malachi constant
Senior Member
- Location
- Minneapolis
Hi all,
I've long been aware that 746W/HP is a good rule of thumb. Somehow I've never taken the time to compare it to the NEC tables. I have a motor application where a UPS sales vendor thought their equipment would be cost competitive with a generator. I sent them the HP loads and they came back with a UPS that I thought was undersized. In exploring that I discovered they used 746W/HP while I was using the NEC tables to get FLA, then multiplying that by 460 * sqrt (3). We are off by nearly a factor of two.
To be more specific, the application is (10) 7 HP motors.
Approach #1: 10 motors * 7 HP/mtr * .746kW/HP = 52.53kW
Approach #2: Per NEC 430.250, at 460V a 7.5HP motor has 11 FLA. 11 FLA * 480V * sqrt(3) * 10 motors = 91.34kVA
7 HP vs 7.5 HP accounts for some of the discrepancy.
I am but a lowly consulting engineer and mentally default to everything having unity power factor (i.e. in my simplistic world kW = kVA).
I do not think this UPS rep does a lot of motor loads so am hesitant to use their number. I could reach out to the generator rep (who I trust understand motor loads better with respect to generator equipment) but thought I would start here.
FWIW these are fan loads and each will be on a VFD. The reason UPS may be cost competive is the generator (or UPS) will located in a code-classified storm shelter, which is buried within the belly of a building - meaning it needs to survive and operate even if the building around it is destroyed - which makes generator intake/exhaust a challenge. The UPS may or may not have similar ventilation issues, haven't got to that exercise yet, just doing a first look on UPS cost and footprint to see if it is close. But them sizing their UPS about half as small as I expected needs to be figured out first.
That last paragraph you can ignore, but included it for those curious as to why I am putting so many motors on a UPS. My primary question for you all is wanting to reconcile the two kW calculation approaches with respect to sizing a power source (UPS or generator). Thanks in advance!
I've long been aware that 746W/HP is a good rule of thumb. Somehow I've never taken the time to compare it to the NEC tables. I have a motor application where a UPS sales vendor thought their equipment would be cost competitive with a generator. I sent them the HP loads and they came back with a UPS that I thought was undersized. In exploring that I discovered they used 746W/HP while I was using the NEC tables to get FLA, then multiplying that by 460 * sqrt (3). We are off by nearly a factor of two.
To be more specific, the application is (10) 7 HP motors.
Approach #1: 10 motors * 7 HP/mtr * .746kW/HP = 52.53kW
Approach #2: Per NEC 430.250, at 460V a 7.5HP motor has 11 FLA. 11 FLA * 480V * sqrt(3) * 10 motors = 91.34kVA
7 HP vs 7.5 HP accounts for some of the discrepancy.
I am but a lowly consulting engineer and mentally default to everything having unity power factor (i.e. in my simplistic world kW = kVA).
I do not think this UPS rep does a lot of motor loads so am hesitant to use their number. I could reach out to the generator rep (who I trust understand motor loads better with respect to generator equipment) but thought I would start here.
FWIW these are fan loads and each will be on a VFD. The reason UPS may be cost competive is the generator (or UPS) will located in a code-classified storm shelter, which is buried within the belly of a building - meaning it needs to survive and operate even if the building around it is destroyed - which makes generator intake/exhaust a challenge. The UPS may or may not have similar ventilation issues, haven't got to that exercise yet, just doing a first look on UPS cost and footprint to see if it is close. But them sizing their UPS about half as small as I expected needs to be figured out first.
That last paragraph you can ignore, but included it for those curious as to why I am putting so many motors on a UPS. My primary question for you all is wanting to reconcile the two kW calculation approaches with respect to sizing a power source (UPS or generator). Thanks in advance!
