How to Measure AC Compressor Inrush with a Clamp Meter

Why this matters

The air conditioner is the load that decides most residential standby sizes, and the number usually fed into that decision is the LRA printed on the condenser plate. LRA is a worst case measured with the rotor held still. The real compressor on that house, on that day, often starts on noticeably less, and sometimes on more. A tech who sizes from the plate alone sells a generator a size larger than the house needs, or quotes a soft starter nobody needed. One who guesses low gets the call on the first hot outage day, when the compressor stalls the engine and the unit shuts down on underfrequency. Ten minutes with a clamp meter replaces the guess with a reading.

Before you open anything

This is live-panel work, and the shop's electrical safety rules govern it. The reading has to be taken while the compressor starts, so the conductors are energized when it matters. Under NFPA 70E (current edition), that is energized-work exposure: a qualified person only, the shop's risk assessment done before the cover comes off, and the arc-flash and shock PPE that assessment calls for. A tech who is not qualified under the shop's program does not take this reading; it goes to one who is, or to the electrician on the job.

Place the clamp dead, read it live. Open the condenser's branch breaker at the panel, lock and tag it per 29 CFR 1910.333(b)(2), or 1926.417 where the job is construction work. Prove the disconnect's terminals dead with a meter tested on a known live source before and after the reading, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024). Only then do hands go into the box. Once the jaws are placed and the panel breaker is closed again, hands stay out of the disconnect until the circuit is locked out a second time.

Keep off the refrigerant side. Gauges, service valves and the control compartment belong to the HVAC tech. You need a conductor, a thermostat and a meter display, nothing more.

Step 1: Confirm the meter can actually do this

Not every clamp meter has an inrush function, and the ones that do do not all measure the same thing.

• Inrush mode triggers on the current step and reports an RMS value averaged over a short window, typically about 100 ms on meters built for motor work. The meter's manual states the window. That is the number to compare against LRA, because LRA is also an RMS figure.
• Peak hold captures the instantaneous crest, often over about 1 ms. On a sine wave the crest is 1.414 times the RMS, and the first half-cycle of a motor start carries an asymmetrical offset on top of that. A start that reads 63 A in inrush mode can read near 89 A, or higher, on peak hold. That number is real, but it is not comparable to LRA.
• Min/max on a normal AC range samples too slowly to catch a start reliably and usually reads low.

Check the meter is rated CAT III 600 V or better for the location, the jaws close fully around the conductor, and the battery is good. If the meter has no inrush mode, borrow one that does. A reading in the wrong mode is worse than no reading, because it goes on the sheet looking authoritative.

Step 2: Read the plate and set up the start

Write down the condenser nameplate before you touch the meter: compressor RLA (rated-load amps), compressor LRA, fan FLA, and whether a hard-start kit is fitted. A hard-start kit is a start capacitor and relay added to the compressor circuit to shorten the start. It changes the current shape, so a reading with one fitted only describes the unit as it stands. Note it either way.

Then set up a start you can trust. The compressor needs to be off long enough for its high and low sides to equalize. A compressor restarted against unequalized pressure, a reciprocating one on a PSC circuit especially, can stall and sit at locked rotor until its internal overload opens, which produces a reading near or above LRA that has nothing to do with a normal start. Most systems carry an anti-short-cycle delay of a few minutes in the thermostat or control board; wait it out, and give the unit at least that long off before each start.

Step 3: Place the jaws on one leg

  PANEL                   DISCONNECT, cover off
  40A 2P breaker          LINE   L1 o      o L2
  [L1]-----------------------------+        |
  [L2]--------------------------------------+
                                   |        |
                         pullout   |        |
                                   |        |
                          LOAD   L1 o      o L2
                                   |        |
                     clamp jaws -->(L1)     |
                     on L1 only    |        |
                                   v        v
                        whip to condenser contactor
                        (compressor + fan both fed)

  meter in INRUSH mode, display turned outward,
  read from beside the box, not in front of it

With the circuit locked out and proven dead, close the jaws around one load-side conductor only. On a single-phase 240 V condenser the same current flows out on L1 and back on L2, so either leg gives the start; clamp both and the fields cancel to near zero. Do not clamp the equipment grounding conductor. Clamping at the disconnect means the reading includes the condenser fan, which starts with the compressor. That is the right number for generator sizing, because the generator sees both.

Set the meter to inrush on a range above the LRA, turn the display so you can read it from the side, and step back. Remove your lock, close the panel breaker standing to the side of the panel rather than square in front of it, so a fault you did not find announces itself away from your face, and close the disconnect if it was opened.

