Continuous duty and non-Continuous duty load

[Ashu]

Hello,
NEC defines continuous load as "A load where the maximum current is expected to continue for 3 hours or more." I am struggling here with term "maximum current". I design machines which use oversized Servo Motors to deal with intermittent peak current requirement. These motors actually run at a current about 30% of their rated motor nameplate current. For example, a motor with 98A rated current, draws only 30A. My question is as none of these motors run at maximum current (Which I consider as motor rated current), should these loads be considered as non-continuous duty loads, per NEC's definition? Thank you!

 

[infinity]

Ignoring the current draw on each motor does this machine draw the same amount of current continuously for 3 hours or more?

 

[Ashu]

Ignoring the current draw on each motor does this machine draw the same amount of current continuously for 3 hours or more?

Hi Rob,
The machine draws the same amount of current for more than 3 hours. For additional information, the total load (most of which is motors) is calculated to around 1000A based on rated motor currents. However, the actual running current of the machine is around 400Amp. Overcurrent protection devices have been set based on the rated current and not based on the actual running current.

 

[Ashu]

Hi Rob,
The machine draws the same amount of current for more than 3 hours. For additional information, the total load (most of which is motors) is calculated to around 1000A based on rated motor currents. However, the actual running current of the machine is around 400Amp. Overcurrent protection devices have been set based on the rated current and not based on the actual running current.

To be clear, when I mentioned overcurrent protection, I meant short circuit protection and not the overload protection.

 

[retirede]

I would define this as a 1000A non-continuous load. I don’t see how it can be anything else.

 

[wwhitney]

It's not what the definitions say, but the rational methodology would be this:

- Graph the device's current vs time over all operational modes.
- The highest current seen is the non-continuous rating.
- The continuous rating is the height of the tallest 3-hour long rectangle that will fit under one of the graphs.

Say you have a device that draws 100A for an hour, then 90A for an house, then 100A for an hour, etc. The current definitions call that a 100A non-continuous load. But it clearly draws more current than a 90A continuous load, which requires larger conductors and breakers than a 100A non-continuous load.

Cheers, Wayne

 

[petersonra]

Aren't motors always considered continuous loads?

 

[wwhitney]

Aren't motors always considered continuous loads?

I would say no--conductors for single motors are sized at 125% of the tabular FLC, but for multiple motors they are sized at 100% of all the FLCs, plus 25% of the largest FLC. I infer that the 125% factor is for startup current for one motor; if the 125% were a continuous use factor, it would apply to all the motors simultaneously, rather than just the largest.

Cheers, Wayne

 

[petersonra]

I would say no--conductors for single motors are sized at 125% of the tabular FLC, but for multiple motors they are sized at 100% of all the FLCs, plus 25% of the largest FLC. I infer that the 125% factor is for startup current for one motor; if the 125% were a continuous use factor, it would apply to all the motors simultaneously, rather than just the largest.

Cheers, Wayne

what about this

 

[wwhitney]

what about this

Ah. Then I infer that the tabular FLC values for continuous duty motors already include any necessary continuous use factors, so a 125% continuous use factor is not applied in field calculations. For example, see 430.24 "Several Motors or a Motor(s) and Other Load(s)." Motor loads and their computational factors are not categorized as continuous/non-continuous there, just the non-motor loads.

But my familiarity with 430 is not extensive, so I may be missing something.

Cheers, Wayne

 

[retirede]

what about this

View attachment 2560021

I think the part of the note that mentions the “nature of the apparatus” is relevant in the context of the OP.

 

[jim dungar]

Ah. Then I infer that the tabular FLC values for continuous duty motors already include any necessary continuous use factors, so a 125% continuous use factor is not applied in field calculations. For example, see 430.24 "Several Motors or a Motor(s) and Other Load(s)." Motor loads and their computational factors are not categorized as continuous/non-continuous there, just the non-motor loads.

This is partly why we typically need to use NEC table values as opposed to actual nameplate currents, when sizing for other than running overload protection.

 

[LarryFine]

The machine draws the same amount of current for more than 3 hours.

If it occasionally draws more, then it's not the maximum current.

