The Multiple Ratings on One Nameplate

Why this matters

Techs treat a nameplate as a list of facts about one machine, which is why the numbers on it look like they should agree and are confusing when they do not. They are not one kind of fact. A plate carries at least six different kinds of number, written by different people for different readers, and two of them contradicting each other is normal rather than a printing error.

Once you can sort a plate's numbers by kind, most of the confusion disappears and so does the most expensive mistake in the trade, which is sizing something from a number that was never a sizing number.

The numbers that are not sizing numbers

Start with the exclusions, because this is where the money is lost.

Locked-rotor, inrush and momentary values. These describe a transient lasting a fraction of a second. They exist so that protective devices can be selected to tolerate a normal start without nuisance tripping, and so that supply behavior during starting can be evaluated. They are not a steady-state expectation and they never size a continuous-duty anything. The reason this one catches people is that it is usually the largest number on the plate, and the instinct under time pressure is to reach for the biggest number as the safe choice.

Nominal designations. Nominal tonnage, nominal voltage, nominal pipe size, nominal horsepower classes. These are identification labels, not measurements. A nominally-rated unit does not deliver its nominal figure at your conditions and was never claimed to. Sizing a system from a nominal designation rather than from a rated capacity at stated conditions is one of the most common causes of equipment that runs constantly and never satisfies.

Capacity at rating conditions. A capacity figure is a design point measured at a stated reference condition, and it is a true statement about a test rather than a promise about your site. Every capacity figure needs its conditions read with it, and used at your conditions it needs the published correction applied.

Any field beginning with maximum. A maximum is a ceiling you stay under, never a target you design toward. Reading a maximum overcurrent device rating as "this is the size it takes" rather than "this is the largest permitted" is the single highest-consequence misread on any plate.

Test and proof values. Some plates and tags carry the pressure or voltage the equipment was tested at, which is deliberately higher than anything it may be operated at. A test value proves the equipment survived a controlled one-time condition under factory conditions; it is not an operating limit and it is not a safety margin you may spend. Operating a vessel or a circuit anywhere near a stated test value is how a component that passed its test fails in service. When two pressure figures appear together and one is noticeably higher, assume the higher one is a test value until the wording proves otherwise, and confirm before you use either.

That is five classes on the plate that will not answer a sizing question. What is left is much smaller than the plate looks.

The six kinds of number

A seventh property cuts across all six: a rating can be conditional, valid only under stated conditions of use from the listing or the manufacturer. A conditional rating that is being used outside its conditions is not a rating you may rely on, whatever its kind.

Why the numbers disagree on purpose

They were written for different readers. The circuit sizing values are written for whoever installs the branch circuit. The current ratings are written for whoever services it. The capacity figure is written for whoever selected it. None of those three people needs the others' numbers, so nobody made them agree.

They describe different subjects. An assembly's plate describes the assembly. A motor inside it carries its own plate describing the motor. A compressor carries a third. All three are correct and none of them describes the same thing, so comparing a component's current rating to the assembly's circuit sizing figure is comparing two different subjects.

They are measured at different moments. A continuous limit and a transient value describe the same machine one second apart. There is no reconciliation to perform.

The practical rule: before you are confused by two numbers disagreeing, check whether they are the same kind, about the same subject, at the same moment. If any of the three differ, the disagreement is expected and means nothing.

Service factor, the trap that looks like headroom

A service factor is a multiplier stated on many motor plates indicating the motor can be operated above its rated output under stated conditions. It looks like free capacity and techs treat it as a design margin. It is not.

Operating in the service factor range is permitted, not free. Motors run in that range typically operate at reduced efficiency and at higher winding temperature than at rated load, and elevated winding temperature is the primary driver of insulation life. A motor deliberately loaded into its service factor as a normal operating condition has a shorter life than the same motor loaded at its rating, and the service factor was never intended as a way to select a smaller motor.

Use it the way it was meant: as tolerance for occasional excursions, and as an explanation for why a measured draw slightly above the rated figure is not automatically a fault. Do not use it as sizing headroom, and be aware that a service factor's stated conditions frequently include an ambient temperature limit, so a motor in a hot space may not have the margin the plate appears to give.

A worked sort

A plate on a piece of packaged equipment carries seven numbers you could plausibly reach for. Say they are: a rated capacity at stated conditions, a minimum circuit ampacity, a maximum overcurrent protective device rating, a compressor rated-load current, a compressor locked-rotor current, a maximum working pressure, and a nominal capacity designation in the model string.

The decision: a clamp meter on the compressor reads high and the tech needs to know whether that is a finding.

Sorting the seven against that decision. The rated capacity is a design point about output, and it is not a current. The minimum circuit ampacity is a floor about the conductor. The maximum overcurrent device rating is a ceiling about the protective device. The maximum working pressure is a limit about the refrigeration circuit. The nominal designation is a label. Five of the seven are about a different subject entirely.

What survives: two of the seven, the rated-load current and the locked-rotor current, are about compressor current. And only one of those two is about a running measurement, because locked-rotor is a starting transient and the clamp is on a running machine. So one number governs the comparison: the rated-load current.

Then the interpretation, which is where the kind matters again. A running draw above the rated-load figure is not by itself a verdict, because rated-load current is a test-condition value taken at stated conditions and a compressor legitimately draws more when supply voltage sags or when it works against elevated head pressure. So the comparison produces a question, not an answer: is the supply inside its stated range, and is the system head where it should be. If the supply is in range and the head is normal, the elevated draw is a real finding about the compressor or its load. If the supply is low, the elevated draw is a symptom of the supply and the compressor may be fine.

What the wrong sort produces. A tech who compares the running draw to the locked-rotor figure finds it comfortably below and concludes everything is normal, because a running draw is always far below a starting transient. That comparison always passes, on every machine, including one that is failing, which is exactly what makes it dangerous rather than merely useless.

How to verify you have sorted a plate correctly

Say the kind out loud with the number. "This is a ceiling," "this is a floor," "this is a design point at stated conditions." Any number you cannot classify is a number you are not ready to use, and the fix is the manual's data section or the listing information rather than a guess.

Check the subject of every number before comparing two. Write the component beside each value. Two numbers about different components do not constrain each other, and a comparison between them will produce a confident conclusion about nothing.

Look for the conditions clause. Design points, service factors and many limits carry stated conditions in small type or in the manual rather than on the plate itself. A number used without its conditions is a different number.

Ask what would make this comparison always pass. If you cannot imagine a machine that would fail the check you just ran, you are comparing across kinds, and the check has no diagnostic value no matter how good the result looks.

References

• Trade-standard practice for equipment nameplate ratings, listing marks and conditions of use from recognized testing laboratories
• Manufacturer documentation practice for stated rating conditions, service factor and published correction factors
• See related: How to Find the Rating That Actually Governs; The Nameplate and What Every Field Tells You; Measuring Current Draw Against the Rating