Continuous duty and non-Continuous duty load

[kwired]

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

Cheers, Wayne

Is it?

Motor draws a starting current briefly, then drops to a fixed level and remains at that level for 3 hours plus - that is continuous by the definition.

Other rules in code that require us to consider nearly all motors as continuous loads goes beyond the definition and is mostly used for conductor selection purposes and not for load calculation purposes.

Then on top of that you may have a motor that starts, continues running for more than three hours but driven load varies and therefore the input current will vary as well.

 

[wwhitney]

Motor draws a starting current briefly, then drops to a fixed level and remains at that level for 3 hours plus - that is continuous by the definition.

Actually, it's not continuous by definition, if you consider the starting current to be the maximum current:

(2017) Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

So that's the flaw: take a continuous load, and add the behavior that once an hour it draws an extra 5A for a minute. Now the NEC no longer considers it continuous by definition. Which makes no sense. [It could still be consider continuous by fiat based on equipment type per some specific section.]

Hence my suggestion that loads should have both a non-continuous and continuous rating. And then under the current 125% regime for continuous loads, the continuous rating would control sizing if it is 80% or more of the non-continuous rating.

Cheers, Wayne

 

[kwired]

Actually, it's not continuous by definition, if you consider the starting current to be the maximum current:

(2017) Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

So that's the flaw: take a continuous load, and add the behavior that once an hour it draws an extra 5A for a minute. Now the NEC no longer considers it continuous by definition. Which makes no sense. [It could still be consider continuous by fiat based on equipment type per some specific section.]

Hence my suggestion that loads should have both a non-continuous and continuous rating. And then under the current 125% regime for continuous loads, the continuous rating would control sizing if it is 80% or more of the non-continuous rating.

Cheers, Wayne

An incandescent lamp has a "starting current" also, as resistance increases as the lamp filament heats up. So if anything maybe NEC definition needs to address such temporary currents as not being part of the circumstances for the definition?

At same time a motor that starts pretty frequently, maybe even "too frequently", can have significant amount of current that becomes a factor for conductor selection and/or even derating of the motor output being necessary if you want it to last for a reasonable time.

 

[wwhitney]

An incandescent lamp has a "starting current" also, as resistance increases as the lamp filament heats up. So if anything maybe NEC definition needs to address such temporary currents as not being part of the circumstances for the definition?

That would work for a lot of loads in practice. It's not sufficient for an arbitrary current vs time pattern. For example a piece of equipment consisting of a constant large load plus a much smaller load at a 50% duty cycle.

We could try something like "Continuous Load. A load where the current is expected to remain substantially at its maximum non-transitory value for 3 hours or more." That of course does not define substantially, and does not reflect the idea that if the sustained current is, say, 90% of the maximum non-transitory current, then the continuous rating should likewise be 90% of that maximum.

Cheers, Wayne