Question for an Electrician...

[Traveler 45C]

I've got to design a ckt to power a 1HP, 115v single phase motor, 1725rpm's that pulls 78amps at start up with a no-load draw of 8.2a. It pops the 20a breaker that I tried to run it on.

What size/type of breaker do I need to use?

 

[SKYCHENEY]

Wow, it draws 10x the load at startup. I think you need some sort of soft start or capacitor setup.

 

[spartonboat1]

Any chance at all that you are on a ground fault circuit and tripping on that basis? Just a thought. I have such a circuit and need to correct it.

 

[Traveler 45C]

I got the specs from the motor manufacturer, 78a at start up is normal for this motor. BTW, full load current draw is 12.8a, I posted the no-load spec originally.

The motor will be driving an oil change pump and it doesn't seem loaded down any.

The manufacturer recommended a 30a breaker. Is the surge current rating the only characteristic to concern myself with?

The ckt I was using wasn't a ground fault circuit.

 

[TedZ]

Soft start is right as far as i am concerned. There are a lot of them and the small ones are cheap. Goggle "soft starter".

Ted

 

[Bill Allen]

All motors I'm aware of have a big spike on starting. ALways a posible problem with compressors. Always start without a load on it. This is also a problem with running motors off a generator, if you can control the start up load a genarartor can run a decent load. That said try it on you breaker see what happens the spike is of very short duration. A 1 hp motor not under load probally won't be a problem. Bill

 

[REBrueckner]

Unless you are pumping,say, a thousand gallons of oil at a time, 1HP is way,way,way more power than is required....

 

[spartonboat1]

Unless you are pumping,say, a thousand gallons of oil at a time, 1HP is way,way,way more power than is required....

I ran a 3/4hp pump through a 30 amp circuit (I assume- it is at the yard's main breaker panel), at the end of 100' of 12ga extension cord (boatyard), but never tripped the breaker. I assume the 100' causes a major voltage drop. Pump never seemed to break a sweat. Pumped 52 qts off, 44 qts in, 40wgt in 40 deg weather; balance in by hand.

 

[krush]

Replace your breaker with a new 20amp and check the wiring before moving on.

 

[MikeP]

Not saying it's necessarily the same but I've got 1HP motors on several items both at home and on the boat. They all run fine on 110 20 amp circuits; in fact two of the 1HP domestic home water pumps here in Mexico are on 15amp circuits. Are you sure the motor is OK or, as Krush noted, that the breaker is OK?

 

[Traveler 45C]

I won the Baldor motor/Oberdorfer pump in an eBay auction. Considering the price I paid for it I’m not too worried if it’s a little overpowered. I got a really good deal, I couldn’t pass it up. I don’t think I’ll have any problems pulling cold oil through it.

From the research I’ve done and if I understand correctly, motors of the same HP may have different startup characteristics depending on the type of motor and the device it’s driving. Some motor may need less current at start up, some may need more. Another consideration is how long the motor will pull that start up current. Some motors spin up quicker than others. My motor/pump assembly is quite beefy, and heavy.

I also learned that when selecting a breaker for a particular motor, you first size the breaker for overload/over current protection. Rule of thumb is to size the breaker at 250% of the motors full load amp draw. From there you adjust the breakers trip point to allow for the start up current. A trial and error approach. Not very scientific, but to my understanding, that’s how to do it. I may be wrong on that and if I am I’ll welcome being straightened out.

250% of full load for my motor worked out to 30.5amps. Couldn’t find a 35a breaker so I used a 40a and the motor worked fine, no binding, sounded smooth. There doesn’t appear to be anything wrong with it but I’ll keep that in mind and check further.

I did a temp hookup yesterday just to see if the motor worked. Before the permanent install I’ll try a 30a breaker and replace the 20a to see if it makes any difference.

 

[krush]

The rule of thumb is to size the breaker to the wiring....and size the wiring to the load

If you still have problems starting it, make sure the pump is unloaded (valves open).

 

[Traveler 45C]

The rule of thumb is to size the breaker to the wiring....and size the wiring to the load .

