I’m taking a practice test, and one question asks for the minimum branch circuit size of a certain 3-phase motor. It does not specify what the motor is for or that it is a continuous duty motor, but the given answer requires you to treat it as one. Is this a rule, and is it somewhere in the code? I want to know if I was wrong or if the test was poorly written.
Are motors assumed to be continuous duty if not specified?
- Thread starter Joe E
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Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
I've seen proprietary motors that as components didn't have the duty cycle, and I know they weren't 100 % duty
ptonsparky
Tom
- Occupation
- EC - retired
Minimum branch circuit size, as in conductor, is usually 125% of NEC table Amp value for the motor. It doesn’t have anything to do with continuous duty.
You would need to fill us in on details of the question.
You would need to fill us in on details of the question.
- Location
- Illinois
- Occupation
- retired electrician
For tests, without any additional information, the answer will be based on the parent text of 430.22. If they want other than continuous duty, they will give you the type of information found in 430.22(E).
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
- Occupation
- Electrician
I could be mistaken but I thought if you added the standard 125% you were covered for continuous duty
Deltaforce
Senior Member
- Location
- India
- Occupation
- Electrical Engineer
The OL relay of motor also to set 125% avoid trip continuous duty. I saw some motor manual say simply set OL relay 100% not stating continuous duty or not in India.
ptonsparky
Tom
- Occupation
- EC - retired
The motors are allowed to run at 25% overcurrent?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Never set OL relays to anything other than what the relay manufacturer recommends. Do not blindly use the OL settings from one manufacturer on a different manufacturer's products.
Some manufacturers already include the 125% factor into their sizing charts if you also add 125% your motor will not be properly protected.
The OP was about the branch circuit sizing not the running OL protection.
Deltaforce
Senior Member
- Location
- India
- Occupation
- Electrical Engineer
This relate motors with service factor but for another thread because
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
This is a common misinterpretation of what the NEC is actually saying, it should not be perpetuated because it usually results in damage to motors. 110.3.B, the “RTFM” clause of the NEC, prevails over this anyway. OL relays must ONLY be set according to what the MANUFACTURER instructions tell you. DO NOT go by that clause in the NEC, because that only allies if you were to attempt to make an overload protective device from scratch, which nobody does. Manufacturers of OL relays ALREADY account for this in their designs, so if you add another 25% in your settings, you are allowing damaging current to flow in the motor if there is an over loading event.
That said, this is a distraction from what the OP was actually asking, so further discussion along this tangent will be stopped. If you want to take it up further, start a new thread.