Current Calculations

[jobrafo]

Calculation Dilemma:
In working with 120/208v systems, when calculating a single phase load on a circuit, the Power (watts) = Voltage(120v) * Current(Equip. rating) (ex. 120v * 4A = 480W)

If the equip. is two pole which calc. would be correct per leg, that is:

120v * 4A = 480W per connected leg or,
208v * 4A = 832vA

I've tried to understand this as Watts and KVA are not exactly the same in that KVA takes into consideration the pf.

Joshua
Raleigh, NC

[ April 01, 2003, 10:18 AM: Message edited by: jobrafo ]

 

[bphgravity]

Re: Current Calculations

To start with Watts does not equal Kva. Kva = 1000 volt-amps whic is equal to 1000 watts or Kw. The volt-amp is the amount of energy that is used which includes the wattage of the load and all losses. As far as your calculation goes, to determine total load of the connected equipment, you would use the connected supply voltage of 208-volt. (208v x 4A = 832va) The actual wattage would indeed need to consider power factor.

 

[dereckbc]

Re: Current Calculations

Josh, with a two-pole KVA = 208*I. KW = 208*I*PF

 

[charlie b]

Re: Current Calculations

Originally posted by bphgravity: The volt-amp is the amount of energy that is used?

I agree with Brian?s answer, except for a minor quibble. The ?volt-amp? is not an amount of energy, it is the rate at which energy is being used. The distinction is the same as that between miles driven and the car?s speed in miles per hour. A unit that measures an amount of energy would be a kilowatt-hour (i.e., you need to multiply by a unit of time).
Charlie

 

[jobrafo]

Re: Current Calculations

How about this.......120/208v system...
208v single ph. equip. rated at 20Amps.
When designing a power panel...how would you determine the single leg Power of each leg?

Would you use 120v*20amp = 2.4kW per leg or
120v*20Amp = 2.4kvA or
208v*20amp =4.16kvA then divide by two for each leg which is 2.08kvA

The problem is to not design a system using Watts to find the Amperage and overload the power panel when I should have used kvA.

 

[charlie b]

Re: Current Calculations

Originally posted by jobrafo: Would you use 120v*20amp = 2.4kW per leg or 120v*20Amp = 2.4kvA or 208v*20amp =4.16kvA then divide by two for each leg which is 2.08kvA

First of all, when you say ?208V single phase,? I infer that you mean a two-pole breaker serves this load, and that the breaker is in a three phase 120/208V panel. Therefore, for this one load, 120V does not enter the picture. Secondly, you did not describe the equipment fully. Is it, for example, a heater (power factor = 1.0) or a motor (power factor around 0.85)? Without knowing the nature of the load, we won?t be able to say anything about ?watts.? Finally, it is inappropriate to speak of ?power per leg.? Power is generated by the source and dissipated by the load. The ?legs? transfer energy from the one to the other, and in the process they consume a nominal amount of energy themselves (i.e., ?energy losses?). The amount of these losses can be calculated, but the information is seldom of any value to anyone.

But the fundamental concept you are trying to find is this: use VA (or kVA) to add up the loads, and then convert to amps at the end. In your example, the power consumed by the load is 120 * 20, or 2.4 kVA. Ignore the power factor, add up the remaining loads, apply any demand factors allowed by the NEC, and divide the result by 360 (this number being 208 times the square root of 3). That will lead you to the correct size for your panel.

 

[jobrafo]

Re: Current Calculations

So, a Kitchen Steamer is 208v, 40amps, 2pole.
8.3kva.
What is the amount of Volt-Amps on each pole?
4150va(8.3kva / 2) or 4800va (120v * 40a)?

Sincere gratitude for the clarity on the matter. BTW, should a Power Panel ever be designed based on Watts or strictly by KVA?

Joshua

[ April 01, 2003, 02:48 PM: Message edited by: jobrafo ]

 

[bphgravity]

Re: Current Calculations

Joshua, a panel is always sized by the volt-amps. The amount of va per pole is irrelevent to a load calculation. If the equipment is 208-volt, 40-amp. Then your total load calculation is 8.3Kva. This means you must use a 40-amp breaker and 40-amp conductors for each line to the equipment. A 208-volt circuit is the operational voltage, not two 120-volt circuits.

[ April 01, 2003, 03:34 PM: Message edited by: bphgravity ]

 

[jobrafo]

Re: Current Calculations

Ok, I'm totally with you.
The NEC gives a %tolerance that each phase can be out by. I'm quite sure that it's 10%. Given that, how do you determine the KVA on each phase.
208v * 40a = 8.32KVA. 40amp breaker 2pole is required.

[ April 02, 2003, 01:40 PM: Message edited by: jobrafo ]