Hey Mike......Situation has come up where we are questioning the correct voltage to use to figure our load calc's for apartment feeders........we have a 120/208V, 3-ph, 4W incoming service to the building.... with 120/208V, 1-PH, 3W services to the apartments......we feel it's appropriate to use 208V in our apartment calc's not 240V suggested by the engineer.........your input would be greatly appreciated.......D. Pellecchia
Multi-family load calcs
- Thread starter dpellx
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jumper
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- 3 Hr 2 Min from Winged Horses
208V for 1P 3W feeds and 208V x 1.732 (360V) for 4W service.
Mike rarely posts.
Welcome to the forum or rather hello for your first post, since you have been here since 2003.
Note 220.54 and 220.55 for service calcs.
Mike rarely posts.
Welcome to the forum or rather hello for your first post, since you have been here since 2003.
Note 220.54 and 220.55 for service calcs.
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In a way, the engineer is partially correct. However, using 208V is the safer approach. You may also want to use both.
Say you have 200A of 120V loads on each line. Your kVA would be 200A ? 240V = 48kVA.
So 48kVA ? 208V = 231A. As you can see, figuring at 208 will give you the safer value for sizing conductors... but you also may end up oversizing them.
The better way is to calculate the amperage of 208V and 120V/120V loads, at 240V, separately and add the ampere values together. If you did the vector math, the line current would be less than the sum just mentioned (I'll add disclaimer here for rare power factor instances).
Say you have 200A of 120V loads on each line. Your kVA would be 200A ? 240V = 48kVA.
So 48kVA ? 208V = 231A. As you can see, figuring at 208 will give you the safer value for sizing conductors... but you also may end up oversizing them.
The better way is to calculate the amperage of 208V and 120V/120V loads, at 240V, separately and add the ampere values together. If you did the vector math, the line current would be less than the sum just mentioned (I'll add disclaimer here for rare power factor instances).
jumper
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Missed that, darn it
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
As Smart stated you could do both. Use 208V for your 3 phase circuits and 240 for the 120V loads. Seems to get messy so I would just use 240 and be done with it.
Actually annex D ex: D5(a) uses 208
Actually annex D ex: D5(a) uses 208
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jumper
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- 3 Hr 2 Min from Winged Horses
Wouldn't that undersize the conductors for 208V circuits?
Actually, it uses both for each dwelling. Divided by 2 legs then divided by 120V/leg equals divided by 240V. It calculates the range at 208V then adds the 120V and 208V values together.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
Actually the OP is asking about the calculation to the apts which are feed with single phase 120/208 so why would you use 240V
jumper
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- 3 Hr 2 Min from Winged Horses
Dennis, using 240V for individual 120/120 loads would decrease the feeder size I think. I believe that was Smart$ point.
Yes, it is... but a decrease in feeder size depends on the result.
It is easy to rationalize. Loads are figured in VA then divided by voltage to determine amperes. Any VA value divided by 240 will be less than that same value divided by 208.
malachi constant
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- Minneapolis
DPell, you are correct, since you have 208/1 in the units you should use 208/1 to calculate the unit panel feeders and any two-pole branch circuits within (elec ranges, elec dryers, etc).
And since you have 208/3 for the overall building service you should use 208/3 to calculate that.
Multifamily calcs are tricky. If you have more specific information as to how you are calculating please post and I can comment. But to answer the question you asked, yes, you are right, use 208V. I would not recommend using 240V as you do not have it in the building.
And since you have 208/3 for the overall building service you should use 208/3 to calculate that.
Multifamily calcs are tricky. If you have more specific information as to how you are calculating please post and I can comment. But to answer the question you asked, yes, you are right, use 208V. I would not recommend using 240V as you do not have it in the building.
There is no requirement to figure 120/208V 1? 3W at 208V only... and technically, figuring at 240V when it is not there is incorrect. Technically it is figuring 120V loads as VA ? 2 legs ? 120V/leg... and that is mathematically equivalent to VA ? 240V... and just the line-to-neutral, 120V loads. Line-to-line loads are always calculated at 208V.
PS: The separation of L-N 120V and L-L 208V can be carried all the way up to calculating the service conductors.
malachi constant
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- Minneapolis
Yes - I agree with all that.
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