208V single phase
- Thread starter rose1981
- Start date
- Status
kwired
Electron manager
- Location
- NE Nebraska
Do you have a single 3000 amp single phase load, or do you have multiple single phase loads that would add up to 3000 amps if supplied from single phase only?
A load(s) with that high of amp draw would be something you normally want to balance across all three phases. If that is what you do have you would cut it down to 1732 per phase if divided across all three phases.
If I am on wrong path here please clarify what you are asking.
A load(s) with that high of amp draw would be something you normally want to balance across all three phases. If that is what you do have you would cut it down to 1732 per phase if divided across all three phases.
If I am on wrong path here please clarify what you are asking.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I agree, your utility will NOT like you very much if you are loading up one phase at 3000A... it will unbalance their transformers big time, all the way up the line.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
And that's why you always work in KVA; KVA does not lie, and it is not distorted by voltage changes. KVA is KVA.
If the panel schedule is totaling amps, it is junk, don't use it.
It sounds like you may be trying to use individual load amps and trying to add them up, when in reality you need to add KVA's and then calculate current.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Even when you use kVA the question comes up (and I think it is the one the OP is asking about) of what to do with line to line loads.
The answer, which gives you the right overall load on the panel and allows you to work your way back to the source to see if adequate KVA is available is this:
Whenever you have a line to line load, assign 1/2 of the load kVA to each pole/breaker. That way when you add up the kVA loads on each phase and then add up the 2 or 3 phase kVA values you will end up with the sum of the load kVA values and not twice that number.
The proper calculation the of the current for each bus or feeder line is subject to disagreement when severely unbalanced loadings are present. Just taking the total kVA and applying the three phase current formula does not give you the same answer as adding up the kVA assigned to each bus and dividing by the line to neutral voltage. The NEC does not seem to care about that.
Accept it.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
The NEC does not seem to care about that.
I do... the NEC is not a design guide, therefore why would the NFPA safety code care about how to add up KVA other than to make sure no one is injured. Doesn't even matter if the electrical system works, just no injuries.
It's as GoldDigger said. Divide the kVA between the two buses, i.e. 624kVA@208V ÷ 2 = 312kVA/bus = 2,600A/bus@120V
The difference is because of rounding error in the calculation, i.e. 208V is not exactly 120V times sqrt of 3.
With that noted, your Revit panel schedule 'autocal' is assuming there will be an equal 208V load placed on the adjacent phases to both sides of the one under discussion... such that the total will be 624kVA/bus.
- Status