The NEC tables are purposely conservative because the NEC can't know the motor Eff, number of poles, power factor etc. of what you are going to use and how you will use it. But the tables in the NEC are really there for determining MINIMUM ratings of conductors and MAXIMUM ratings of OCPDs.

Don't know your purpose in trying to determine RLA, it's not really good for anything as it relates to the NEC. But from those formulae, notice that the one for RLA from HP and FLA from HP are identical. The difference is in the fact that for FLA, you are looking at the rating of the MOTOR, RLA you are looking at what the LOAD uses. So to determine RLA, the HP that you use would be the actual SHAFT HP under the rated operating load. If you don't know that, then you can't calculate RLA.

In some cases though, the mfr of a piece of equipment will determine an RLA value by measuring the shaft power (or measuring the shaft torque and speed, which would GIVE you the ability to calculate shaft HP). OEMs do that when they don't want people to know or get confused by HP values. A common place you see this is in Chillers , because the Chiller mfr. doesn't want people swapping out their motors by looking at a HP value on the nameplate since they size motors based on short time surges and long cooling periods that they themselves control. But when an OEM only provides RLA, then they have an added responsibility to provide the installing electrician with values for MOCP (Maximum Over Current Protection) and MCA (Minimum Circuit Ampacity), because technically, the NEC insists on an electrician sizing conductors from Table 450.250, which needs a HP value. If a piece of machinery comes with an MOCP and MCA, the electrician then just installs based on manufacturer instructions.

 

Thanks and cheers

The difference is in the fact that for FLA, you are looking at the rating of the MOTOR, RLA you are looking at what the LOAD uses. So to determine RLA, the HP that you use would be the actual SHAFT HP under the rated operating load. If you don't know that, then you can't calculate RLA.

Thank you, got it completely know. Love the simple way of explanation. Now I totally grasp it. :smile:

 

It's more complicated than your formulas suggest. The motor flux is basically constant with voltage and almost 100% inductive (power factor = 0, 90 degrees out of phase, all vars). Other than heating, there is no work. The load torque Is almost purely resistive (power factor 1.0, all kw). In practice flux is a few percent of total kVA at full load but dominates at no load. So as you can tell power factor drops off nonlinearly and rapidly as the load decreases. So you can calculate RLA but you need either the full motor model parameters or a power factor chart so you might as well just have a torque vs. current chart. You can estimate the motor model from a set of performance tests (locked rotor, no load, 25/50/75/100 dynos) using IEEE standard 111 but the calculations are long and tedious. From there you can make torque/current charts but most motor manufacturers have it all in a car database and can send you all this if you ask. So unless you need to calculate starting time or something else not typically done, or you are rewinding the motor to a new configuratio nobody uses that level of detail.

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