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Old 09-09-2008, 09:57 PM   #16
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Understanding how to size a circuit conductor


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Originally Posted by gp_wa View Post
My case, assuming what? (sorry) A/C, motor, welder, none of the above?
As Stubbie explained. All/any of the above given the information provided.

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Answers based on the 2008 & 2011 NEC.
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Old 09-09-2008, 09:59 PM   #17
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Understanding how to size a circuit conductor


Thanks Stubbie. That one helped. I did not recognize the difference between minimum ampacity and full load ratings.

So the fact that this may or may not be a time machine, or an A/C unit, is irrelevant because it has a stated minimum ampacity. I would size the conductors based on ampacity rating, even if the proper breaker capacity might exceed the capacity of that conductor? That's the part that's confusing, because it is conceivable that a piece of equipment could draw almost, but not quite enough current to trip the breaker, which might be a higher current than the conductor is rated for...
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Old 09-09-2008, 10:45 PM   #18
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Understanding how to size a circuit conductor


It is common for breakers to be sized greater than the minimum ampacity of a motors branch circuit conductors. This is due to starting currents being much higher than running amps. The breaker is oversized to allow the motor to start without tripping, not always necessary but is common to do so given the right circumstances and type of motor.

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So the fact that this may or may not be a time machine, or an A/C unit, is irrelevant because it has a stated minimum ampacity. I would size the conductors based on ampacity rating, even if the proper breaker capacity might exceed the capacity of that conductor?
Yes but you need to understand the equipments requirements and it's overcurrent parameters. Very few motors state a minimum ampacity, this is only common in A/C HVAC , refrigeration... Motors have a table (NEC 250.52) that we go by for maximum overcurrent protection...can be as much as 250% above the rated FLC HORSEPOWER TABLE 250.148. Sizing conductors and over current is not necessarily a read the label and thats all worksheet. Overload devices are used upstream of ocpd in the event the equipment does not have thermal overload as an integral part of the motor.

In your example a motor with 52 amps is the calculated ampacity of the equipment under load... if you size the conductors at 55 amps and put those on the 60 amp breaker you will not exceed the temperature rating of the wire if the equipment draws 59 amps due to overload. Either the overload protection internal to the motor will trip before damage or your upstream overload seperate from the motor will open....eventually. Motors depending on service factor will carry a percentage of overload for sometime before damage occurs.

Last edited by Stubbie; 09-09-2008 at 10:47 PM.
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Old 09-10-2008, 07:15 PM   #19
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Understanding how to size a circuit conductor


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You're missing my question. I am asking which number is of relevance to this calculation, 52 (minimum ampacity), or 60 (maximum protection).

I understand why you'd use a 60 amp breaker as apposed to a 52 amp one...

Hmmmm....my point was that you would HAVE to use the 60 because the next choice is a 50 which is smaller than the 52 required.

If you had access to a 52 amp breaker and wire rated for 52 amps, you could use it.
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Old 09-10-2008, 07:21 PM   #20
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Understanding how to size a circuit conductor


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Originally Posted by 220/221 View Post
Hmmmm....my point was that you would HAVE to use the 60 because the next choice is a 50 which is smaller than the 52 required.

If you had access to a 52 amp breaker and wire rated for 52 amps, you could use it.
I have at no point asked what breaker I should use. I have asked for discussion on how to properly determine conductor size, given a minimum ampacity (or calculated ampacity) and maximum breaker size.
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Old 09-10-2008, 11:26 PM   #21
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Understanding how to size a circuit conductor


So how we doing so far in getting the confusion cleared up?

Would a graphic and example help?

If a minimum circuit ampacity is given then you simply pick a conductor from table 310.16 that has that ampacity or greater and is suitable for the application... but you need to know about your conductor and what column you can use in that table.

Conductors in general require you to consider the following
  1. Are they for Continuous loads
  2. What are the terminal temperature ratings
  3. What is the conductor insulation
  4. Conductor ampacity
  5. Any special application (like an arc welder)
  6. System voltage...120/208Y or 120/240 etc...
  7. Motors

Last edited by Stubbie; 09-10-2008 at 11:52 PM.
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Old 09-11-2008, 12:49 AM   #22
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Understanding how to size a circuit conductor


I can see I'll need to buy a copy of the NEC, if I am ever to be satisfied that I know how to hook electric thingies to load thingies without burning the house down.

That'll just raise more questions, you realize



Thanks for all the answers guys

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