Step 4: Call for cooling and capture three starts

Arm the meter's inrush capture, then have someone at the thermostat call for cooling with the setpoint well below room temperature. The contactor pulls in after any delay, the compressor starts, the meter latches a value. Write it down with the time.

Let the unit run about a minute and read the running current on the same leg in normal AC mode; it confirms you are on the right conductor and gives a sanity check against RLA. Then end the call, wait out the anti-short-cycle delay plus enough off time to equalize, and repeat. Three starts is the minimum. One reading can catch an unequalized start or a missed trigger; three that agree within a few amps describe the compressor.

When you are done, lock out at the panel again, prove dead the same way as before, remove the jaws, refit the disconnect cover, and restore. The finish check on the restore is a normal call for cooling that runs, so the household is not left with an air conditioner that stayed off because a pullout went back in loose.

Step 5: Compare with the plate, and know why they differ

The measured start is almost always below LRA, for three reasons that all point the same way:

• The rotor does not stay locked. LRA is the current with the shaft held, at rated voltage, after it settles. A real compressor starts turning in the first cycles and the current falls as it accelerates. An averaging window of about 100 ms already includes part of that fall.
• Voltage sags during the start. The utility transformer, the service drop and the branch wiring all drop voltage under a large current, and lower voltage means lower starting current. On a generator the sag is deeper, so a utility-fed reading is, if anything, slightly higher than what the generator will see for the same start.
• An equalized compressor starts lightly loaded.

It can come out above LRA too. A reading over the plate, or one that keeps climbing on repeated starts, points to an unequalized restart, a failing run capacitor, low voltage, or a compressor beginning to bind. That is a finding for the HVAC tech before it is a sizing input.

Worked example: a 3-ton condenser on a site walk

The plate reads compressor RLA 16.0 A, LRA 82.0 A, fan FLA 1.0 A. No hard-start kit. Outdoor temperature 88 F. The meter's manual gives a 100 ms inrush window.

Spread: 63.0 minus 59.8 is 3.2 A, so the three agree. Running current: about 15.3 A for compressor plus fan, against 16.0 plus 1.0, or 17.0 A on the plate; a unit running below plate on an 88 F day is normal. Worst start to LRA: 63.0 divided by 82.0 is 0.77, so this compressor starts on about 77 percent of its plate figure.

Carry the worst start, not the average. The generator has to survive the hardest start the compressor makes, so the sheet gets 63.0 A. In starting kVA at 240 V, 63.0 x 240 is 15,120 VA, about 15.1 kVA, against 82.0 x 240, or 19,680 VA, about 19.7 kVA from the plate. The difference is 19.7 minus 15.1, or 4.6 kVA. How that number turns into a generator choice, and how the NEC code letter bounds it, belongs to the sibling card. See related: Motor Starting kVA and the NEC Code Letter.

What the sheet records: nameplate RLA and LRA, three inrush readings and which was carried, the running current, the meter mode and its window, outdoor temperature, hard-start kit absent. A second tech must be able to tell from the sheet that 63.0 A was an inrush-mode reading, not a peak-hold one.

When a soft starter is worth quoting

A single-phase compressor soft starter is a control installed in the condenser that ramps the start current rather than letting the compressor take it all at once. Makers commonly claim reductions on the order of half or more; treat that as a claim and measure before and after.

Quote one when the measured start, not the plate start, is still the thing forcing a larger generator. On the example above, if the candidate generator's published motor-starting capability (read the way the sibling card describes) clears 15.1 kVA at an acceptable dip, there is nothing to fix; a quote for a soft starter here would solve a problem the house does not have. If the candidate falls short of 15.1 kVA and the next size up is the alternative, a soft starter that brings the start inside the smaller unit is usually the better answer for the customer. It is also the answer where two condensers could start together and the load manager cannot keep them apart.

What changes the call. A hard-start kit already fitted means the reading includes it; a soft starter normally replaces it rather than adding to it, per the soft starter maker's instructions. A compressor reading above LRA gets diagnosed before anyone quotes a starter for it. And the soft starter is HVAC equipment: the HVAC tech installs it to the compressor maker's and starter maker's instructions, and the equipment maker's warranty terms are checked first.

Close the loop. After any soft starter goes in, repeat the three-start measurement in the same mode on the same leg and put both sets on the record. A sizing decision built on a claimed reduction is still a guess.

References

• NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120 and the energized-work requirements; 29 CFR 1910.333 and 1926.417
• The clamp meter maker's manual for the inrush function and its averaging window
• The condenser nameplate and the compressor and soft starter makers' installation instructions
• See related: Motor Starting kVA and the NEC Code Letter; How to Size a Standby Generator From a Site Walk; Generator Sizing for Residential