Circuit breaker size for motor
- Thread starter Fluke631
- Start date
Started a brand new 5 HP compressor today. Incoming rush 109 amps. Customer was speechless. He had a 30 amp breaker with 12 or 14 gauge wire. Before I left he was going to run a 50 amp breaker with 8 gauge wire. Should be much better for him in the long run. Just food for thought.
The question is missing a few pieces of data to properly calculate wire gauge. Assuming your motor is a 230V 5HP motor your FLC (NEC table Table 430.248) should be about 28A and as such you may be able to use 10AWG copper wire.
The wire is sized based on FLC (code book). Your over load protection should be sized based on FLA nameplate on the motor. Although I suspect a compressor should already have overload protection.
The wire is sized based on FLC (code book). Your over load protection should be sized based on FLA nameplate on the motor. Although I suspect a compressor should already have overload protection.
Maybe you should educate me a bit. Let say I do use a 8 gauge wire, to put a 30 circuit breaker will not hurt my system right, breaker will trip if there is a short or overload, like I said maybe I am wrong.
So why a bigger circuit breaker?
I just need to understand
A run of 8 gauge and 30 amp breaker will work just fine. I run my 3hp tablesaw on a 30 amp breaker with 10 gauge wire without any trouble of a breaker tripping.
Bigger is not always better, if the motor does not have overload protection built in then the motor windings will become the fuse and not trip the breaker. I have had to rewind many motors in an industrial setting because the electricians up the overloads higher than they should to stop nuisance trips, when there are other problems.
Fluke631, if you are really concerned about the wiring you can get an electrical permit and have it inspected by the ESA.
Magicman
Well-known member
Lifetime Member
Equipment
M4900 Utility Special 4WD e/w FEL & 1530 John Deere "Traveling Man"
It always surprises me when I read/hear people who believe a circuit breaker protects the appliance or that the appliance wiil be damaged by an improperly rated CB.
It’s time for everyone to understand that a CB’s only purpose is to protect the WIRE which serves the appliance. So a CB should be rated based upon the gauge of WIRE. The CB does NOT protect the appliance.
The gauge of WIRE should be sized/rated appropriate to the expected electrical demand of the appliance. Good practices dictate that the wire should be rated 120% above the max electrical demand of the appliance.
So,... if the appliance needs 21.5 Amps then the next size wire ga UP should be selected for 120% of 21.5A which is 25.8A Here’s a chart to assist you:
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/
Notice that the ambient temperature should be considered.
Notice that the choice of conductor can affect matters (copper vs aluminum).
I suggest 8ga wire and a 30 Amp CB.
OP, I see you think you have “on hand” a 30A breaker. I hope you realize 230 volt circuit will require a “double” breaker.
Keep in mind that old breakers can lose their calibration. Some old breakers (and their cabinets) should be CONDEMNED (such as Federal-Pacific. See: https://www.angieslist.com/articles/are-federal-pacific-circuit-breaker-panels-safe.htm )
Hope this helps.
It’s time for everyone to understand that a CB’s only purpose is to protect the WIRE which serves the appliance. So a CB should be rated based upon the gauge of WIRE. The CB does NOT protect the appliance.
The gauge of WIRE should be sized/rated appropriate to the expected electrical demand of the appliance. Good practices dictate that the wire should be rated 120% above the max electrical demand of the appliance.
So,... if the appliance needs 21.5 Amps then the next size wire ga UP should be selected for 120% of 21.5A which is 25.8A Here’s a chart to assist you:
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/
Notice that the ambient temperature should be considered.
Notice that the choice of conductor can affect matters (copper vs aluminum).
I suggest 8ga wire and a 30 Amp CB.
OP, I see you think you have “on hand” a 30A breaker. I hope you realize 230 volt circuit will require a “double” breaker.
Keep in mind that old breakers can lose their calibration. Some old breakers (and their cabinets) should be CONDEMNED (such as Federal-Pacific. See: https://www.angieslist.com/articles/are-federal-pacific-circuit-breaker-panels-safe.htm )
Hope this helps.
Thanks a lot for clarifications, yes I do have a 30 breaker (double post of course) on hand and 8 gauge wire. Because I am not an electrician I figure I could ask here and to be honest I was a bit confused about the proper size circuit breaker after reading some posts. Thanks a lot for the great information.
I would be more worried about the wire size and length of wire ran to the machine. Smaller wire causes voltage drop. I have also seen breakers that were not sized properly and overtime heated up internally causing an electrical arc or melt down. I was recommending to you what I have seen in the field that works the best with little to no issues. Everyone has their own opinion. For a typical home compressor this is what I recommend to every home owner and they have had years of dependable equipment. I watched a man run 14 gauge wire 200 feet from his house to the compressor and wondered why it didn’t run. The voltage dropped from 230 to 115 before tripping the breaker. Hope this helps.
On another note always watch what size air hose you use as well. I have seen a lot over the years. A customer had 100’ of 3/8 line outside diameter hose to the da sander, it would not maintain proper rpm. The hose was too long, too small and caused pressure loss making the machine lose its speed. I shortened his line to 20’ and 1/2” and wallah he did not have any problems. Similar logic to electrical wires.
GreensvilleJay
Well-known member
Equipment
BX23-S,57 A-C D-14,58 A-C D-14, 57 A-C D-14,tiller,cults,Millcreek 25G spreader,
the 'hoses' numbers also apply to gardening !! Wife didn't understand that ONE 1000' length of 1/2" weeping hose wouldn't water her garden. I broke it into 8 , 150+-' runs, fed from 1 1/2" header from 2" feed from tank.
Exactly! Good point Jay
I also hate hose reels, they leak, there reduced in size to mate to the machine, and they are expensive. I love a nice Ralley Wheel painted grey with chrome caps and rings to hang my hose from. Just makes the garage lol.
Last edited:
The link I posted above also provides for length of wire-run.
It is highly UN-likely that a residential installation will run into this problem.
Also, the recommendation I offered (8ga/30A CB) would more than cover up to 100’ wire runs, (because as can be seen in that link above, but even more specifically in this one, a typical 50’ run only requires a 10ga wire and I was recommending an 8ga wire): https://www.cerrowire.com/products/resources/tables-calculators/voltage-drop-tables/
If the OP already has on-hand 8ga copper and a 30A double breaker he should be just fine if he keeps the wire-run 100’ or less.
It is highly UN-likely that a residential installation will run into this problem.
Also, the recommendation I offered (8ga/30A CB) would more than cover up to 100’ wire runs, (because as can be seen in that link above, but even more specifically in this one, a typical 50’ run only requires a 10ga wire and I was recommending an 8ga wire): https://www.cerrowire.com/products/resources/tables-calculators/voltage-drop-tables/
If the OP already has on-hand 8ga copper and a 30A double breaker he should be just fine if he keeps the wire-run 100’ or less.