# 1-phase vs 3-phase current calculation

If we have a piece of equipment that comes in two models of power supply connection:

• Model A: 208V/20A-1P
• Model B: 120V/30A

Measurements of the model gives its max load as 4400W at a p.f. of 0.9.

1. What is the current in each case?
Is it for:
Model A: $$\4400 / (208 \times 0.9) = 23.5A\$$
Model B: $$\4400 / (120 \times 0.9) = 40.74\$$
2. What does 208V/20A-1P mean (specifically the 1P) 1 pole? Does this mean we connect between any two phases? So we consider it as a simple phase load and calculate as $$\4400 / (208 \times 0.9) = 23.5A\$$?
3. Can we tell if this is a 1-phase or 2-phase supply?
4. If it were 3-phase, what would be the calculation to determine phase current?

What does 208V/20A-1P mean?

It means single-phase 208 V - 20 A.

Model A: Single-phase 208 V - 20 A.

Power = V * I * Cos Ø = 208 * 20 * 0.9 = 3.7 kW

It is presumed that Model B is three-phase, since it is not marked 120 V / 30 A - 1 P.

Consider a three-phase model (Model X) having the same power rating (3.7 kW).

Model X: Three-phase 120 V - 20 A.

Power = √ 3 * V * I * Cos Ø = √ 3 * 120 * 20 * 0.9 = 3.7 kW

Model B: Three-phase 120 V - 30 A

Power = √ 3 * V * I * Cos Ø = √ 3 * 120 * 30 * 0.9 = 5.6 kW

It's apparent that the three-phase Model B has a higher power rating.

• Thanks so much for that clarification So you mean: a) Power = V * I * Cos Ø = 208 * 20 * 0.9 = 3.7 kW is the maximum it can draw? i.e. basing on the values for Model A: 208V/20A-1P, we can tell the maximum power drawn b) It would thus seem that in "Model A: 208V/20A-1P" the 1P refers to 1 phase? So if Model B were to have 3 phase we would expect it to be described as: "120V/30A-3P"? Nov 3, 2023 at 21:02
• Anytime, Dennis! Yes, 1-phase power is (V * I * Cos Ø) and 3-phase power (√ 3 * V * I * Cos Ø). In 'Model A: (208V/20A-1P)', 1P refers to 1-phase. Likewise, 3P may refer to 3-phase. Generally, a 1-phase source is designated as (1 x 120 V ~ 50 Hz) and a 3-phase one (3 x 208V ~ 50 Hz). A 1-phase source may also be designated as (120 V ~ 50 Hz) and a 3-phase one (208 V ~ 50 Hz - 3 wire) or (208 V ~ 50 Hz - 4 wire). Another way to designate a 3-phase source is 208V / 120 V ~ 50 Hz where 208 V is the line voltage and 120 V the phase voltage. Nov 4, 2023 at 3:01
• Thanks again! Just a new thought developed. Model A draws: 4400/(208×0.9)=23.5A If I have Qty5 of Model A and I distribute them on my panel as: PhaseA- Connect Qty2 PhaseB- Connect Qty2 PhaseC- COnnect Qty1 Will a simple vector sum work: Additional load on PhaseA: 2*23.5=47A Additional load on PhaseB: 2*23.5=47A Additional load on PhaseC: 1*23.5=23.5A Nov 6, 2023 at 1:23
• Should the loads be delta-connected to a 3 x 208 V source, the current would be double in the two phases with two loads as compared to that in the phase with one load. The line currents would be the vector sum of the related phase currents. Nov 6, 2023 at 5:14