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capacitor conductors/motor loads 1

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tulum

Industrial
Jan 13, 2004
335
When applying capacitors on the secondary of a motor circuit disconnecting means the capacitor cables must be sized to 1/3 the motor conductors.

1. I believe this is because when the motor is disconnected, the motor ends up supplying vars to the cap... there will be an increase in voltage and more current through the capacitor wires. How did the code come up with 1/3 the size?

2. Is it 1/3 the motor conductors or motor load? Why?

Thanks in advance
Tulum
 
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Suggestion: If adherence to NFPA 70-2002 National Electrical Code (NEC) is required, then NEC Article 240-21 Feeder Taps, 25 Feet and 1/3 Ampacity Tap Rule applies.
 
After talking to a senior engineer this is my understanding...

The cable feeding the capacitor needs to be sized for 1/3 the upstream protective device amperage rating (1/3 the motor cable is just a more conservative short cut). This is for protection of the cap.

 
Sorry last line should read protection of cap cable...
 
Suggestion: If the capacitor happen to be drawing more current than 1/3 cable size of motor feeder cable size can safely carry, then the capacitor cable size has to be increased. Also, the 1/3 capacitor cable size is the minimum size. It can obviously be 1/2 or higher in its size.
 
Sorry Jbartos...

I was just adressing the the 1/3 rule. The other rule used is conjuction with the 1/3 rule is that the conductors must be sized for 1.35 x the current being draw by the capacitor... which ever is greater.


I was also told by a co-worker that the 1/3 rule does not apply if on the line side of the contactor. This does not make sense to me, if my assumptions above are correct? Any thoughts Jbartos? Are my assumptions correct?



 
Suggestion to the previous posting: Reference NFPA 70-2002
1.
460.8 Conductors.
(A) Ampacity. The ampacity of capacitor circuit conductors shall not be less than 135 percent of the rated current of the capacitor. The ampacity of conductors that connect a capacitor to the terminals of a motor or to motor circuit conductors shall not be less than one-third the ampacity of the motor circuit conductors and in no case less than 135 percent of the rated current of the capacitor.
2.
(B) Overcurrent Protection. An overcurrent device shall be provided in each ungrounded conductor for each capacitor bank. The rating or setting of the overcurrent device shall be as low as practicable.
Exception: A separate overcurrent device shall not be required for a capacitor connected on the load side of a motor overload protective device.
 
Suggestion to the original posting: Please, post a reference what your advisors are using. This Forum does not necessarily cover NFPA 70 NEC only.
 
Jbartos,

ref: CE code book, Canadian Standards Association, rules 26-208, and 26-218...

 
I will throw out a guess.

Very rough numbers:

Motor reactive current is similar magnitude to no-load current.
20%-60% but for 2 , 4, 6-pole motors usually <33% of full load current.

PF correction must be limited to somewhere <90% of motor kvars to prevent resonant circuit at frequency below line frequency when power is disconnected, leading to overvoltage.

Sizing the cable at 1/3 the motor current would in most cases ensure the cap cable is adequately sized. Sized at 1/3 motor cable size or protection setting would provide slighly more margin. Presumably there are other sizing checks required and this this minimum 1/3 is applied as a backup to the calculations?


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I was assuming wye-connected pf correction caps.

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Electricpete,

Thanks for the post. This sounds reasonable to me. To answer your question No, no backup checks, if it meets the code it fits the bill...I am looking into this for my own interest.

So to echoe back what you said (will seem obvious):

When a motor is being de-energized the frequency drops, and since the motor is not doing any work only reactive amps are present (approx. 1/3 motor amps). If a capacitor is attached to the circuit as described, there will be an exchange of energy between the two. If the frequency hits the resonant frequency Xc will equal -Xl and the impedance will be zero, causing all the reactive amps from the motor to flow to the cap, producing a corresponding overvoltage.
 
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