Non-linear loads

[SmithAndrewR]

Here's the background. The facility is a Garage/Storage area with a small office. I have a 4C w/G 4/0 TC-er cable feeding a 225 MB panel being fed from a 250A generator disconnect. All the rules of the outside tap rule apply. An inspector says that due to derating at 80% counting the neutral as a CCC 4/0 is only rated for 184 amps. My response was that the neutral does not count as a CCC as it is a 3p 4w system. He explained to me that computers and fluorescent lamps make it a non-linear load. In my head :rant: of course those are non-linear loads but they only make up a small fraction of the load there are (4) 5hp roll up door motors and and (3) 10 ton ECU's that make up the MAJOR portion of the load. Upon asking him about the motors he told me because of the capacitor for start-up they also had to be counted as non-linear. I've never heard of this. Your thoughts.

 

[Besoeker]

Upon asking him about the motors he told me because of the capacitor for start-up they also had to be counted as non-linear. I've never heard of this. Your thoughts.

I can't answer your points about code requirements - I'm a Brit.
But a capacitor start motor as a non-linear load? Don't see how.
For me, non-linear loads are those that take non-linear current.

 

[Smart $]

I can't answer your points about code requirements - I'm a Brit.
But a capacitor start motor as a non-linear load? Don't see how.
For me, non-linear loads are those that take non-linear current.

I find many have a hard time understanding that linear AC load current isn't actually linear, the waveform being sinusoidal. Perhaps non-sinusoidal is a better term than non-linear. Another way some can understand the difference is to use the comparison of passive and active components, representing linear and non-linear respectively.

 

[Besoeker]

I find many have a hard time understanding that linear AC load current isn't actually linear, the waveform being sinusoidal. Perhaps non-sinusoidal is a better term than non-linear.

You have a point. To some extent it is perhaps a bit sloppy calling linear when we mean there is a linear relationship between current and voltage. But that's a bit cumbersome and more so if you add in constraints like saturation.

 

[Jraef]

Your inspector is a nincompoop. He is thinking that the definition of a non-linear load (regardless of how incorrect that term may be) is one which contains a capacitor. A motor starting capacitor is in the circuit for such a brief moment that it would not affect neutral loading. That would be the equivalent of saying that you must size all CCCs for motor circuits for LRA, not FLA. Ridiculous.

 

[Smart $]

... there are (4) 5hp roll up door motors and and (3) 10 ton ECU's that make up the MAJOR portion of the load. Upon asking him about the motors he told me because of the capacitor for start-up they also had to be counted as non-linear. ...

Out of curiosity, are these 1?, line-neutral-connected loads?

I ask because 1? L-L, and 3? (L-L-L) loads do not contribute to neutral current, yet they do figure into the total load. Only the non-linear L-N-connected loads count toward being a major portion of the [total] load.

 

[charlie b]

. . . he told me because of the capacitor for start-up they also had to be counted as non-linear.

To what the others have said I will add that a capacitor is a linear load. That is, if you impose a sinusoidal voltage across it, you will get a sinusoidal current going through it. That is enough, by itself, to prove that your inspector is wrong. The fact that the capacitor is only in the circuit briefly (as Jraef has mentioned) is even stronger proof.

Regarding the use of the word, "linear," I don't have a problem with it. It speaks not to the shape of the current or voltage waveform by itself, but rather to the relationship of the current waveform to the voltage waveform, as I mentioned above.

 

[Joethemechanic]

Why do 3 phase motors need caps for starting?

(4) 5hp roll up door motors and and (3) 10 ton ECU's that make up the MAJOR portion of the load. Upon asking him about the motors he told me because of the capacitor for start-up

 

[brian john]

Your inspector is a nincompoop. He is thinking that the definition of a non-linear load (regardless of how incorrect that term may be) is one which contains a capacitor. A motor starting capacitor is in the circuit for such a brief moment that it would not affect neutral loading. That would be the equivalent of saying that you must size all CCCs for motor circuits for LRA, not FLA. Ridiculous.

While I find the terminology in the first sentence to be a bit technical for my mind, it seems to fit the situation PERFECTLY.

 

[Jraef]

Why do 3 phase motors need caps for starting?

I was just assuming they were single phase motors. But it's a good point. If they are 3 phase, that inspector is even more of a moron.

Side note: I have observed that the existence of capacitors in a piece of equipment is often used by the ill-informed to identify a "non-linear load" because they associate the bridge rectifier with the bus filter caps as being something they always see on those kinds of loads. So they extrapolate that if all non-linear loads have caps, that means all loads with caps are non-linear! I'm not implying they are right, merely explaining some of the obsevations I have come across. That's a fairly common one.

I can imagine that this inspector took some sort of one day course in which they told him to watch out for neutral loading on non-linear loads, he raised his hand and said "How can I tell if a load is non-linear?" They said "It will have a rectifier and a set of capacitors.". All he remembers is "capacitors".

 

[SmithAndrewR]

The motor loads are all 3 ph 208 volts. To be an inspector here in Afghanistan you have to pass an interview (you can't apply they randomly choose for electricans on ground). The good news for me is the inspectrs findings are by no means final and it will be a quick over turn. I just didn't want to be missing somethig before I stated my case to his boss. Thaks for the consult.

 

[Joethemechanic]

The motor loads are all 3 ph 208 volts..

If they are 3 phase, that inspector is even more of a moron.

:rotflmao: