Motor Starting kVA and the NEC Code Letter

Why this matters

A standby sized on running watts carries the house all afternoon and then fails the first time the air conditioner calls. The compressor that runs at about 4.5 kVA asks for about 25 kVA for the fraction of a second it takes to break away, the alternator voltage sags, contactors chatter, and either the compressor stalls on its internal overload or the generator drops out on undervoltage. The customer calls it a bad generator. It was a load sheet that never asked how the biggest motor starts. This card owns that arithmetic for the shelf; the whole-house load method lives in 'Generator Sizing for Residential'.

Why locked rotor, not running watts, is the number

An induction motor at standstill has no back-EMF. Until the rotor turns, the winding looks to the supply like a transformer with a shorted secondary, and current is limited mostly by winding impedance. That locked-rotor current is commonly several times the running current, and it flows until the rotor comes up to speed.

Two things make it the controlling number on a generator rather than on the utility:

• The utility is stiff, the alternator is not. A pole transformer serving a few houses shrugs off a 100 A inrush with a small flicker. A residential alternator has far less short-circuit capacity, so the same inrush pulls its terminal voltage down hard, and the voltage regulator and the engine governor both need time to recover.
• Locked-rotor power factor is low. Most of the starting current is magnetizing and reactive, so the kW the engine sees during a start is only a share of the kVA the alternator must deliver. The alternator's voltage dip, which tracks starting kVA, is usually what fails first, though on a small air-cooled unit the engine can bog and trip on underfrequency too. A generator with plenty of kW can still dip a compressor contactor open.

That is why a maker's spec sheet publishes motor-starting capability as its own line, stated at a named voltage dip, separate from the kW rating. The kW rating answers "can it carry the house"; the starting line answers "can it start the biggest motor while carrying the rest".

Two ways to read starting kVA off a nameplate

From locked-rotor amps (LRA). Hermetic refrigerant compressors are marked with rated-load current and locked-rotor current rather than horsepower; NEC 440.4, in substance, requires that marking (check the current edition as adopted by your jurisdiction). Single phase:

starting kVA = LRA x volts / 1,000

Three phase multiplies by 1.732 as well.

From the code letter. A general-purpose motor rated in horsepower carries a code letter that brackets its locked-rotor kVA per horsepower. NEC 430.7(B) requires the letter and its Table 430.7(B) gives the ranges, as adopted by your jurisdiction:

starting kVA = hp x the table's kVA-per-hp for that letter

The table gives a range for each letter, not a single value. Size to the top of the range, because the motor in front of you may sit anywhere inside it.

That is the middle of the table, the rows most residential motors land on; letters A through E sit below it and P through V above it. Read the full table in the edition your jurisdiction has adopted.

Worked: the two plates on one site walk

Here is what the tech photographed at a ranch house with a 4-ton condenser and a submersible well pump. Values are this example's, read off these plates.

OUTDOOR UNIT DATA PLATE
  Volts 208/230   Phase 1   Hz 60
  COMPRESSOR   RLA 18.6     LRA 104
  FAN MOTOR    FLA 1.2      HP 1/4
  MIN CIRCUIT AMPACITY  24.5
  MAX FUSE/BREAKER      40

WELL PUMP MOTOR (control box label)
  HP 1.5   Volts 230   Phase 1
  CODE LETTER K     SF 1.15

STARTING KVA, WORKED AT 240 V GENERATOR OUTPUT
  Compressor: 104 A x 240 V = 24,960 VA
              = 25.0 kVA
  Compressor running: 18.6 A x 240 V = 4,464 VA
              = 4.5 kVA
  Pump, letter K = 8.0 to 8.99 kVA per hp
    low:  1.5 x 8.0  = 12.0 kVA
    high: 1.5 x 8.99 = 13.5 kVA  <- size to this

The compressor. 104 A times 240 V is 24,960 VA, so 25.0 kVA. Running, 18.6 A times 240 V is 4,464 VA, about 4.5 kVA. Check it the other way: 4.464 kVA times 5.6 is 25.0 kVA, so this compressor starts at about 5.6 times its running kVA. Using 240 V against a plate rated 208/230 V overstates the figure by about 4 percent (240 divided by 230 is 1.043), which is the conservative direction and the right one for sizing.

The pump. Letter K brackets 8.0 to 8.99 kVA per hp. At 1.5 hp that is 12.0 to 13.5 kVA. As current at 240 V, 12,000 VA divided by 240 is 50.0 A and 13,485 VA divided by 240 is 56.2 A, so this pump's locked-rotor draw sits somewhere from 50 to 56 A. Size to 13.5 kVA.

