Question about continuous vs non-continuous loads

[bryanbdp]

I have been reading up on sizing equipment branch circuits for our shop.
I have some questions about determining whether an machine is a continuous or non-continuous load... :blink:

For example, most welders have a duty cycle, and it left on but not used, draw very little power.
So during heavy use, they still need to cool down according to their duty cycle, thus non-continuous IMHO.
If left on for long periods, they only draw a few amps of parasitic power...
So would this be a Con or Non-con?

Spot welder - can really only be operated for a few seconds at a time, can't be left on.

Cord connected devices (non-fixed) appear to be rated at a blanket 125%.

Granted, any motor draws a fair amount of power during startup, but many, once running, draw very little.
For instance, some motors drive a fairly constant and large load, say a big blower or circulator pump. Fine, those sure seem like continuous loads. So does an electric oven, or air compressor under a lot of use.

Now lets look at a 7hp 3ph table saw. Big startup load, but then very little load until you cut something. But the most you can cut is usually 8 to 16 feet or so, before you have to stop, and reposition for another cut. Kind of works out like a duty cycle. I don't see any way to continuously load that machine, short of some sort of automated feeder etc.

CNC machines - It may be possible to run these machines at near full power during some operations, but generally they have to pause slightly between passes. In addition, many have variable frequency drives, not sure if these draw power in the same manner as a normal motor, or if they're a non-linear load, or what.

Last stupid question of the day. You are supposed to calculate lighting loads based on the exterior building size and VA per square foot. But, I have a completely engineered LED lighting plan that uses a small fraction of the normal, historical power.
So do I still have to use the prescribed lighting power calculations? Any how do you apply that? Do you have to take the total requirement, divide it by the voltage to get the amps, and then divide it up into X number or circuits, even though they are very lightly loaded? For example, the LED plan calls for 14,920 watts. As best I can tell, the rating for this space is 2 VA per SF x 17,000 sf = 34,000 VA. So the LED lighting is about 44% of the calculated amount.

Thank you in advance to anyone that answers....

My brain is smoking right now..
Bryan

 

[iwire]

I have been reading up on sizing equipment branch circuits for our shop.
I have some questions about determining whether an machine is a continuous or non-continuous load... :blink:

For example, most welders have a duty cycle, and it left on but not used, draw very little power.
So during heavy use, they still need to cool down according to their duty cycle, thus non-continuous IMHO.
If left on for long periods, they only draw a few amps of parasitic power...
So would this be a Con or Non-con?

Spot welder - can really only be operated for a few seconds at a time, can't be left on.

Cord connected devices (non-fixed) appear to be rated at a blanket 125%.

Granted, any motor draws a fair amount of power during startup, but many, once running, draw very little.
For instance, some motors drive a fairly constant and large load, say a big blower or circulator pump. Fine, those sure seem like continuous loads. So does an electric oven, or air compressor under a lot of use.

Now lets look at a 7hp 3ph table saw. Big startup load, but then very little load until you cut something. But the most you can cut is usually 8 to 16 feet or so, before you have to stop, and reposition for another cut. Kind of works out like a duty cycle. I don't see any way to continuously load that machine, short of some sort of automated feeder etc.

CNC machines - It may be possible to run these machines at near full power during some operations, but generally they have to pause slightly between passes. In addition, many have variable frequency drives, not sure if these draw power in the same manner as a normal motor, or if they're a non-linear load, or what.

Last stupid question of the day. You are supposed to calculate lighting loads based on the exterior building size and VA per square foot. But, I have a completely engineered LED lighting plan that uses a small fraction of the normal, historical power.
So do I still have to use the prescribed lighting power calculations? Any how do you apply that? Do you have to take the total requirement, divide it by the voltage to get the amps, and then divide it up into X number or circuits, even though they are very lightly loaded? For example, the LED plan calls for 14,920 watts. As best I can tell, the rating for this space is 2 VA per SF x 17,000 sf = 34,000 VA. So the LED lighting is about 44% of the calculated amount.

Thank you in advance to anyone that answers....

My brain is smoking right now..
Bryan

See article 100 definitions and look up continuos load.

Then keep in mind motors a single motor always gets 125% added to its FLA regardless of how long it runs.

 

[mwm1752]

Basically Lighting calculations for sizing services -- it is a set standard for the calculation of service conductors -- no specific lighting design is anticipated for this initial calculation and is considered a minimum standard IMO -- With energy codes restricting the watts per foot on identified occupancies the NEC has exceptions to the rule, see 220.12

 

[bryanbdp]

Thank you

Thank you

OK, so you would use the calculated lighting load to calculate the service size, but could wire up the actual lighting as needed? That makes sense...

The definition in section 100 lists a continuous load as one expected to be AT FULL LOAD for three hours or more.
However, since most motors seem to be calculated at a minimum of 125%, it seems like a moot point. And I'm assuming you treat a "machine" which contains a motor, like a drill press, as the motor it contains, since any other electrical functions are incidental (laser, work light).

So should all the loads I listed be considered continuous?
thank you,
Bryan

 

[Carultch]

OK, so you would use the calculated lighting load to calculate the service size, but could wire up the actual lighting as needed? That makes sense...

The definition in section 100 lists a continuous load as one expected to be AT FULL LOAD for three hours or more.
However, since most motors seem to be calculated at a minimum of 125%, it seems like a moot point. And I'm assuming you treat a "machine" which contains a motor, like a drill press, as the motor it contains, since any other electrical functions are incidental (laser, work light).

So should all the loads I listed be considered continuous?
thank you,
Bryan

If the dominant load is a motor, I'd treat it as a motor. Like a power tool with a negligible load for a work lamp.

If the dominant load is something else, I'd treat it as that something else. Such as a soldering iron which is primarily heat, and might have a minimal load motor-driven fan. Maybe that isn't the best example, as that is usually just a general receptacle load. Consider a hypothetical similar example on a larger scale. I'd say ovens, but those already have specific treatment in the NEC.

 

[Strathead]

OK, so you would use the calculated lighting load to calculate the service size, but could wire up the actual lighting as needed? That makes sense...

The definition in section 100 lists a continuous load as one expected to be AT FULL LOAD for three hours or more.
However, since most motors seem to be calculated at a minimum of 125%, it seems like a moot point. And I'm assuming you treat a "machine" which contains a motor, like a drill press, as the motor it contains, since any other electrical functions are incidental (laser, work light).

So should all the loads I listed be considered continuous?
thank you,
Bryan

No.

A motor that runs all day will be a continuous load. Like a typical warehouse exhaust fan. A motor that doesn't run for three hours at a time, will not be a continuous load. The calculation of motors at 125% is a little more complex. One motor on one circuit is calculated at 125% of the full load current from the motor charts in article 340. Two or more motors on a single circuit will only be 125% of the largest motor and 100% of the remaining motors. This also extends to panels and the service. As in, only the largest motor on the entire panel needs to be calculated at 125%.

 

[Smart $]

... So should all the loads I listed be considered continuous?

No.

Actually, as far as load calculations go, motor loads are neither continuous or noncontinuous loads. Services, feeders, and some branch circuits are calculated in accordance with...

430.24 Several Motors or a Motor(s) and Other
Load(s). Conductors supplying several motors, or a motor(
s) and other load(s), shall have an ampacity not less
than the sum of each of the following:

(1) 125 percent of the full-load current rating of the highest
rated motor, as determined by 430.6(A)

(2) Sum of the full-load current ratings of all the other
motors in the group, as determined by 430.6(A)

(3) 100 percent of the noncontinuous non-motor load

(4) 125 percent of the continuous non-motor load.

...in general, as I omitted the exceptions for brevity.