~125% confused...

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mgphill

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Lawrence, KS, USA
Hello, I am a college senior trying to complete a (commercial) building power systems senior design project as NEC-accurate as I can.

In seemingly every example panelboard schedule I have come across (with a demand totaling section at the bottom), I find that, while most types get a demand factor of 100%, some loads, such as lighting and "largest motor", are given a demand factor of 125%.

This 125% doesn't smell like any demand factor from Article 220.

What relation does this 125% "demand factor" have to the NEC 2008, 215.2(A)(1) feeder conductor minimum ampacity of (1.25 x Continuous) + Non-Continuous?

Is it the same thing? If yes, could someone possibly give a thorough-ish explanation of what is going here?

thank you
 
jumper

jumper

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Location
3 Hr 2 Min from Winged Horses
It applies to any loads that be on for three hours or more. The loads must be at full ampacity for the whole time period.

From Article 100 definitions:

Continuous Load. A load where the maximum current is
expected to continue for 3 hours or more..

Loads that are intermittent, such as ovens and other thermostatically controlled are not continuous.

Some loads such as water heaters are considered continuous for branch circuit sizing only.

422.13 Storage-Type Water Heaters. A fixed storage-type
water heater that has a capacity of 450 L (120 gal) or less shall
be considered a continuous load for the purposes of sizing
branch circuits.

Annex D, example D3 has some continuous loads in its service calculation.
 
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Kartracer087

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Location
Milwaukee WI
mgphill said:
Hello, I am a college senior trying to complete a (commercial) building power systems senior design project as NEC-accurate as I can.

In seemingly every example panelboard schedule I have come across (with a demand totaling section at the bottom), I find that, while most types get a demand factor of 100%, some loads, such as lighting and "largest motor", are given a demand factor of 125%.

This 125% doesn't smell like any demand factor from Article 220.

What relation does this 125% "demand factor" have to the NEC 2008, 215.2(A)(1) feeder conductor minimum ampacity of (1.25 x Continuous) + Non-Continuous?

Is it the same thing? If yes, could someone possibly give a thorough-ish explanation of what is going here?

thank you
Click to expand...

The real reason why this is so is because generally speaking, overcurrent devices are rated at 80 percent of their marked rating, UNLESS the manufacturer notes that the device is tested and certified for use at 100 percent. If you do have a 100 percent rated device, then it becomes the sum of the continuous plus the sum of the noncontinuous load without adding any 1.25 percent factor to the continuous load.

See 215.2 (A) (1) Exception No 1

If the panel feeds multiple motors, then it is always 1.25 times the largest plus the sum of the remaining motors on the panel/circuit. See 430.24.

Any lighting load should be taken at 1.25 times as well.
 
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Smart $

Smart $

Esteemed Member
Location
Ohio
Kartracer087 said:
The real reason why this is so is because generally speaking, overcurrent devices are rated at 80 percent of their marked rating...
Click to expand...
Even though you stated "generally speaking", that's still not quite correct, or at the very least, misleading. You can load an overcurrent device to 100% with a non-continuous load... but not with a continuous load or combination continuous and non-continuous. Common overcurrent devices have a time vs. current trip curve. Loading with a 100% continuous load puts the device too close to non-fault tripping.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
mgphill said:
... could someone possibly give a thorough-ish explanation of what is going here?
Click to expand...
How about a simple-ish explanation? ;)

Where 125% factoring is required, it is generally a scheme to prevent component and conductor insulation degradation. Think of it as a "headroom" factor.
 
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mgphill

Member
Location
Lawrence, KS, USA
Thanks for your comments everyone.

I think I might just have an issue with the use of the term "demand factor" to describe the 125% applied to continuous
loads when sizing circuit components.

I have always thought of a "demand factor" as a value less than one, that is applied to a
calculated load BEFORE multiplying by 1.25 to size components.

The NEC deals with "demand factors" in the load calculations section chapter.

My understanding is basically the following two paragraphs

NEC 220.40
"The calculated load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, as determined by Part II of this article, after any applicable demand factors permitted by Part III or IV or required by Part V have been applied."

NEC 215.2(A)(1)
"Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load."

So, while the NEC does not consider the 125% a "demand factor", I have read a lot where people are calling this a demand factor, and calling
1.25 x kVA an "NEC Demand Load" (or similar phrasing)
and
1.25 x FLA an "NEC Demand Current" (or similar phrasing)

As a student just trying to hack my way through all of this, I have found such wording confusing, considering how much it sounds like it is related to Article 220, when it is not.

I do understand the need for an actual, designated noun to call the 125% though...maybe something like "continuity factor" or "sizing factor"

sorry, for the long post. Just wanted to communicate fully what my issue is.

Thanks again.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
You are correct. Demand factors (in the NEC) are always less than 1. The 125% factor in Chapter 2 is not a demand factor, even though some call it that.

The 125% factor accounts for tolerances in utility voltage that can cause loads to draw more than nominal current. Resistive loads will draw more current when voltage is high. Motors loads draw more current when voltage is low. As long as the source voltage is within the range of 85-110% of nominal, your connected equipment can't overload its conductors.

We should probably petition the NFPA to define a name for that 125% factor so people can stop calling it the "Demand."
 
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JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
jumper said:
Why do I have a problem with that statement?
Click to expand...

Let's do the math:

Imagine a 240V, 4.8 kW, 1? heater. It's nominal current is 20 amps. The utility is allowed to maintain voltage in the range of -15% to +10%. What is the current at the top end of this range?

The current in a purely resistive load is proportional to the voltage, so I2 = I1 x V2/V1 = 20 x 264/240 = 22 Amps, or 110%. If we had protected this circuit at 100% of the connected load, the breaker will trip when the utility voltage is higher than nominal, but within tolerances.

There are probably other elements that add to the Conductor Rating Factor, like manufacturing tolerances in motors and heaters and variations in ambient temperature, metal-fatigue in the wire and so forth. It would be interesting to see all the things that went into consideration when the code was written. I'm sure it's not just a totally arbitrary number.
 
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