Energy Meter Multiplier Formulas
Energy meter formulas: the multiplier of a CT and VT meter, the load from the pulse rate and the energy from the difference of two readings.
The formulas
| To find | Formula |
|---|---|
| Multiplier | M = (CT primary / CT secondary) × (VT primary / VT secondary) |
| Power from pulses | P = 3600 × N / (t × Kc) |
| Load | Pload = Pmeter × M |
| Energy | Eload = (reading difference) × M |
N is the number of pulses counted in t seconds, Kc the meter constant in pulses per kWh and M the multiplier that scales the meter value to the real value in the primary circuit. A meter with a direct connection to the supply has a multiplier of 1, and a meter on a current transformer alone has a VT ratio of 1. Some meters show the primary value and apply the ratios inside, so read the nameplate or the display before applying M again. The meter constant, the ratios and the readings here are examples.
Worked example: pulses on a meter with a 400 / 5 CT
Kc = 1,000 pulses per kWh, and 10 pulses in 20 s.
- At the meter, P = 3600 × 10 / (20 × 1000) = 1.8 kW
- M = (400 / 5) × 1 = 80
- Load = 1.8 × 80 = 144 kW
Worked example: a meter on a 200 / 5 CT and an 11,000 / 110 V VT
The reading rose by 12.5 kWh.
- M = (200 / 5) × (11,000 / 110) = 4,000
- Energy = 12.5 × 4,000 = 50,000 kWh
Multiplier of a CT-only meter by CT ratio
| CT ratio | Multiplier | Primary load for 1 kW at the meter |
|---|---|---|
| 100 / 5 | 20 | 20 kW |
| 200 / 5 | 40 | 40 kW |
| 400 / 5 | 80 | 80 kW |
| 800 / 5 | 160 | 160 kW |
Frequently asked questions
Why does a meter need a multiplier?
The meter sees a small current and voltage from the instrument transformers, so its reading is a fraction of the real value. The multiplier restores the full value.
What is the meter constant?
The number of pulses that the meter output gives for each kWh. It is on the nameplate.
How can I check the load with a stopwatch?
Count the pulses over a known time, apply the formula, and multiply by the multiplier. The method suits steady loads.
Does the multiplier change the billing?
Yes. The billed energy is the reading difference times the multiplier, so a wrong multiplier gives a wrong bill.
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