 

[wwhitney]

If it occasionally draws more, then it's not the maximum current.

Right, which is a flaw in the definition of continuous load.

Cheers, Wayne

 

[Ashu]

It's not what the definitions say, but the rational methodology would be this:

- Graph the device's current vs time over all operational modes.
- The highest current seen is the non-continuous rating.
- The continuous rating is the height of the tallest 3-hour long rectangle that will fit under one of the graphs.

Say you have a device that draws 100A for an hour, then 90A for an house, then 100A for an hour, etc. The current definitions call that a 100A non-continuous load. But it clearly draws more current than a 90A continuous load, which requires larger conductors and breakers than a 100A non-continuous load.

Cheers, Wayne

Hello Wayne,
Thanks for your suggestions, so are you suggesting to take the graph of the actual running current of motors and not the FLC/Motor's rated current? FYI-Most of these motors run 100% of times.

 

[GeorgeB]

When this electro-hydraulics guy was supporting electric servo systems (Vickers owned Italian Polymotor and el.ge for a while in the early 1990s), systems with which I was involved typically used high currents only during acceleration (positive and negative). MOST systems, averaged over a minute or 2 (or hour or 2 as well) used perhaps 10% or 20% of the Wh as during fraction of second accelerations. Applications varied. CNC operations often were very high, and we water-cooled the motor flanges in some applications.

NEC doesn't consider this AFAIK. As with VFDs, our customers sized to the drive, not the motor. The motor was usually the limit; minimal inertia was often the #1 criterium.

 

[wwhitney]

Thanks for your suggestions, so are you suggesting to take the graph of the actual running current of motors and not the FLC/Motor's rated current? FYI-Most of these motors run 100% of times.

My comments were more theoretical than practical. If you are making a listed piece of equipment, I would think the listing requirements would tell you how to do the calculations and label it. If this is more of a one off site built piece of equipment, then I would think NEC Article 430 would cover it, in particular 430.24 may be of interest.

Cheers, Wayne

 

[Ashu]

My comments were more theoretical than practical. If you are making a listed piece of equipment, I would think the listing requirements would tell you how to do the calculations and label it. If this is more of a one off site built piece of equipment, then I would think NEC Article 430 would cover it, in particular 430.24 may be of interest.

Cheers, Wayne

Thanks again for your efforts to help me here. I want to make sure that I am pointing the right concern here. My question is not how much percentage of continuous and non-continuous load I should consider while calculating the conductors sizes as described in 430.24. However, my question is on the definition of the continuous load itself. My machine has several motors and I need to calculate the feeder overcurrent protection device per NEC 215.3. For this, I need to calculate a 125% of the continuous motors and 100% of non-continuous motors. So, in my case, all motors run continuously and draw a very less percentage of their rated FLA/FLC because they are oversized. So, if the sum of the rated current of all continuous duty motors is 1000Amp, will this current value be considered as continuous load for calculating the feeder protection OR the term "maximum current" in NEC's definition of the "Continuous load" means that I need to get the maximum running current of each motor, rather than the rated current of the motors. Hope it is clear. Thanks again.

 

[wwhitney]

My machine has several motors and I need to calculate the feeder overcurrent protection device per NEC 215.3.

I'm not sure of the NEC citation chain that gets you there, but my understanding is that sizing a feeder that supplies motors is governed by Article 430, not by 215.2 and 215.3. 430.24 governs the conductor size, and 430.62/430.63 governs the OCPD size.

Cheers, Wayne

 

[Ashu]

I'm not sure of the NEC citation chain that gets you there, but my understanding is that sizing a feeder that supplies motors is governed by Article 430, not by 215.2 and 215.3. 430.24 governs the conductor size, and 430.62/430.63 governs the OCPD size.

Cheers, Wayne

Thanks Wayne! I was aware about 430.62/63 but wasn't sure if I should apply 215.3 or or 430.62/63. It makes sense to apply 430.62 as the loads contain motors and other loads. My number is still close to what I had earlier but good thing is that I don't need to worry about calculating continuous and non-continuous loads separately. I appreciate your help!