Watt-Hour Meter CT / PT Multiplier

A transformer-rated meter does not read energy, it reads a scaled fraction of it, and the multiplier that converts the register to real kilowatt-hours is the product of two instrument transformer ratios and a register constant. The multiplier is a pure product, and that is exactly why it goes wrong: swapping a 200:5 current transformer for a 400:5 during a load upgrade doubles the correct multiplier, and if the billing record is not changed with it the customer is billed half, indefinitely. The same happens with a potential transformer changed on a voltage conversion. Neither error announces itself, because the meter keeps working and the register keeps advancing -- there is nothing to see in the field, and a billing error of this kind commonly runs for years in either direction before anyone catches it. THE CHECK THAT CATCHES IT IS DIMENSIONAL RATHER THAN CLERICAL. Take the metered demand, multiply it out to the primary, turn it into a current at the service voltage, and hold that current against the current transformer that is supposed to be carrying it. A multiplier off by a factor of two puts the implied load somewhere the service physically cannot go, and the discrepancy shows up in one line of arithmetic rather than in an audit. A current transformer is not passing two or three times its primary rating in normal service, so an implied current above it means the multiplier, the CT record, or the reading is wrong. This is the multiplier arithmetic for a transformer-rated installation and the plausibility check on it. It does not verify instrument transformer accuracy class, burden, or polarity: a reversed CT polarity, or a current transformer paired with the wrong phase's potential transformer, produces a wrong reading that no multiplier fixes and that this cannot see. It does not detect a meter wired to the wrong phase, a shorted CT secondary, or a blown PT fuse, which are the field failures. Ratio-correction and phase-angle-correction factors from the instrument transformer test report are not applied, and transformer-loss compensation, used where the metering sits on the low side of a customer-owned transformer, is a separate adjustment. ANSI C12.1, the instrument transformer test reports, and the utility's metering standard govern.

CT ratio = CT primary / CT secondary; PT ratio = PT primary / PT secondary; multiplier = CT ratio x PT ratio x register constant; billed energy = register reading x multiplier; implied primary current = implied demand / (sqrt(3) x line-to-line service voltage), held against the CT primary rating.

The transformer-rated metering multiplier relation, by name, with ANSI C12.1 named. Accuracy class, burden, polarity, ratio-correction and phase-angle-correction factors, and transformer-loss compensation are not applied. ANSI C12.1, the instrument transformer test reports, and the utility's metering standard govern.

A product of two ratios and one plausibility division; no metering standard text is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: ct_primary_a, ct_secondary_a, pt_primary_v, pt_secondary_v, register_constant, register_reading_kwh, demand_register_kw, service_voltage_kv, ct_ratio, pt_ratio, multiplier, billed_kwh, implied_demand_kw, implied_current_a, ct_utilization_pct

• Ratio arithmetic only accuracy class, burden and polarity are not verifiedthe instrument transformer test reports
• Balanced three-phase at the entered voltage the plausibility current assumes itthe utility's metering standard
• No transformer-loss compensation low-side metering of a customer-owned transformer is a separate adjustmentANSI C12.1