Horsepower vs Current Rating

Hi Group!
Wondering if anyone in the group can explain the difference between a
current rating and a horsepower rating on magenetic starters, disconnect
switches and MCC switch compartments.
TIA
Fred
Reply to
Fred
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If I understand your question
Current is amps that a given device can withstand. HP rating is the largest motor that the device can withstand. Motors can draw up to 6 times running current or FLA to get started. Really bad on contacts if not seated properly.
Does not matter if the device is a disconnect or a motor starter. Most electrical stuff in the US is sold based on the fact you will use the switch/disconnect/starter to interrupt or make under the maximum condition.
Reply to
SQLit
Generally, when you size the above equipment, you do so based upon it supplying a motor of some design horsepower. The relationship between this horsepower and the equipment's current (and voltage, of course) rating is made assuming a conservative (i.e. low) value for efficiency. This is because there is the possibility that a motor might be replaced with one having a lower efficiency, resulting in higher full load and starting currents, which could overload its controls without this margin of safety.
When the motor overload protection is set, it is based upon the specific motor's full load current. This must be set as close to the actual motor's nameplate rating and changed if the motor is replaced with another of different specs. Since a highly efficient motor will draw less current at maximum mechanical load, the overload protection must be set to this lower level or it may be damaged in the event of a mechanical overload.
Reply to
Paul Hovnanian P.E.
One Horsepower = 746 watts
Technically Speaking, this usually refers to the actual power consumption or true power. Electric Meters (like the kind your utility provides for your house) will always read true power. A fancy commercial meter might read more parameters.
The VA rating (Volts x Amps) will always be higher for a motor depending on the p.f. (powre factor). This is also called apparent power since it reflects a higher current flow that is used to establish the magnetic fields within the motor. During each sinusoidal AC cycle, extra power is extracted from the source and then returned to the circuit as the field alternately expands and contracts.
pf = 100 x (True Power WATTS/ Apparant power (VA))
A perfect resisitance load will have power factor of 100%.
A motor or any load with coils will always have a power factor of less than 100%. With small scale residential loads, the utilities usually don't care, but in a commerical or industrial billing situation, you usually have to pay a penalty for having a low power factor.
Beachcomber
Reply to
Beachcomber
I think a one horsepower rated motor is rated to PUT OUT one (mechanical) horsepower. The 'actual power consumption' (electrical) will be higher by an amount that depends on the efficiency of the motor (factors like core and copper losses, windage, slip). And, as you pointed out, input current will be even higher due to reactive power. Particularly with small single phase motors, the current can be way higher than what is calculated from (mechanical horsepower/line voltage). Here in Canada the CEC gives figures for design purposes, the currents are huge, I believe it gives 16A for a 1hp single phase 120V motor. That 1920VA is nearly triple (or 257% or so) what the simple calculation would give.
j
Reply to
operator jay
Most
But a rating on a disconnect (or similar) in Amps would correspond to the load's the nominal rated input current (such as a motor nameplate FLA 'full load amps' or RLA 'running load amps' value and not the inrush). Correct?
The real maximum condition would be a fault condition...
j
Reply to
operator jay
hi!. try to check this site. heres the link,
formatting link
might got the answer to ur queston. i hope that one helps, even if not for your problem but on other situations!
Fred wrote:
Reply to
nooks!!
Disconnects and controllers, in general, have to be rated for the motor HP so they can withstand opening at the starting (locked rotor) current. (They will have a horsepower rating.) Disconnects, in general, also have to be have a current rating of 115% of the motor full load amps to withstand the operating current.
Incidentally, the circuit is protected from overload by the "motor overload protector" which is typically a motor starter with overload heaters. Fuses/circuit breakers protect the circuit from short-circuit current and can be much larger than the motor FLA.
Correct, which is the available fault current, which can be thousands to hundreds of thousands of amps. The overcurrent protection (fuses, circuit breakers) have to be rated for the available fault current. This is a rating in addition to their normal operating current.
The "maximum conditions" referred to by SQLit was the locked rotor current.
Bud--
Reply to
Bud--
Jay makes a good point re the motor OUTPUT versus INPUT. Most of the 3 phase MCC sections I've come across with combination disconnect/starter sections have a Horsepower Rating label affixed. Seems this is perhaps a UL requirement...wondering how it is arrived at and what factor may be being used for an Amp vs Hp cross reference?
Fred
Reply to
Fred
The amp rating is a resistive rating (pf=1). The hp rating takes into account the power factor, inductive nature of the load, starting current, service duty, and the contact life in the device. It is not as simple as a formula relating the two. Look at the difference in size between an IEC and a NEMA device rated for the same horsepower. The NEMA device will be larger, because they use different assumptions about the above factors.
Ben Miller
Reply to
Ben Miller

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