# Tri rated cable calculations



## paulengr (Oct 8, 2017)

Jdot96 said:


> Hi is anyone familiar with the factors involved with motor starter cable calculations? I’m trying to find out what derating factors to apply.


1A If it is HVAC and has FLA printed on the whole unit, use that.
1B For all others look up FLA in tables in 430. If it is nonstandard use name plate FLA.
2. If it is intermittent duty apply the multiplier in the table in 430. If it’s a fire pump multiply by 200%. Otherwise treat it as continuous duty (125%) even if it’s not for the largest motor plus 100% of any other motors (multiple motors in a single contactor).
3. Treat the cable as any other load.

That’s for cable. Other rules apply for OCPD and contactors. Do not mix them up because each type of equipment has its own rules and factors.


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## Jdot96 (7 mo ago)

thanks for your reply. What grouping factors should be applied for a panel running a single 3 phase motor.


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## paulengr (Oct 8, 2017)

Jdot96 said:


> thanks for your reply. What grouping factors should be applied for a panel running a single 3 phase motor.


None of the multipliers in NEC are called “grouping factors”. The only place NEC really talks about grouping is with parallel conductors. And it is clear on how to do it.

The only “grouping factor” that applies is multiple motors. Beyond this I can’t say. I’m going to be dealing with testing motors today that require up to four 500 MCM conductors per phase. Also some motors require multiple connections by design (wye-delta starter, two speed, part winding). Some have extra control wiring (heaters, internal temperature sensors). All of this may require one or multiple raceways.

Once you determine ampacity a motor is no different from any other loads once you have the ampacity. You just move on to sizing it as per the Neher-McGrath method. Since you only refer to a three phase motor with no specifics about the conductors, raceway, or ambient temperature you can’t determine sizing for conductors.

So is your question about motors or wiring? If it’s wiring, you need a lot more details. Sizing wiring and raceways is the most complex task an electrician is expected to do. There are several tables involved and it tends to be iterative. Usually an entire chapter of most training material is dedicated to this single subject.


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## CMP (Oct 30, 2019)

You might want to take notice where the OP is registered from.


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## paulengr (Oct 8, 2017)

CMP said:


> You might want to take notice where the OP is registered from.


IEC is same. The biggest difference is they go straight to name plate amperes.


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