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.
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
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
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
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.
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.
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...
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.
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?
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.