Hmmm, perhaps you should discuss it with these guys:
http://www.electriciantalk.com/f28/new-motor-ckt-breaker-use-10428/

 

[krush]

Hmmm, perhaps you should discuss it with these guys:
http://www.electriciantalk.com/f28/new-motor-ckt-breaker-use-10428/

I see no disagreement between my comments and that thread. Intermittent load is different than fault protection. Wiring can easily handle higher amperage intermittent loads.

You don't want a 40 amp (continuous) breaker on wire that is only capable of carrying 20 amps (continuous).

Intermittent load is a different argument.

 

[MikeP]

"Originally Posted by Traveler 45C
Hmmm, perhaps you should discuss it with these guys:"

Reading their thread makes it sound like they all disagree anyway!

The wiring books, including Calder, all point out, as K did, that the breaker is to protect the wiring. So if you install a 40A breaker because that's what you need to run the motor, you have to run whatever size wiring is necessary for the 40A load over the distance involved to eliminate the fire hazard of undersized wire.

 

[Mako440]

Mike is correct.
On residential applications, the "minimum" wire size for the 20 amp breaker is "12".
What you have done, by replacing the 20 amp with the 40 amp breaker, is basically the same as putting a penny under a fuse when it blows.
Depending on certain factors, you may have to switch the wiring to a size "8" in order to be safe. You may be able to get by with a size "10", but, I would call a licensed electrician to determine exactly what size you need.

As to the previous poster removing the G.F.I., it is there for a reason. If it continuously trips, either the receptacle is faulty, you have to many things on that circuit, or something on that circuit is drawing too much power. Correct the problem.

 

[Traveler 45C]

Right, but there isn't going to be a continuous 40a load. The full load spec for the motor is 12.2a. It pulls higher amps at start up but that's only there for an instant.

Didn't I mention I was designing a new ckt? I'll be running new wires for this motor, not installing a 40a breaker in a ckt designed for 20a.

 

[MikeP]

"Right, but there isn't going to be a continuous 40a load."

But that doesn't matter. If the breaker can sustain 40A at 120v and the wiring can only handle 20 or 30A (or whatever A less than 40) then a short circuit anywhere in the wiring beyond the breaker can cause a fire because the wire will become red hot and ignite the insulation while the breaker just sits thinking everything is OK because the load to burn out wiring sized for 20 or 30A will not trip the 40A breaker.

It's all based on fire hazard for the wiring.

 

[Mako440]

Didn't I mention I was designing a new ckt? I'll be running new wires for this motor, not installing a 40a breaker in a ckt designed for 20a.

Sort of!

In your first post, you said you were designing a circuit, what size breaker do I need. Later, you stated that the circuit you were using had a 20 amp breaker, which was tripping and was not G.F.I. protected. I think that is where the confusion came in.

If you are running a new circuit, I'd be curious how many amps you have available at your main.
That must be one special pump to require a 40 amp breaker. My sprinkler pump is 2 hp and only has a 30 amp breaker.

I would think that a 30amp breaker with a #10 wire would be sufficient in your case. As I said earlier, have a licensed electrician come out and look at your electrical system to see what is appropriate.

 

[Traveler 45C]

True statement but a short in the ckt, should that happen, will pull a whole lot more than 40a. True also that the wiring must be able to carry 40a if you have a 40a breaker and will be using that much current to drive a load.

In my case, my motor pulls the same current as my vacuum cleaner, 12a full load. The difference between them is the start up current. The motor is more beefy than the vacuum and more powerful, thus pulls more current at start up. Sizing the breaker and wiring the same as a 15a/20a household ckt just won't do, the smaller breaker trips before the motor spins up and settles down, reducing it's current draw. Sizing up the breaker to 40a allows the instantaneous current for a longer period of time.

As this will be a dedicated ckt for the motor only, there's no risk of overloading the wiring running other equipment. After the initial inrush of current, the motor settles down, pulling the same current as the vacuum, 12a. Should a short ckt or a locked rotor occur, the current draw will be well above the breaker rating.