Good power review starts by knowing which system you are actually looking at. DC, single-phase AC, and balanced three-phase AC do not use the same assumptions, and many field mistakes come from mixing their formulas or units. This guide keeps those relationships clear for common U.S. voltage classes and 60 Hz equipment.
DC and AC Do Not Behave the Same Way
Direct Current
In steady DC work, voltage and current do not reverse polarity periodically.
Core relationship:
P = V x I
Common DC examples:
- batteries
- DC control circuits
- electronics
- PV strings before inversion
Alternating Current
In AC systems, voltage and current vary with time and can include a phase angle between them.
That means AC review often needs:
- RMS voltage
- RMS current
- power factor
- real power
- apparent power
- reactive power
For utility-distribution and most building power work in the United States, the nominal frequency is 60 Hz.
Real, Apparent, and Reactive Power
In AC systems, these terms should stay separate:
- kW = real power
- kVA = apparent power
- kVAR = reactive power
They are related by the power triangle:
S² = P² + Q²
Where:
- S = apparent power
- P = real power
- Q = reactive power
Power factor is:
PF = P / S
If power factor is 1.0, real power and apparent power are equal. As power factor drops, current rises for the same real-power output.
Single-Phase Power in U.S. Work
Common single-phase and split-phase nominal values include:
- 120 V
- 240 V
- 120/240 V split-phase
Basic Relationships
For DC or a purely resistive AC load:
P = V x I
For single-phase AC with power factor:
P = V x I x PF
S = V x I
Q = V x I x sin(phi)
Example 1: 240 V Single-Phase Load
- Voltage: 240 V
- Current: 24 A
- Power factor: 0.92
Real power:
240 x 24 x 0.92 = 5,299.2 W = 5.30 kW
Apparent power:
240 x 24 = 5,760 VA = 5.76 kVA
This is why a single-phase motor or transformer load cannot be reviewed correctly with volts times amps alone unless power factor is known.
Three-Phase Power in U.S. Work
Common U.S. three-phase voltage classes include:
- 208Y/120 V
- 240 V delta
- 480Y/277 V
For balanced three-phase systems:
P = sqrt(3) x V_LL x I_L x PF
S = sqrt(3) x V_LL x I_L
Q = sqrt(3) x V_LL x I_L x sin(phi)
Where:
- V_LL = line-to-line voltage
- I_L = line current
Example 2: 480 V Three-Phase Motor Load
- Voltage: 480 V line-to-line
- Line current: 18 A
- Power factor: 0.86
Real power:
1.732 x 480 x 18 x 0.86 = about 12.88 kW
Apparent power:
1.732 x 480 x 18 = about 14.97 kVA
Again, kW and kVA are not the same value unless PF equals 1.0.
Line-to-Line and Line-to-Neutral Must Not Be Mixed
One of the most common mistakes in three-phase review is using the wrong voltage reference.
Examples:
- a 208Y/120 V system has 208 V line-to-line and 120 V line-to-neutral
- a 480Y/277 V system has 480 V line-to-line and 277 V line-to-neutral
If the load is connected line-to-neutral, use the phase voltage that the load actually sees. If it is a balanced three-phase load, use the three-phase line-to-line relationship.
Split-Phase Is Not Two-Phase
A typical U.S. dwelling service is usually 120/240 V split-phase from a center-tapped transformer.
That means:
- 120 V from either hot leg to neutral
- 240 V from hot to hot
It is not a true two-phase 90-degree system. Treating it that way leads to bad current and power assumptions.
Why Three-Phase Is Common for Larger Loads
Balanced three-phase systems are widely used because they:
- deliver power smoothly to rotating equipment
- reduce current for the same real power compared with a comparable single-phase arrangement
- help limit conductor size and voltage drop for larger loads
That does not make three-phase "better" for every application. It means it is usually the practical choice for larger motors, distribution equipment, and commercial loads.
Measurement Practice
When reviewing power values in the field, confirm:
- whether the meter reports real power or apparent power
- whether the voltage is line-to-line or line-to-neutral
- whether the current is per phase or totalized
- whether the system is balanced enough for simplified three-phase formulas
- whether the load is AC or DC
On unbalanced or distorted systems, measured data is more reliable than idealized hand formulas alone.
Common Mistakes
- Using the DC formula for AC loads with significant phase angle.
- Treating kW and kVA as if they are interchangeable.
- Using single-phase formulas on three-phase equipment.
- Mixing line-to-line and line-to-neutral voltage.
- Calling split-phase service "two-phase."
Practical Review Checklist
Before solving a power problem, verify:
- AC or DC
- single-phase or three-phase
- nominal voltage class
- measured or assumed current
- real power vs apparent power
- actual or assumed power factor
Summary
Power review becomes much more reliable when the system type is identified first:
- DC usually uses direct voltage-current power relationships.
- Single-phase AC needs power-factor awareness for most real equipment.
- Balanced three-phase AC uses the square-root-of-three relationship with line-to-line voltage.
- kW, kVA, and kVAR each describe different parts of AC behavior.
- U.S. voltage classes and 60 Hz assumptions should stay matched to the actual system being reviewed.