motor conductor ampacity vs duty cycle

[Chris Friesen]

I commented on this in another thread, but nobody has responded so I thought I'd separate it out.

It's been commented multiple times by various people that in a hobby woodworking environment most motors cannot be used in such a way that they would be considered as "continuous duty" and thus draw the full FLA. Because of this, the claim is that it is not necessary to use conductors rated for at least 125% the FLA of the motor.

Let's look at the code:

Section 430.22(A) says that continuous duty applications mean that the conductors need an ampacity of at least 125% of the FLA. So far so good.

430.22(E) Says that conductors for motors used for short-term, intermittent, periodic, or varying duty should use the percentages in table 430.22(E). However, according to that table if the motor itself is continuous-load rated (even if it's not being used in a continuous manner) that table specifies at least 140% of the FLA, and for "varying duty" it specifies 200% of the FLA!

By my reading, with a continuous duty rated motor (which most woodworking equipment uses) you're better off treating it as a continuous duty application and using the 125% value. Nowhere do I see a way for a continuous duty rated motor to be supplied with conductors rated for 100% of the FLA.

Can someone please point out what I'm missing? I'd love to know where I'm going wrong in my reasoning, cause it just doesn't make sense to me that a non-continuous duty cycle requires heavier-duty conductors than a continuous one.

[Jason Roehl]

I would guess that it's because start-up current in any motor is higher than the running current, even at FLA. So, if you're constantly cycling a motor on and off, you're probably heating the conductors to a point where it's the equivalent of running at 100+% of FLA.

Other than that, oh, boy, here we go again...we'll be bombarded with the recommendations that everyone install #6 wire with 50A breakers for all their 1 HP motors... :rolleyes:

[Rick Christopherson]

Yes, I did notice your posting in the other thread, and yes, I did ignore it because I don't feel like taking the hours of digging through code that it will take to prove to you that this is not applicable.

However, just because I am too lazy to do the digging does not mean that this is applicable--it just means that I am too lazy :eek: to dig into a topic that no one would consider otherwise. It is not applicable nevertheless.

So barring a full investigation, which I am not willing to do at the moment, consider a touch of reality instead. If this section of code was applicable, then every 120 volt contractor's saw would require a 30 amp circuit. Even your kitchen blender would require a 30 amp circuit, and your much-loved router would require a 30 amp circuit.

The biggest problem with citations such as this and a few others, is that someone takes information from code, without realizing that it is a sub-section to another aspect of code, and all of the exceptions and applications in previous sections of that section negate the information being quoted.

Unfortunately, this is one of the primary causes for otherwise code-knowledgeable people to mis-apply or mis-quote code requirements. What makes code so difficult to understand for most people is that it is written in a "legalese" format where information in a dominant section carries through to subordinate sections.