NEC interpretation table 430.22(E) 1

[cmelguet]

Hi, I am trying to understand the NEC 2011 table 430.22(E). I would apreciate if someone can clarify.

From the table I understand NEC classify motors in Continouos duty, intermittent duty, short time, periodic and varying duty. Also each duty is classify in 5 min rated, 15 min rated, 30-60 min rated and continouos rated. (Are these ratings and duties specify in the Nameplate?).

For example if i have a 100 A motor intermittent load 5 min rated, i need to use a 85A rated cable. If i have a 100 A motor interminttent load, but the motor is rated continuous, then i need to use a 140A cable. This is confusing since a continuous motor should run cooler than a 5 min rated motor if both are running with the same intermitent load.

Regards

 

[LionelHutz]

That's the section for sizing the cable, right? I believe the rating means it's on for 5 minutes every hour. This means the cable gets heated for 5 minutes then cools for 55 minutes. How hot the motor gets is irrelevent.

 

[electricpete]

In the interest of learning more about the code, I looked up the table. Here is a copy of table 430.22E (perhaps older version):

http://books.google.com/books?id=A9...AEwAw#v=onepage&q=nec table 430.22(E)&f=false

Each column corresponds to a different motor type (5-minute rated... continuous etc).
Each column corresponds to a different application type (short-time, intermittent, etc).

As Lionel said, it provides cable sizing requirements for single-motor branch circuits... non-continuous.

My first thought: why does the cable sizing care what type of motor it is (it should only depend on the application). I think I can answer that: probably the overload settings are adjusted according to motor type in some other section of the NEC, so overload setting and cable size need to be coordinated to assure cable remains protected. Although I didn't see any mention of motor type within the overload setting discussion 430.31 (is it addressed somewhere?)

My second question: how is it that some entries in this table end up being greater than 140% (which is the cable size that would be required for continuous-duty motor in continuous application per 430.22A). That doesn't make sense to me.


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[electricpete]

Correction in bold:

Each column corresponds to a different motor type (5-minute rated... continuous etc).
Each row corresponds to a different application type (short-time, intermittent, etc).

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[cmelguet]

Electricpete,

1) isnt 125% of the motor FLC the ampacity of the cable that supply a continuous duty, continuous rated motor as per 430.22 (A)?

2) the table is confusing. The rating of motors (5 min, 15 min, 30-60 min) means that the motors run for 5,15, 30, etc min and then stop and rest for up to an hour, and start again. So this make sense for an intermittent load with rating motors up to 60 min. What does it mean an intermittent load with a continouos rated motor?. If the motor run continouosly then it is not an intermittent load.

Regards

 

[electricpete]

1) isnt 125% of the motor FLC the ampacity of the cable that supply a continuous duty, continuous rated motor as per 430.22 (A)?

You're right, 125% is what I meant. The question remains: how can we end up with requirement for cable rating greater than what we would apply to continuous duty motor in continuous application?

What does it mean an intermittent load with a continouos rated motor?. If the motor run continouosly then it is not an intermittent load.

A continuous rated motor is specified/designed/manufactured to handle full load continuously, without any damage to the motor. It is a description of the motor, independent of the application. So again, we can see the table captures the characteristics of the motor (columns) and characteristics of the application (rows).

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