For a three phase fan powered box with a single phase motor load, is the single phase motor load assumed to be on phase A for load calcs? Example, 12 kW fan powered box at 480V with .5 HP fan at 277V. Divide 12 kW by three and add to phase A the single phase load of the motor only?
Fan powered box load
- Thread starter cppoly
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kwired
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What do you mean by fan powered box?
Is the single phase load line to neutral or line to line? I would say for load calc's add this load to which ever line(s) it is connected to and not just assume phase A.
texie
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I"m guessing the OP is referring to a fan powered VAV terminal.
It is a 3 phase heater with a single phase (line to neutral) fan blower installed to provide heat through duct work. I'm the engineer so I don't know which phase will actually be connected to the single phase motor. So for instance I show:
5900 VA phase A
4000 VA phase B
4000 VA phase C
On a project with a lot of these, assumption of phase A is multipled by many fan powered boxes
iceworm
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cp -
Just a suggestion. The mfg literature shouldn't be too hard to find. You might even consider giving them a call. Might find the fan can be connected up to any of the phases. Unless, of course the project has not yet selected a mfg.
I'm a bit surprised the phase the motor is connected matters. Since it does, consider adding a motor to each phase. This would give 1.5hp per fan instead of .5hp. However I wouldn't think the system would be sized so tight that this would matter.
ice
scotteng
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When we design circuits for FPBs, we tell the contractor to alternate the phase that the blower motor is connected to so as to maintain balance. It is not uncommon for us to string together six FPBs, so we assume two blowers motors per phase.
That makes sense. I like that idea.
kwired
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So is the fan 1900 VA? Seems like a pretty large fan for a 12000VA heater. That is near 2.5 hp. Most units of that capacity will only have 1/2 hp blower motor - maybe 3/4 max.
I agree, even though the blower is there, it is not that significant as compared to the heat load and usually one is not calculating the total load so tight that the blower would matter that much. If there are enough units for unbalancing to be a concern, then maybe try to make sure they get phase connections rotated around enough to balance the blower loads across all three phases.
It's 277 volt single phase 3/4 HP @ 6.9A. That's 1900 VA
kwired
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Never mind the 2.5 hp I mentined - we need to consider efficiency and power factor before we can say say it is 2.5 hp. That is still a little high of a current for a 277 volt motor though. My motor slide calculator that is supposed to be based on NEC tables says FLA for a 3/4 hp @ 240 volts is 6.9 amps.
Actually, that is a good point. That brings up my next question. NEC table 430.248 for single phase motors only goes up to 230V. If you have a 277V single phase motor, you have to use the current listed for 230V single phase. Is the actual power consumed 277V * (amperage listed for 230V)? The motor will run with lower current at 277V so it is lower power though. How do you determine the current for 277V single phase motor?
iceworm
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(2011) NEC 430.22 points you to 430.6.A.1. There is an exception for shaded pole or split capacitor blower motors and an exception for listed motor operated appliances.
If you are looking for a design issue that is a different problem.
ice
kwired
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If a motor is not in the NEC tables then I believe you have no choice except to use the motor data plate. Most cases this will be definite purpose motors, OEM's, etc. and not NEMA standard motors - which is what the NEC tables contain (I think). What you have is likely in that definite purpose or OEM category, and as iceworm mentioned it is also part of a listed appliance, so when doing calculations you are looking at the appliance nameplate and not the motor nameplate.
Interesting. I see 277V single phase motors quite a lot so this is helpful. Thanks. The good news is if I've been using FLC table for max of up to 230V single phase, the current is a conservative value compared to a 277V motor.
kwired
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I guess that depends on what you mean by conservative. All characteristics otherwise the same, a motor rated 277 volts should draw less than a 230 volt motor. This should give you high enough ampacity conductors for those types of calculations, but if you go by a 230 volt rating for overload protection, your selection will allow overloading of the motor.
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