I was having a discussion the other day with a guy who seemed to be quite knowledgeable. While we were talking it came up about a motor that was once tripping and he said he came to the conclusion that the pipe was too small going to the motor and that once he upsized the pipe he never had a problem with the motor tripping again. So my question is if you have a continuous duty motor and I look at the slide chart to size my pipe and use that chart am I messing up? Thanks in advance for any opinions or answers.
Continuous duty and pipe size
- Thread starter slimjimmy
- Start date
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
That seems far fetched but I guess if the pipe was filled much beyond capacity then heat would build up and destroy the insulation on the wire. Not sure it could actually trip a breaker. My guess is he replaced the wires and one had a short in it.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I agree with @Dennis Alwon . I don't see a plausible mechanism for conduit size to directly cause a motor to trip, and suspect that there was another problem which got fixed when the pipe was changed.
What causes a motor to 'trip'? Commonly the system measures supply current and trips if the current is too high for too long. Insufficient voltage to the motor can cause excess current. I guess conduit size could impact conductor temperature, changing voltage drop. Conduit material can also impact voltage drop. But a loose connection or defective conductor is IMHO a more likely culprit.
Some motors have internal temperature sensors and will trip if the temperature gets too high. Conduit size might impact motor cooling, but I'd suspect whatever got changed around the motor when a larger conduit gets installed. If a bunch of mess gets cleared out to make room for the pipe, then the motor might have better airflow and run cooler.
Jon
What causes a motor to 'trip'? Commonly the system measures supply current and trips if the current is too high for too long. Insufficient voltage to the motor can cause excess current. I guess conduit size could impact conductor temperature, changing voltage drop. Conduit material can also impact voltage drop. But a loose connection or defective conductor is IMHO a more likely culprit.
Some motors have internal temperature sensors and will trip if the temperature gets too high. Conduit size might impact motor cooling, but I'd suspect whatever got changed around the motor when a larger conduit gets installed. If a bunch of mess gets cleared out to make room for the pipe, then the motor might have better airflow and run cooler.
Jon
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
No, what this person told you is not based on anything real if the conductors are sized correctly.
Fred B
Senior Member
- Location
- Upstate, NY
- Occupation
- Electrician
More of a question than an answer. If "pipe" is of a Ferrous type could the wiring be inside in such a way that it could induce a choke point? Just a wild thought.
Another issue could simply as already stated a damaged conductor that when pulled out and even if reused it now lays in the pipe differently and no longer making contact. All it would have to be is a small slice or nick that when under load shorts out.
Another issue could simply as already stated a damaged conductor that when pulled out and even if reused it now lays in the pipe differently and no longer making contact. All it would have to be is a small slice or nick that when under load shorts out.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Take a look at NEC table 9 for conductor properties. The resistance and reactance of large conductors can be noticwably different when the conductors are in different conduit types.
The table doesn't report if different conduit sizes change the values, but it is plausible that they could make a slight difference.
For this to actually make a difference on a motor tripping out, we'd be talking about a motor wired with large conductors already right on the edge anyway.
Jon
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Not with all of the conductors (and thus, current) contained in the same conduit.
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
Saying that the conduit has anything to do with the motor tripping the OCPD is ridiculous. There's a billion miles of conduit and conductors feeding motor with no tripping issues. Regarding tripping maybe that the issue with the person who came up with this idea. 
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