I am working with a fractional hp motor which has a handy chart in its
install paperwork. I put it on my Web site for general use. It shows wire
sizing to use per horsepower per voltage per length of wire run. For example,
a 2hp motor at 115V at 50 feet should be wired with 4 gauge wire. - GWE
It's pretty conservative. By this chart a 1/4 hp motor
couldn't be run on the common 15A circuit. And it is
certainly more conservative than other data I have on wire
size, length, and acceptable voltage drop. Perhaps, it is
intended for direct wired motors that are heavy duty and
George, Are you sure you're reading it right?. I haven't connected to
Grant's web site to look at the chart - however *my* chart says a 1/4 hp
motor draws 5.8 amps fully loaded, on a 115 v circuit.
Yes, his doesn't give amps. It gives wire size and feet for
various motors sizes. It says a 1/4 hp 115V needs 14 gage
for 25 feet, 10 gage for 50 feet, and 8 gage for 100 feet.
15 amp circuits are often 100 feet long and certainly more
than 50 feet and use 14 gage wire.
It says a 1 hp needs 10 gage for 25 feet and 6 gage for 50
feet. My old Craftsman saw with a Sears 1 hp 12.2 amps
motor is built on a movable base that has a 25 foot 16 gage
wire extension wire. Currently, it is plugged into an outlet
that is at least 25 feet from the panel (and probably 14
gage wire). My father used this saw extensively from the
1950's to late 1980s with this same cord and never had motor
One curiosity. He apparently got the extension cord from
Sears at the same time that he got the saw as the cord is
marked 16/2 300V Motor Cord.
Charts of this type giving motor hp or motor kva instead of amps are usu.
based on the amount of current a motor would "draw" if working fully loaded.
The wire sizes given in such charts are that which will cause a certain
percentage of voltage drop (usu. 2%) over a given length of wire. For
example, a fully loaded 1/4 hp motor would draw about 5.8 amps from a 115 v
single phase circuit. A chart of mine for *110 v* says that 6 amps drawn
from a 110 v circuit would require #14 wire for a 2% v drop over 50 feet.
This goes to #12 wire if the distance is 75 feet and to #10 for 100 feet.
This is very close to the information you mentioned from Grant's chart. So,
Grant's chart is conservative - it may be based on a 1% voltage drop or on
less efficient (imported ?) motors. It may also be "code". There are many
variables that cause recommended wire sizes to vary, but you will never go
wrong if you always stay on the conservative side.
Perhaps Grant's chart is based on starting current which is typically
quite a bit higher than run current, even at full load. While to brief
in proper operation to overheat the wires, I have seen a case where a
small generator could keep an appliance running but couldn't start it.
The chart says it is from Dayton Electric Co., so I guess
some one could ask them. Based on just the figures given
for 1 hp and under single phase 115V, we know it isn't for
2% voltage drop. And it isn't based on inefficient motors,
reasonable starting currents, and practical knowledge, based
on other published data and experience. For example, 1/8 hp
motors don't require 14 gauge wire, they run just fine on 16
gage and even 18 gauge wire. They even run just fine on 14
gauge extension cords that are 200 feet long.
Another example is my compressor is rated at 5 hp, but we
know that is a lie and is probably close to 1 hp. It runs
just fine from a building that is supplied by about 100 feet
of 10 gauge wire. That is in stark contrast to the chart
that indicates 4 gauge wire would be required for a 1 hp
Your description of "runs just fine" is not quite an accurate appraisal. In
fact it is quite vague. Just because a motor runs doesn't mean that it is
wired correctly or safely. "Runs just fine" doesn't imply what load is
imposed on the motor. Phrases, like "fully loaded", "2%", "1%",
"efficiency", etc. have exact meanings more or less as they are used to
describe conditions of operation. Before you poo-poo Grant's chart or any
other published standards, you are going to have to come up with a better
argument, than "runs just fine".
You're right about one thing - 14 gage is the minimum wire size this
chart goes down to. I think it says you can't wire a circuit with
smaller wire than 14 gage and still be to code. - GWE
George E. Cawth>
Excuse me! Runs just fine doesn't me just runs; you could
accurately assume that runs just fine means it runs as
intended with no problems. Runs just fine also means that
the motor runs within the temperature intended. But, excuse
me, next time I run my compressor I'll measure the voltage,
will that tell you anymore about how my motor is running?
How the hell would I measure the load on the compressor
motor. The load is the compressor, if the load is 90, 80,
or 70 percent of capacity, what do I care, that's determined
by the manufacture. You think they reduced profits by
installing a motor that runs at 50 percent capacity?
There is no safety issue, if the load is too much for the
circuit, the breaker will trip and protect the wire, if the
motor get too hot, the internal fuse or break will switch it
off and protect the motor.
I didn't poo-poo the chart, for all I know it is correct for
its intended purpose. What I did was dispute some of the
possible bases for the chart. If you can't figure out that
a 1 hp motor won't run fine on 10 gage wire when 4 gage is
require, then you shouldn't bother with replies.
It's funny that you didn't offer any real information or
Good point. The Dayton chart doesn't say, but if it is for
aluminum, that could explain things. All the rest of my
information is for stranded copper. Does it make a
difference if it is hard copper or soft copper? Where do
you get house wire in different hardnesses, and why would
You are probably right, the chart is just for installed
wiring. However lots of motors of 1/8 hp or greater
capacity are run on extension cords that are as long or
longer than the maximum run listed for 14 gage wire.
George sez: " It's funny that you didn't offer any real information or
The "real" information is in the standard wiring data and charts that are
published for the same purpose as Grant's chart. Grant was kind enough to
put forth information for us to use. There is really nothing to be gained
if you contest such standard information with examples that appear to break
the rules and still work ok. Now as for insight - I'm afraid that insight,
like beauty, is pretty much in the mind of the beholder. I can't give it to