3 Hp Sigle Phaze Motor Breaker Size

[rbjscott]

What breaker size would be correct? I come up with 20 amp breaker. What do you think?

 

[John Hasler]

What breaker size would be correct? I come up with 20 amp breaker. What do you think?

That'll work.

 

[mksj]

There are various types of breakers and fuses, so there are a number of factors and allowances (range) for breaker and fuse sizing. Based on Nema Motor Data Calculator (see link) they specify a 35A Branch Breaker or 25A dual-element time delay fuse for a 230V single phase 3Hp motor. Probably want a type D Curve which have a 10-20x instantaneous trip current. This assume it is a full rated HP, and not "Peak Hp".

http://www.schneider-electric.us/do...-and-online-tools/nemamotordatacalculator.xls

One 3 HP, 230 volt single phase AC motor with a temperature rise of 40°C using an inverse time breaker and THW conductors.
Step 1: Motor FLC
17 A
Step 2: Running Overcurrent Protection
17 x 1.25 = 21.25 A minimum
17 x 1.40 = 23.84A maximum
Step 3: Branch Circuit Conductors
size conductors at a minimum of 125% of FLC
17 x 125%- 21.25 = #12 THW
Step 4: Branch Circuit Protection (Varies by fuse-Breaker 150-250%, round up to nearest size breaker)
17A x 150% = 17 x 2.50 = 25.5 A or 30 amps
17A x 250% = 17 x 2.50 = 42.5 A or 45 amps

 

[4GSR]

In home shops, I would go with 10ga wire instead of 12 so that way you are still in code with house wiring using a 30 amp breaker.
That's how my shop is wired up.

 

[coolidge]

Also depends on the intended use for the motor. I had a 2hp single phase 220v motor on a Grizzly cyclone dust collector, it frequently tripped the Grizzly recommended 20 amp breaker. I put a Fluke on it and it was pulling north of 50 amps at start up. I re-wired it for a 30 amp breaker and all was well.

 

[jim18655]

In home shops, I would go with 10ga wire instead of 12 so that way you are still in code with house wiring using a 30 amp breaker.
That's how my shop is wired up.

Motors are treated differently when it comes to overcurrent protection and wire sizes. There's a note in the code book that limits 14, 12, and 10 AWG to 15, 20, and 30 amps respectively, unless permitted else where in the code. Motors fall into "elsewhere' when sizing overcurrent protection. The breaker or fuse needs to be large enough to allow the motor to start without the inrush current tripping or blowing the overcurrent device. The larger breaker, up to 800% full-load-amps (FLA) for instantaneous trip, 250% for inverse time (usual type in panel) or fuse (Time delay 175%, non time delay 300%) will allow the motor to start HOWEVER continuous duty (3 hours or more) motors over 1HP are required to have overload protection based on the motor nameplate FLA. Smaller motors usually have built-in thermal protection. This is what protects the motor and limits the branch circuit current to protect the wire. The larger fuse or breaker only provides short circuit protection for the wires while allowing the motor to start. Motors and transformers are almost a "dead short" on the system until inductance limits the current draw to design specs. Without overload protection the branch circuit wires are at risk of burning up if the motor is loaded beyond its rated FLA, especially if the wire size is marginal.

Without knowing how long of run it is from the panel to the motor, how many wires in the conduit, and whether or not the OP has thermal protection on the motor I would also recommend 10 AWG to lessen voltage drop although the 12 AWG would be code compliant.

 

[mksj]

The Schneider Electric recommendation is based on 100 feet wiring run, that being said I would also use 10G to minimize voltage drop, but you could use 12G for a power cord to the machine/motor. Overload protection for continuous duty motors is usually 125% of nameplate depending on the service factor of 1.15 or greater. It can get more complex with different types of motors. A 20A breaker at a branch circuit will most likely trip at start-up. I use a 30A breaker for my 2Hp and 3 hp motors.

 

[4GSR]

................ I would also recommend 10 AWG to lessen voltage drop although the 12 AWG would be code compliant.

I tend to agree in a industrial environment. But in a home environment, an inspector was to find a 12 ga. wire tied to a 30 amp breaker, that would be a violation! He wouldn't care about that 3 HP motor amp rating at the other end of that wire. Right?

 

[jim18655]

The NEC applies to all installations, except power generation plants and distribution. As long as proper overload protection is provided you'd be covered. There isn't any phrase in Article 430 that limits it to industrial uses only and if you show the inspector the circuit he should be OK with the installation. The overloads protect the wire and the breaker is for short circuits.

 

[4GSR]

Jim,

I agree with you 100% with what you are saying. But what I am saying is, there are local building inspectors that will reject wiring in a "home environment" if the breaker does not match the wire size. OR at least the wire size is equivalent or larger than necessary for the breaker size. If course, the inspector will want to check out your motor protection at the control panel on the machine, too.

Personally, myself, I oversize the wiring by one size just to eliminate problems later down the road. And some day after I'm gone to the magic machine shop in the heaven's, The new home owners won't be left with a mess to deal with, just to get the home insured or to pass inspection when they find a 12 ga. wire tied to a 30 amp breaker. And the other end of the wire ends in a junction box, worse yet, hanging down from the ceiling, not connected to anything.