The Demand Factor is not 125%

[JoeStillman]

mgphill recently posted a question about sizing conductors. He's an EE student with a pretty good grasp of the concepts in the NEC. My hat is off to him for not being led astray by references he has found to a "demand factor" of 125%. The 125% multiplier in chapter 2 of the NEC is not a demand factor. Demand factors are always less than 1. (See the definition of "demand" in article 100.)

So if the 125% factor we all know and love is not a "demand factor", what is it? It is similar to the derating factors of Table 310.15(B)(2)(a)-"Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable". Now that's a mouthfull. Most people refer to these as the "derating factors", although that term is used elswhere for something a little different (310.60) and not included in the definitions.

If all the derating factors are less than 1, then our 125% factor can't really be a "derating factor." It's really the reciprocal of a derating factor, so I propose we call it the "Rating Factor". All in favor of calling it the "Conductor Rating Factor" raise your hands.

Now if we could only get the NFPA to include it in article 100....

 

[petersonra]

It has little to do with the conductors themselves.

It is more about dealing with people who run their motors at more than FLC. A lot of people routinely run them at close to the SF.

IMO, it does not serve much purpose and never really needed to be there in the first place. Conductors have plenty of spare ampacity as is.

 

[JoeStillman]

... it does not serve much purpose and never really needed to be there in the first place. Conductors have plenty of spare ampacity as is.

Seriously?

I was taught that the 125% factor (it first appears in 210.19 and turns up all over chapters 2 and 4) 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.

If we want people to stop calling it a demand factor we have to call it something else. My modest proposal is "Conductor rating factor" or "Rating Factor" for short.

 

[charlie b]

Why does it need a name? For example, 430.22(A) tells us that the conductors that serve a single motor shall have an ampacity of 125% of the full load current. It does not say to look up the FLA and multiply it by a "demand factor" of 125%. I can do the math, without having a name assigned to each factor within the calculation.

It is worth noting, since you pointed out the article 100 definition of "demand factor," that the NEC does not use the phrase "demand factor" in the manner in which that phrase is defined. The definition speaks of the maximum demand as compared with the connected load. But when we apply demand factors (e.g., as allowed under 220.53), we are not dealing with a "maximum demand." Rather, we are being given permission to count a load at less than 100% of its connected load, by virtue of the probability that not all loads will be running at full load all the time. So it is an accounting process, not a measurement of actual demands.

 

[petersonra]

Seriously?

I was taught that the 125% factor (it first appears in 210.19 and turns up all over chapters 2 and 4) 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.

If we want people to stop calling it a demand factor we have to call it something else. My modest proposal is "Conductor rating factor" or "Rating Factor" for short.

I don't see any reason to have a name for it. Conductor ampacity is very conservative anyway. I don't see any real good reason to add in these kind of safety factors. if the voltage is so low that a motor starts to draw too much current the overloads will trip.

High voltages will indeed cause most resistive loads to increase their current but virtually all resistive loads are intermittent giving the conductors time to cool off so the insulation is not harmed. In any case, very few loads are sized right at the limit of the conductors ampacity anyway.

 

[JoeStillman]

Why does it need a name?

Only because I don't want people to be confused by this and "Demand Factor." It needs its own name so that people who don't know any better will not be wrong about what a demand factor really is. I have seen a few posts here in the forum where folks get mixed up about what we're doing and why.

It is worth noting, since you pointed out the article 100 definition of "demand factor," that the NEC does not use the phrase "demand factor" in the manner in which that phrase is defined. The definition speaks of the maximum demand as compared with the connected load. But when we apply demand factors (e.g., as allowed under 220.53), we are not dealing with a "maximum demand." Rather, we are being given permission to count a load at less than 100% of its connected load, by virtue of the probability that not all loads will be running at full load all the time. So it is an accounting process, not a measurement of actual demands.

When the HVAC guys calculate their cooling loads, they refer to this concept as "Diversity." We have diversity in The Code too, but it isn't defined. "Diversity" appears in article 310.15(B)(2), Fine Point Note No.1, then again in the annex. I haven't really thought about what it means in that context.

 

[JoeStillman]

I don't see any reason to have a name for it. Conductor ampacity is very conservative anyway. I don't see any real good reason to add in these kind of safety factors. if the voltage is so low that a motor starts to draw too much current the overloads will trip.

Are you just trying to rattle my cage? Do you follow the code in your designs?

