Continious Loads

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Bowhunter

Member
Location
Illinois
215.3 Says the overcurrent device shall be 100% of the noncontinious load plus 125% of the continious load prior to the application of adjustment and correction factors.

If I have 100 amps of non-continious load and 150 amps of continuious load.
100 amps + (150 amps x 1.25) = 287.5 amps. OCPD selected = 300 amp. as permitted by 240.(B).
My question is since this is prior to adjustment and correction factors. If i have to increase the size of the wire because there are 6 wires in the conduit and a temperature of 40 degrees C. I beleive I would have to use a 500 kcmil conductor. Do I have to increase the size of the overcurrent device or is the 300 amp. OCPD Protecting the 500 kcmil wire?

500 kcmil- 90 degree column =430 x .8 x .91 = 349.44 amps
 

david luchini

Moderator
Staff member
Location
Connecticut
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A 300A OCPD would protect a conductor with an ampacity of 349A.

In your case, however, you would only need a 350kcmil 90 degree rated conductor for your load.
 

Smart $

Esteemed Member
Location
Ohio
215.3 Says the overcurrent device shall be 100% of the noncontinious load plus 125% of the continious load prior to the application of adjustment and correction factors.

If I have 100 amps of non-continious load and 150 amps of continuious load.
100 amps + (150 amps x 1.25) = 287.5 amps. OCPD selected = 300 amp. as permitted by 240.(B).
My question is since this is prior to adjustment and correction factors. If i have to increase the size of the wire because there are 6 wires in the conduit and a temperature of 40 degrees C. I beleive I would have to use a 500 kcmil conductor. Do I have to increase the size of the overcurrent device or is the 300 amp. OCPD Protecting the 500 kcmil wire?

500 kcmil- 90 degree column =430 x .8 x .91 = 349.44 amps
The only time I'm aware of that you may have to increase an ocpd rating is if it trips when a motor starts, and there are limitations on doing that.

As for sizing, the following is what I get using my Excel "calculator".



You can download the Excel file:

https://docs.google.com/file/d/0By0rzU3UuNh7eHYwdS1QdTRCNHc/edit?usp=sharing
 

infinity

Moderator
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Location
New Jersey
Occupation
Journeyman Electrician
After everything is said and done he needs a feeder conductor that will be a minimum of 287.5 amps. This is only the minimum, he could certainly use a larger size feeder say 400 amps with larger conductors.
 

Smart $

Esteemed Member
Location
Ohio
After everything is said and done he needs a feeder conductor that will be a minimum of 287.5 amps. This is only the minimum, he could certainly use a larger size feeder say 400 amps with larger conductors.
A lot of it depends on the perspective in which you are stating the minimum. For example, I can correctly say the minimum ampacity of the conductor only has to be 251A after derating. This is minimum ampacity permitted to be protected by a 300A OCPD [240.4(B)], and greater than the actual load of 250A.
 
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infinity

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Location
New Jersey
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A lot of it depends on the perspective in which you are stating the minimum. For example, I can correctly say the minimum ampacity of the conductor only has to be 251A after derating. This is minimum ampacity permitted to be protected by a 300A OCPD [240.4(B)], and greater than the actual load of 250A.
How does that work when his calculated load is 287.5 amps?

If I have 100 amps of non-continious load and 150 amps of continuious load. 100 amps + (150 amps x 1.25) = 287.5 amps.
 

Smart $

Esteemed Member
Location
Ohio
How does that work when his calculated load is 287.5 amps?
Read 215.2(A)(1) and consider the two sentences as mutual but otherwise independent requirements. You will also have to acknowldge that a load calculation does not include the extra 25% for the continuous load. That actually comes from the second sentence, not Article 220. Actual conductor ampacity is always after derating.
 
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infinity

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Location
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So if this were a test question you're saying you would choose the answer based on the first sentence and not the second?

The question from the OP:

If I have 100 amps of non-continuous load and 150 amps of continuous load.
What is the minimum feeder size?
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
The only time I'm aware of that you may have to increase an ocpd rating is if it trips when a motor starts, and there are limitations on doing that.

As for sizing, the following is what I get using my Excel "calculator".