What the pair tells you. The condenser is the largest starting load on the site at 25.0 kVA, nearly twice the pump's worst case of 13.5 kVA. The generator must start the condenser while everything else already runs. On a well house, that "everything else" includes the pump, which is why the pump goes into the running total, not the starting one.

How the starting figure feeds the sizing

The working rule: the generator must carry the running total of every load except the largest motor, plus that motor's starting kVA, without dipping past the voltage the maker rates its starting capability at. Running load for the rest of the house comes from the site walk ('How to Size a Standby Generator From a Site Walk' and 'Generator Sizing for Residential'). The starting figure is then compared against the motor-starting line on each candidate unit's spec sheet, in the unit and at the dip that line is stated in, never against the kW rating.

NEC 702.4, as adopted by your jurisdiction (the subsection numbering inside it has moved between editions), in substance requires that where automatic transfer equipment is used, the standby source can supply the full load it picks up, or that a load management system limits what it picks up. That rule is written about load. It does not tell you the alternator will hold voltage through a compressor start, which is why the starting check is a separate line on the sheet and not something the code calculation covers.

Where the number changes

Two motors starting together. The working rule assumes motors start one at a time. At transfer they may not: when the switch closes onto the generator, the condenser, the pump and the refrigerator can all be calling at once. In this house that is 25.0 plus 13.5, or 38.5 kVA, momentarily. A thermostat's compressor short-cycle delay or a load management module that holds the condenser off after transfer is what keeps the real peak at 25.0 kVA. Confirm which one the site has rather than assuming.

A soft starter on the compressor. It reduces peak inrush by ramping the voltage or current. Use the figure the soft starter maker states for that compressor, or measure it after install ('How to Measure AC Compressor Inrush with a Clamp Meter'); do not apply a generic percentage.

A hard-start kit is not a soft starter. A start capacitor and relay add starting torque so the compressor breaks away sooner on a sagging supply. That shortens the start; it does not cut the peak the way a soft starter does, and a load sheet that credits it with a reduction is wrong.

Three-phase motors. Multiply LRA times volts by 1.732 as well. Forgetting it understates a commercial compressor by 42 percent.

A tight or tired compressor. LRA is the current at locked rotor, so a healthy motor passes through it quickly. A compressor with worn bearings, liquid in the crankcase or high head pressure against an unequalized system stays near locked rotor longer, and a start that worked on a cool morning fails on a hot afternoon.

Reading errors that ship a wrong number

• MCA used as starting current. Minimum circuit ampacity is a wire-sizing figure, built from 125 percent of the compressor RLA plus the other motors. At 24.5 A on this plate it understates the start by a factor of four.
• The fan's figure read as the compressor's. Data plates list both; the fan here is 1.2 FLA and has no LRA printed at all.
• The bottom of the code-letter range. 12.0 kVA instead of 13.5 kVA on this pump is 1.5 kVA of headroom the sheet claimed and did not have.
• The "three times running" rule of thumb. On this compressor it would give 13.4 kVA (4.464 times 3), against a real 25.0. The multiple varies motor to motor; read the plate.
• A plate that has been painted over or replaced. A compressor changed under warranty may not match the outdoor unit's data plate. Where the compressor's own label is reachable without opening a live panel, read it; where it is not, the outdoor plate is the record and the commissioning start test below is the check.

How to confirm the number on site

The plate gives a ceiling; the start gives the truth. On the utility, a clamp meter with an inrush mode on the compressor common lead reads the actual peak. Expect the peak at or under the LRA; a reading well above the LRA means the meter or the lead is wrong, and a start that stays near locked-rotor current for longer than a fraction of a second points at the compressor's condition. Clamping a running condenser lead is energized work: it is done with the service panel of the condenser removed only by a tech qualified for that exposure, in the PPE the shop's NFPA 70E program sets.

At commissioning, the second check is the generator itself. With the house on the generator, let the condenser start on a thermostat call and read the generator output voltage on a meter set to min hold. A dip that recovers within a second or so and leaves the contactor pulled in is a pass; a contactor that drops out, a compressor that trips on internal overload, or a generator undervoltage shutdown is the starting calculation telling you it was wrong. The case where that happened is written up in 'The New Standby That Could Not Start the Heat Pump'.

References

• NFPA 70, National Electrical Code, as adopted by your jurisdiction: 430.7(B) and Table 430.7(B) (code letters), 440.4 (hermetic compressor marking), 702.4 (optional standby capacity); check the current edition.
• NFPA 70E, current edition, for energized measurement work.
• See related: Generator Sizing for Residential; How to Size a Standby Generator From a Site Walk.
• See related: How to Measure AC Compressor Inrush with a Clamp Meter; The New Standby That Could Not Start the Heat Pump.