Electric motor horsepower & circuit ampacity & wire sizes vs. motor horsepower.

This article explains how to calculate the horsepower of an electric motor if we know its wattage or current draw in amps.

The article gives a simple formulat to convert electrical amps to horsepower and explains what data are actually needed to make an accurate horsepower calculation or rating.

We include wiring tables showing the circuit ampacity and wire sizes needed for electric motor circuits including for long wiring runs.

We explain the relationship between rated load amps (RLA), and full load amps (FLA) and lock rotor amps (LRA) ratings on electric motors.

These electric motor horsepower formulas, definitions, and ratings are suitable for determining the horsepower of an air conditioner compressor motor, a fan motor, a well pump motor and similar equipment.

How to Calculate Electric Motor Horsepower

Question: relationship of RLA and electrical motor horsepower

(Aug 2, 2014) Eng.M.Mossad2010@gmail.com said:

From the part of " FROM EQUIPMENT RLA # - Air Conditioning Equipment Capacity from Equipment RLA Numbers " in the article above can we find a relation between the equipment's RLA and the HP "horse power " for the Compressor motor?

This question was originally posted at COOLING CAPACITY, RATED

The steam operated electric motor generator shown here is described

at STEAM BOILERS GENERATORS CONTROLS, PRATT

Reply: relationship of RLA, LRA and FLA motor ratings & converting to horsepower

Mossad:

for a specific air conditioner or heat pump motor I'd obtain the horsepower rating first by looking at the BTUh rating on the data tag or encoded in the model number. For example, 18,000 BTUh is equivalent to 2 HP (horsepower) or 24,000 = 2.5 HP.

You can find the basic rating data for an air conditioner compressor motor (or for other HVACR motors) by noting information on the data plates we describe in detail

at DATA TAGS on AIR CONDITIONERS

Let's also note for other readers that RLA = running load amps - the current drawn normally during the operation of an electric motor.

RLA will always be a smaller number than LRA which is lock rotor amps - the start-up current draw. FLA = full load amps, a fixed number that will also always be higher than RLA. Refrigeration engineers generally state that if we know LRA we can estimate RLA as follows:

RLA = LRA / 5.56

(for most electric motors LRA is about 5 times greater than its RLA/FLA and some may be 7 times greater.

An accurate answer to the question of translating RLA into horsepower requires more data: the voltage leel and the number of phases of electrical current. And comparing HVAC compressor motors by amperage ratings alone may be misleading.

Compressors need to be compared for the same rated conditions and same area of application, as motor designs and uses vary.

But for typical residential air conditioner systems and heat pump systems we can assume we're talking about 240V and 2-phases of power.

Since the maximum rotating power or force exerted by an electrical motor is that needed to get the motor spinning we should look at the motor's LRA or FLA rating. The RLA or running load current draw in amps will always be a smaller number than the motor's maximum. We'll give the motor horsepower as its maximum in our examples below but you can choose any amperage rating to calculate an equivalent horsepower number.

Details about locked rotor amp ratings and LRA FLA data tables are given

at TIGHT or SEIZED AC COMPRESSORS

Formula to Convert Electrical Amps to Horspower

1 HP (horsepower) = 745.7 Watts (round to 746 W)

To express watts in amps or current is not reasonable since these are two different measurement scales (Watts in power and amps in coulombs per second).

But if we know any two of the three basic electrical measures (amps, watts, and volts) we can calculate the third value. We can write the famous (and simplified) equation in any of the three ways using simple algebra:

AMPS = Watts / Volts  OR Watts = Amps x Volts OR Volts = Watts / Amps

E.G. 120 Watts / 120 Volts = 1 Amp of current.

