This "horsepower inflation" really needs to be reigned in. A few
years ago, we started seeing 15 amp @220VAC compressors being
rated at 5HP, then 6HP and now I see them rated at 6.5 HP.
As Randall brought up, this should be impossible for a 15 amp,
115VAC motor to be rated at 4.5HP (i.e., with 746 watts equal to
1 "electrical" HP).
These inflated ratings are possible (tolerated?) due to
electrical motors being able to output more power for short
periods of time without overheating/stalling against a temporary
obstacle (like a knot in a large log in a wood chipper, or
something like that).
Assuming 100% efficient power conversion (which is impossible
above absolute zero), maximum HP/current specs would be:
load 120VAC 220VAC
15 amp 2.4HP 4.4HP
20 amp 3.2HP 5.9HP
25 amp 4.0HP 7.4HP
30 amp 4.8HP 8.6HP
In order for a 120VAC compressor to realistically output 4.5HP,
it would need to draw around 30 amps.
The reason manufacturers can get away with these inflated ratings
is due to something called a "special purpose" or "definite
purpose" specification. If a motor does not have this type of
specification on it, it must be able to to deliver its rated
power 24 hours a day, 7 days a week.
If you look at the plate on your compressor motor, it most likely
says "definite purpose compressor motor," or something very
similar.
If motors are rated realistically, most are able to output 1.5 to
2 times their rated output for brief periods (e.g., a 2.25
horsepower motor could overload and produce 3.4 HP for maybe 15
minutes, and 4.5 HP for maybe 1 minute).
So, assuming that this 115 VAC, "4.5HP" compressor motor actually
is a continuous rated 2.25HP motor, if overloaded momentarily
(starting up against a compressed tank, or just before kicking
out at the maximum rated PSI), a 15 amp motor could draw 25 - 30
amps momentarily.
It also is not uncommon for some types of electrical motors to
produce 4 or 5 times as much torque at startup, than when running
at full speed (naturally the amperage consumed at startup will be
much higher, and the motor can overheat if it does not reach full
speed quickly).
So, the long and short of this is, I suppose, that when wiring
for electrical motors, you really cannot assume the current
ratings on the motor, and you should assume significantly higher,
short-term loads may occur.
When running wire for larger motors, a rule of thumb is to assume
125% of rated amperage when computing minimum wire size and
breakers-- so in this case, your 15 amp motor would require a
18.75 amp supply, and this would mandate a 12AWG run.
Length of run is important -- one way distances for a 2% drop at
120VAC:
amp 14awg 12awg 10awg
15 30ft 45ft 70ft
20 20ft* 35ft 55ft
The asterisk on the 20 amp/14awg run indicates that it does not
have sufficient ampacity for the load.
You should be fine with 12awg cable as long as the distance from
the breaker is less than 40ft. If you sustain a prolonged
voltage drop of greater than 2%, the motor's output and lifespan
will be diminished. Technically, for electrical motors, you
should not have more than a 1.5% voltage drop.
This is why one listed noted that he did not trip the breaker
when using an outlet close to the breaker, while he did when
using one further away -- the additional length of copper wire
turned enough current into heat on the way to the motor that it,
combined with the motor's draw, exceeded the breaker's capacity.
If you want to actually test the circuit, place a meter on the
line before starting the motor, and then again while the motor is
running (e.g., if you see a full 125 VAC at the motor terminals
before it starts, you should no less than 123 VAC while the motor
is running). When the motor is starting, it is permissible to
see up to a 7.5% voltage drop, depending on the motor.
This probably is more than you wanted to read, and I apologize
for that, but wiring electrical motors properly is more involved
than wiring for incandescent bulbs.
regards,
doug
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