rectifier. (they're not just transformers)
But, the UPS is rated by it's output capacity, not it's input. And since the UPS is not 100% efficient, 8kW at the output means more power on the input. Since you should have oversized your UPS, then this should already accounted for and 40A at the 208 side would be sufficient.
There is no 80 amps flowing anywhere in that system, as the higher 120v legs split and drop the current. You do have to ensure that the legs are balanced, though, so distribute your load evenly when connecting devices to the UPS.
Source(s): 30 years engineering