High voltages will indeed cause most resistive loads to increase their current but virtually all resistive loads are intermittent giving the conductors time to cool off so the insulation is not harmed. In any case, very few loads are sized right at the limit of the conductors ampacity anyway.

I'm so old I remember when light bulbs (a continuous load) were purely resistive, and you could fit your code book in a coat pocket.

 

[don_resqcapt19]

...
If we want people to stop calling it a demand factor we have to call it something else. My modest proposal is "Conductor rating factor" or "Rating Factor" for short.

You have until Nov 7, 2014 to submit a proposed change for the 2017 code

 

[petersonra]

Are you just trying to rattle my cage? Do you follow the code in your designs?

Just because I do not see a good reason to do something that is in the code, does not mean I ignore the code.

Conductor ampacity is already very conservative. Why do we add to it.

BTW, if the reason for the 125% is for voltage fluctuations, why are we not required to size everything at 125%? Feeders are not sized that way. If the reason was to account for voltage fluctuations would not every feeder need to be sized at 125% to account for the chance that it might be needed?

 

[kwired]

It is just a "multiplier for continuous loads".

My understanding is the main reason is the design of overcurrent protection devices, and to be able to operate at 60C or 75C (which ever applies) the conductor must be sized at 125% of the maximum designed load. This is because the overcurrent device "sinks" heat into the conductor and therefore would raise the termination temperature if the conductor were only sized at 100%.

All the places where it tells you to size the conductor at 125% also has an exception followning that allows you to size at 100% if the device is rated for 100%. These 100% rated devices do not "sink" heat into the conductor. Most modern 100% devices are electronic sensing and not thermally sensing AFAIK, and makes sense they would not create the heat a thermal device would create.

 

[suemarkp]

Likewise, for feeders you can also skip the 125% factor on the grounded conductor if it doesn't land on an overcurrent device. If it was a wire issue, it wouldn't matter if it was the ungrounded or grounded conductor since the current in each is the same.

Seems like this is just to keep circuit breakers cooler and not push the thermal trip mechanism.

 

[charlie b]

How about calling it the "125% factor"? :happyyes:

 

[kwired]

For motors you usually have higher rated breakers or fuses, but overload protection is usually allowed to be up to 125% of motor full load current, so this would still put overload protection of the conductor approximately at same level as conductor ampacity, and the fuse or breaker takes care of short circuits and ground faults.

 

[steve66]

I'm fine with calling it a demand factor. IMO, It doesn't seem to create any extra confusion.

IMO, it does not serve much purpose and never really needed to be there in the first place. Conductors have plenty of spare ampacity as is.

I won't pretend I know how much spare capacity the conductor ampacity's have, but surely it makes sense to tread continuous loads a little different than intermittent loads.

Steve

 

[G._S._Ohm]

My two cents:

Naming things does not do any harm that I know of. Game Theory would call this a 'dominant strategy' - you have the name handy if you need it but you don't have to use it if you don't need it.

 

[petersonra]

I'm fine with calling it a demand factor. IMO, It doesn't seem to create any extra confusion.



I won't pretend I know how much spare capacity the conductor ampacity's have, but surely it makes sense to tread continuous loads a little different than intermittent loads.

Steve

We can't even for sure figure out why we would do so in the first place, so why does it make any sense?

It also makes no sense that we would make CBs that are only good for 80% of their rating on a continuous basis.

BTW, if I am using a non-100% rated breaker, why can't I use it at 100% if I make the wires bigger to carry off more heat?

Or this - I can use (2) 1/0 wires or (1) 350MCM for 300A ampacity on a 100% rated breaker. (2) 1/0 wires is like 1/2 the copper of a single 350. How does that figure into the heat xfer numbers?

 

[kwired]

We can't even for sure figure out why we would do so in the first place, so why does it make any sense?

It also makes no sense that we would make CBs that are only good for 80% of their rating on a continuous basis.

BTW, if I am using a non-100% rated breaker, why can't I use it at 100% if I make the wires bigger to carry off more heat?

Or this - I can use (2) 1/0 wires or (1) 350MCM for 300A ampacity on a 100% rated breaker. (2) 1/0 wires is like 1/2 the copper of a single 350. How does that figure into the heat xfer numbers?

I can't answer all your questions, but the (2) 1/0 would have more surface area to dissipate heat than a single 350 would have. Circumference of a 350 is 7.75 mm, circumference of 1/0 is 5.9346 mm x 2 = 11.8692. And 1/0 is nearly 1/3 the cross sectional area as 350.