The 25% "thing".

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steve66

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OK, I don't want to hijack the transformer thread, so I'm starting a new one for this. So we are discussing load calculations, and the requirements in 210.20A and 215.3.

Two of you have basically said the same thing:
IIRC, the 25% thing only applies to ampacity of conductors and not directly to sizing OCPD.
The 25% "thing" doesn't apply to conductor ampacity... it applies to minimum conductor size... and it does apply directly to OCPD sizing where not 100%-rated equipment.

Why do you say it doesn't apply directly to the sizing of the OCPD? The code section specifically says "the rating of the overcurrent device shall not be less that the noncontinuous load plus 125 percent of the continuous load.
 

Smart $

Esteemed Member
Location
Ohio
Second quote is my statement. It is pretty much the exact opposite of the first. Not the same thing.
 

iwire

Moderator
Staff member
Location
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Great question, I hope you don't mind that I add this from the UL general directory.

CIRCUIT BREAKERS, MOLDED-CASE AND
CIRCUIT-BREAKER ENCLOSURES (DIVQ)
USE

<SNIP>

Unless otherwise marked, circuit breakers should not be loaded to exceed 80% of their current rating, where in normal operation the load will continue for three hours or more.

<SNIP>

From GUIDE INFORMATION FOR ELECTRICAL EQUIPMENT THE WHITE BOOK 2010
 

kingpb

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and the reciprocal of not loading to more than 80% is to size it at 125%.

1/0.80 = 1.25

In general, I ignore the non-continuous 100% loading and simply consider everything not fed from a 100% rated device to be continuous.

I'm all about easy.
 
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topgone

Senior Member
25% and the 80% --one and the same thing

25% and the 80% --one and the same thing

If we do the math, those figures relate with each other perfectly! For 100 samples, 25% is 20 units, and you will be left with 80 out of 100 (80/100 = 0.80 or 80%)if you take 20 from 100! Conversely, you multiply 1.25 (derived by dividing 1 by 0.8 or 80%) to any figure in order to pad up that number by 25%-->making the original number just 80% of the new number!

In our trade, we use wires/conductors that can carry 25% more than what the current passing thru them is, to assure they don't heat up and get damaged. By doing that, we also need to use an overcurrent protective device that will cut off the circuit should more than 100 +25% = 125% of the rated current pass thru these wires/conductors!

OTOH, if by convention, we can agree with using a standard current carrying capacity allowance of 30%, then we will be using a fudge factor of 1/0.7 = 1.4285714285714285714285714285714 as our new norm! And that's hard to remember, compared with 25%, or 80% and 1.25. :D
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Second quote is my statement. It is pretty much the exact opposite of the first. Not the same thing.

Smart:

Sorry, I misread your statement a couple of times. I thought I saw a "doesn't" when you actually said "does".

Now I see what you are saying. The code section only applies to the Overcurrent protection device. But if we have to increase the breaker size to match 125% of the continuous load, we may have to increase to wire size if necessary to comply with other paragraphs in 240. So it may indirectly affect wire size, but not directly.

So the 1st quote had it backwards. It should have said it only applies to the sizing of the OCPD, and not directly to the conductors.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
and the reciprocal of not loading to more than 80% is to size it at 125%.

1/0.80 = 1.25

In general, I ignore the non-continuous 100% loading and simply consider everything not fed from a 100% rated device to be continuous.

I'm all about easy.

I won't hold that against you. Easy is good, and its always nice to have some spare capacity.

But do you figure your transformer sizes the same way?
 

Smart $

Esteemed Member
Location
Ohio
... It should have said it only applies to the sizing of the OCPD, and not directly to the conductors.
But the extra 25% for continuous does apply directly to minimum conductor SIZE.

It applies indirectly to minimum conductor AMPACITY, because the OCPD rating (which is affected directly) must protect the conductor.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
I won't hold that against you. Easy is good, and its always nice to have some spare capacity.

But do you figure your transformer sizes the same way?

Pretty much, can't say always cause I probably haven't at least once, but certainly that would be the minimum transformer size anyway.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
But the extra 25% for continuous does apply directly to minimum conductor SIZE.

It applies indirectly to minimum conductor AMPACITY, because the OCPD rating (which is affected directly) must protect the conductor.

It seems you guys make this way harder than it needs to be. In general, figure out the load in KVA, calculate amps per phase. Disregarding VD, # of CCC in a conduit, specialty circuits, etc. for this conversation, multiply the FLA by 1.25%, pick your cable, round up if needed to select breaker.

I'm old and lazy so 100% non-continuous and 125% continuous is a bother; too many things to remember.
 

iwire

Moderator
Staff member
Location
Massachusetts
It seems you guys make this way harder than it needs to be. In general, figure out the load in KVA, calculate amps per phase. Disregarding VD, # of CCC in a conduit, specialty circuits, etc. for this conversation, multiply the FLA by 1.25%, pick your cable, round up if needed to select breaker.

I'm old and lazy so 100% non-continuous and 125% continuous is a bother; too many things to remember.

Yes, I think we can all agree it is easier to oversize than be precise.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
[2011] 210.19(A)(1) second sentence for branch circuits; 215.2(A)(1) second sentence for feeder circuits

OK, I'm with you there. So the OCP and conductor size are designed for 125% of continuous loads.

Still not following you here:

But the extra 25% for continuous does apply directly to minimum conductor SIZE.

It applies indirectly to minimum conductor AMPACITY, because the OCPD rating (which is affected directly) must protect the conductor.

You can't change a conductor size without changing its ampacity, or vice versa, so I'm not sure why you would say it affects one directly and the other indirectly.
 

Smart $

Esteemed Member
Location
Ohio
...You can't change a conductor size without changing its ampacity, or vice versa, so I'm not sure why you would say it affects one directly and the other indirectly.
Not absolutely true. You can use a conductor of the same size and change a condition of use which affects its ampacity, such as using a different insulation rating. The sections I cited have a two-part requirement: 1) size is determined using NC+125%C [coordination with 110.14(C)], and 2) ampacity must equal or exceed NC+C. The latter indirectly because the OCPD rating must protect conductor at its ampacity rating [240].
 
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