Mike Holt Videos - Conductor and Overcurrent Protection Calculations

[Shawn McCarter]

I'm having a bit of difficulty in the way that calculated minimum ampacities are being presented in the Mike Holt courses and am wondering if anyone here has found this to be true or if I'm simply over-thinking things.

The NEC states that to size a conductor or an overcurrent protection device at 125% of the continuous load PLUS 100% of the non-continuous load. The presentation will show this in two different ways. 1. 200A continuous x 1.25 = 250A minimum, and 2. (100A continuous x 1.25) + 100A non-continuous = 225A minimum.

Any assistance understanding why it is being presented both ways would be appreciated.

 

[ptonsparky]

I'm having a bit of difficulty in the way that calculated minimum ampacities are being presented in the Mike Holt courses and am wondering if anyone here has found this to be true or if I'm simply over-thinking things.

The NEC states that to size a conductor or an overcurrent protection device at 125% of the continuous load PLUS 100% of the non-continuous load. The presentation will show this in two different ways. 1. 200A continuous x 1.25 = 250A minimum, and 2. (100A continuous x 1.25) + 100A non-continuous = 225A minimum.

Any assistance understanding why it is being presented both ways would be appreciated.

They are two different scenarios. The first is all continuous load, the second is not.

 

[ptonsparky]

Now a question for you. Assume three conductors.
What size wire would you use for these loads?
PVC Conduit size?
Subject to damage?

 

[Shawn McCarter]

They are two different scenarios. The first is all continuous load, the second is not

I guess I should have been more specific, sorry about that. On the Unit 6 - Conductor Sizing and Protection Calculations DVD, from 3:45:00 to 3:50:00, the panel is discussing feeder protection and conductor sizing. Mike Holt first calculated a 200A feeder at 125% of continuous load equaling 250A. He then went on to an example for sizing a feeder conductor at (100A continuous load x 1.25) + 100A non-continuous load equaling 250A. He then jumped back and stated that he should have calculated the feeder protection at (100A continuous load x 1.25) + 100A non-continuous load = 225A. He did this so that he could jump to Tbl 240.6A to show the existence of a 225A circuit breaker.

I guess I'm simply going to have to assume that, on a test, if I'm not presented with both a continuous and a non-continuous load, that I will base my calculation on the information provided.

I just find it confusing the way that it's all being presented. I'm a math oriented person and when I'm told that a calculation consists of (A + B) x C, I expect to add A to B before multiplying that product to C, and not skip over that step. Also, If I'm told that a continuous load exists, I have to assume that a non-continuous load is also present at times, and if so, I should always consider the non-continuous load when calculating amperage for a protective device or conductor.

 

[Fred B]

Tom is correct, and this also shows how the similar presumed 200AMP when calculated by the 2 methods can result in very different wiring requirements. And how if you can determine continuous vs non-continuous loads it can save on sizing requirements.