For complete details about these basic electrical formulae

see DEFINITIONS of ELECTRICAL TERMS

How to Calculate Horsepower from Motor Amperage

We can easily express Amps in a formula to obtain horsepower as follows:

AMPS = ( Horsepower x 746 ) / Voltage

Example: For a 220V electric motor with a 40-Amp LRA or FLA rating

40A = (Horsepower x 746) / V

40 = (HP x 746) / 220

(40 x 220 ) = HP x 746

(40 x 220) / 746 = Horsepower

11.8 = Horsepower

So this is a 12 horsepower motor based on its LRA or FLA rating.

Here is a different example:

At 220V a 240kBTUh A/C unit will draw around 8 amps when running. Using the same calculation as above but finding the horsepower exerted when the air conditioner motor is running at normal load (that is not at start-up and not with abnormal head pressure),

8 A = (HP x 746) / 220 which calculates out as 2.4 HP

So the horesepower being delivered by our air conditioner when running at normal load is around 2.4 HP

Electric Motor Circuit Wiring: Horsepower Voltage, Starting Amps, Run Amps, Circuit Size

Notes to the table above

1. Watch out: the electric motor wire size and ampacity ratings in the table above are for lengths no greater than 25 ft. (10m) - for longer circuit runs see the next table below and also
   
   see SE CABLE & WIRE SIZES FOR LONG RUNS
2. Adapted from Sears, "Circuit Size for Individual Single-phases Motors", in Electrical Wiring Handbook, Sears, Roebuck & Co., April 1977
3. Definition of Electric Motor RLA or Rated Load Amps: the current the motor should draw at its rated horsepower when running under full load.
4. Definition of Electric Motor LRA or Locked Rotor Amps: the starting current drawn by an electric motor when it is first turned on
5. Older Definition of Electric Motor FLA or Full Load Amps: the current the motor should draw at its rated horsepower when running under full load, changed in 1976 to RLA - given above.
6. Watch out: Consult the wiring instructions and circuit amperage requirements from the manufacturer of your specific electric motor brand and model

Use Larger Wire Sizes for Electric Motors on Long Wire Runs

Notes to the table above

1. Run length from electrical panel to motor, copper wire
2. Other models of electric motors may offer the same horsepower at lower Amps requirements than shown in the table above
3. Other models of electric motors may offer greater horsepower at the same Amps requirements as shown in the table above
4. Electric wire size increase requirements for long wiring runs is discussed at
   
   SE CABLE & BRANCH CIRCUIT WIRE SIZES vs AMPS
   
5. Voltage drop in electrical circuits is discussed at
   
   SE CABLE & BRANCH CIRCUIT WIRE SIZES vs AMPS
6. Watch out: Consult the wiring instructions and circuit amperage requirements from the manufacturer of your specific electric motor brand and model

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Reader Comments on Electric Motor Horsepower Ratings & Calculations

On 2017-03-13 by (mod)

Excellent points, Steve, thank you. We'll keep your comments in the article.

If you want to be identified as a technical reviewer/contributor use the page top or bottom CONTACT link to send me an email note.

Daniel

On 2017-03-13 by Steve - Horsepower is OUTPUT POWER. The electrical requirements is INPUT POWER.

Horsepower is OUTPUT POWER. The electrical requirements is INPUT POWER. To make those two things comparable you need to consider motor efficiency in the equation.

If you assume the efficiency is about 80% (pretty safe), then the Input power would be 1 / 0.8 or 1.25x the output horsepower, in order to deliver that much power. Also if the motor is three phase you can reduce the amps by 1.73 when converting from watts to FLA. There is more but this is the basics.

On 2016-02-25 by Anonymous - RLA = Rated Load Amps, not Running Load Amps.

RLA = Rated Load Amps, not Running Load Amps.

If motor is driving the max load it is allowed, it will run at RLA. Most compressor motors will run less than RLA because they are not typically fully loaded.

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Continue reading  at ELECTRIC MOTOR DIAGNOSTIC GUIDE - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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• SE CABLE & BRANCH CIRCUIT WIRE SIZES vs AMPS
• SE CABLE & WIRE SIZES FOR LONG RUNS

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