You can download the Excel file:

https://docs.google.com/file/d/0By0rzU3UuNh7eHYwdS1QdTRCNHc/edit?usp=sharing
Looks like the ambient temp correction is based on the 2008 code cycle and not the 2011 if I'm reading this correctly. In the 2011, instead of a correction factor of .91 it is 1.0, unless I missed something.
 

Smart $

Esteemed Member
Location
Ohio
So if this were a test question you're saying you would choose the answer based on the first sentence and not the second? ...
No. I'd choose the answer based on compliance with both sentences. The question is what is the minimum feeder size, not what is the minimum ampacity with or without conditions.
 

Smart $

Esteemed Member
Location
Ohio
Looks like the ambient temp correction is based on the 2008 code cycle and not the 2011 if I'm reading this correctly. In the 2011, instead of a correction factor of .91 it is 1.0, unless I missed something.
In 2008 and prior NEC's, the ambient correction factors were part of the allowed ampacity tables. In 2011 NEC, those ambient correction factors were removed from the allowable ampacity tables and provided as separate tables. You use Table 310.15(B)(2)(a) for ampacity tables based on 30?C ambient, while you use Table 310.15(B)(2)((b) for ampacity tables based on 40?C ambient.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
In 2008 and prior NEC's, the ambient correction factors were part of the allowed ampacity tables. In 2011 NEC, those ambient correction factors were removed from the allowable ampacity tables and provided as separate tables. You use Table 310.15(B)(2)(a) for ampacity tables based on 30?C ambient, while you use Table 310.15(B)(2)((b) for ampacity tables based on 40?C ambient.
Looks like my ignorance is being displayed ....... again (looking at 310.15(B)(2)(b), duh.

Thanks money.
 
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david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
What is the minimum feeder size?
The minimum feeder size is 350mcm. From the second part of 215.2(A)(1): 100A + (1.25*150A) = 287.5A. From Table 310.15(B)(16), 300mcm has an allowable ampacity of 285 (too small) and 350mcm has an allowable ampacity of 310. This would make 350mcm the minimum feeder size.

How does that work when his calculated load is 287.5 amps?
The calculated load is 250A, not 287.5A. From the first part of 215.2(A)(1), you need a conductor with an (adjusted/corrected) ampacity not less than 250.

A 350mcm, 90 deg conductor has an allowable ampacity of 350. The final ampacity for the conditions is: 350A * 0.8 * 0.91 = 254.8A, which is large enough to carry the load.

(Using a 75 deg conductor, you would need a 500mcm conductor to supply the load: 380A * 0.8 * 0.88 = 267.5A)

As Smart$ noted, your adjusted/corrected ampacity will also need to be 251A or greater to be properly protected by a 300A OCPD using the next standard size up rule.
 

Bowhunter

Member
Location
Illinois
Thank You fellow Instructors,

So based on the two sentences in 215.2(A)(1) and the two exceptions, the continious load is a phantom load if the over-current device is rated for 100% of its load. However it must be considered if its a regular over-current device that you can only load to 80% of its capacity.
But I still feel I'm suppose to consider the extra 25% when sizing the conductors because of the adjustment and correction factors. Or is it a phantom load then too?
I thought this all had to do with bringing heat back to the terminals on the breaker. (75 degrees)

Bowhunter
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
But I still feel I'm suppose to consider the extra 25% when sizing the conductors because of the adjustment and correction factors. Or is it a phantom load then too?
I thought this all had to do with bringing heat back to the terminals on the breaker. (75 degrees)
I'd say its a phantom load then too. The feeder will only see 250A but the OCPD is sized at 300A and the min conductor size is 350mcm. So the OCPD and terminations are sized larger than the load current.

The adjusted/corrected ampacity need only be large enough to carry the actual load current.
 

Smart $

Esteemed Member
Location
Ohio
I'd say its a phantom load then too. The feeder will only see 250A but the OCPD is sized at 300A and the min conductor size is 350mcm. So the OCPD and terminations are sized larger than the load current.

The adjusted/corrected ampacity need only be large enough to carry the actual load current.
...or the minimum protected by the OCPD, if greater than the actual load current.
 
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