As applied to 215.2 when do I apply 80% as vs 125%

[Fred_Jarvis]

I am studying for the Master exam in florida. My question relates to when do I use 1/1.25 or the 80% rule vs when to multiply by 1.25 as stated in 215.2? An example of a question as follows "What is the maximum store parking lot lighting load in kw for a #4 THW conductor at 120 volts?" I know the answer is 8.16kw, I need help in understanding the why of the 1/1.25 in the formula.

 

[realolman]

I'm sorry. I really gotta start reading the posts... I answered something that had nothing to do with your post.

 

[Fred_Jarvis]

Clarify the question

Clarify the question

Matbe this will help. When I answered the question I used 1.25 and multiplied to come up with the answer. It was wrong. He said I had to use the 1/1.25 or 80% rule as continuous (3 hrs or more) was indicated in the question. My problem is that I don't see where he came up with the "divide" rather than the multiply. 86a x 1.25 = 106.25a vs the correct answer of 85a x .80 = 68a then 68a x 120v = 8.16kw

 

[Smart $]

Matbe this will help. When I answered the question I used 1.25 and multiplied to come up with the answer. It was wrong. He said I had to use the 1/1.25 or 80% rule as continuous (3 hrs or more) was indicated in the question. My problem is that I don't see where he came up with the "divide" rather than the multiply. 86a x 1.25 = 106.25a vs the correct answer of 85a x .80 = 68a then 68a x 120v = 8.16kw

The simplest answer is to remember the entire formula:

Continuous Load (in amperes) ? 125% = Ampacity (in amperes)

 

[Fred_Jarvis]

Simple answer?

Simple answer?

Unfortunately I still don't know why he multiplied by .8 rather than 1.25. He does that in other problems (multiple by 125%) so how do I know when to use each formula? I am forced to work both ways and try to match up the answers. As long as both are not there I can figure it out. Don't want to do this at test time. What theory am I missing?

 

[resistance]

The reciprocal of 1.25 is .80. So 1/1.25=.80

 

[resistance]

How about this:


Example:OCPD's are sized no less than 125% of the continuous load-[plus 100% of the non-continuous. We will ignore this for your problem]--> 200A continuous load (125%)= 250A Now: continuous loads are limited to 80% of the OCPD. SO: 250 amps is already sized for the continuous load, all you need to do is limit the OCPD to 80% of it's rating. SO: 250(80%)= 200A

Your test question: 86A (This is already sized at 1.25 of the continuous load) So all you have to do is now limit the continuous load to 80% of the OCPD. So: 86a(.80)=68.8A or 70A per section 240.6(A).

I wish I knew what the question asked exactly. If it asked: What size OCPD is needed for a 86A continuous load. Then: 86(1.25)=107.5A

Post the exact question.

 

[realolman]

What I answered actually DID have something to do with your post after all.

They're pretty much the same thing, only from a different perspective.

It's a math thing, not an NEC thing .

One is the reciprocal of the other, or , 1 / by one = the other

1/1.25 = 0.8 ............................1 / 0.80 = 1.25

100 X .8 = 80............................ 100 / 1.25 = 80


The conductor ampacity must be 1.25 or 125% of the continuous load.

The load must not be more than 0.80 or 80% of the conductor ampacity.

...pretty much the same thing... except for being different :smile:

 

[Smart $]

Unfortunately I still don't know why he multiplied by .8 rather than 1.25. He does that in other problems (multiple by 125%) so how do I know when to use each formula? I am forced to work both ways and try to match up the answers. As long as both are not there I can figure it out. Don't want to do this at test time. What theory am I missing?

a ? 125% = a ? 80%, where a is any value.

Continuous Load (in amperes) ? 125% = Ampacity (in amperes)
Continuous Load (in amperes) ? 80% = Ampacity (in amperes)
Continuous Load (in amperes) = Ampacity (in amperes) ? 125%
Continuous Load (in amperes) = Ampacity (in amperes) ? 80%

 

[Fred_Jarvis]

Different formulas

Different formulas

Understanding comes to those who wait and ponder. I now see the difference, Realolman, it is the load vs the conductor that makes the difference.
Load = 125% vs the wire = 80%
Thank you both (Realoman and Smart $)

 

[resistance]

Understanding comes to those who wait and ponder. I now see the difference, Realolman, it is the load vs the conductor that makes the difference.
Load = 125% vs the wire = 80%
Thank you both (Realoman and Smart $)

UR welcome!

 

[realolman]

UR welcome!

U can have mine... I didn't really deserve it anyway

 

[resistance]

U can have mine... I didn't really deserve it anyway

Thanks, but I'll pass! :wink: