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Richard Cummins
Electrical Manager
Lectrus Corporation
(832) 295-1200
Richard Cummins is Electrical Manager for Lectrus Corporation (formerly Metal Systems), a Chattanooga-based industry leader that engineers and builds fully-integrated, electrical equipment centers, operator centers and custom enclosures for the power, oil & gas...more»»
Article 440.32: BranchCircuit Selection Current
Posted by James from Islip, NY, US on June 3, 2009
This is what I believe to be the next code change to be administered and would like to know how to do it as soon as possible for everybody’s winfall. (IE natural resources and cost of doing business to help people save money in this time of recession.)
Art. 440.32 States that “Branch circut conductors suppling a single motor-compressor shall have an ampacity not less than 125 percent of “either” the motor-compressor rated-load current or the branch-circut selection current, whichever is greater.
I have been doing central air conditioner wiring for multiple companies for over ten years now and have come to realize (Through 8 hour code enhansement seminars that was required by Nassau in the past that I still attend even though it was supasssed) that every residential outside compressor unit name plate has the compressor-motor RLA as well as the fan motor RLA stamped on their name plates and thier total is always 80% of what they term on their name plate as “minimum circut ampacity”
First of all, I am not concernerd with the fact that in any article of the code wether it be in art. 430 0r 440 they both state that when you have multiple motor loads we all know that you take the sum of all the motor FLA plus 25% of the largest motor to rate the branch circut current.
Every A/C compressor company that I have been dealing with for the past ten years (Reeme, Ruud, Wheather King, Carrier, Kenmore, Ect.) takes both the compressor FLA and the fan motor FLA adds them together then adds 25% to that and then stamps that total on thier name plates as “Minimum circut ampacity” !!!!!!!
I guess my question and concern is, Is this the “branch-circut selection current” of which we have to apply another 25% to or is this our minimum circut ampacity to which we should size our conductors to?
Or, (on the more intelligent side) should we make a motion to rewrite the code to elliminate the word “either” in the first sentance of Art. 440.32?
If not than please explain to me why we have to rate “Hermetically sealed, thermally protected motors and their assoiciated cooling fan at 150% of their combined FLA?
Sincerly yours,
James F Mayott
Master Electrician, JFM Electric Corp.
The name plate current is approximately 120% of the Running Load Amperage (RLA). The name plate current is based on Locked Rotor Amperage (LRA), which is always higher than RLA. LRA is often referred to as the motor starting amperage. I believe that NEC requires an additional 25% to the total LRA is to cover powering the control voltage transformer, relays, wire losses in the HVAC unit and to add a safety factor. Hope this helps!
Circuit Breaker Terminology
Posted by John from Warwick, RI, US on May 23, 2009
What is “companion trip” circuit breaker as referenced in NFPA 70 (2008) 550.11(C)?
The companion trip circuit breaker is made up of two common trip breakers tied together. It looks like there are four single pole breakers tied together. If anyone of the breakers trip, they all trip.
Conductor Allowable Ampacity
Posted by frank from upland, CA, US on May 19, 2009
I have three parallel runs that have been installed in a temperature contolled room inside a 4 inch conduit per Article 310.4. Each run feeds a UPS system and contains six 500MCM THHN. The conductor in the conduit is EMT and bonded at both ends using the correction factors per Article 310.15. The temperature limitations of the insulation type are per Article 310.16. What would be my allowable ampacity after all the correction factors?
There are two pieces of information missing. The voltage used and the controlled room temperature. I will assume less then 2000 VAC and a room temperature between 78F and 86F. The 4" EMT can carry up to (9) 500MCM conductors. Per B.310.16 the ampacity of the 500MCM is 430 Amps. The correction factor in B.310.15(B)(2)(a) is 80%. The temperature correction is 1.0 per Table B.320.16. Since these are balanced loads we do not need further corrections per Table B.320.11. The ampacity is 430 x 0.80 = 344 amps.
VFD drives
Posted by John from Laurel, MD, US on May 19, 2009
I have been told that VFD drives are causing problems with either pitting of the ball bearings or deformation or the bearings of an electrical motor causing the life expectancy of the motor to be greatly reduced. Could you shed some light on this subject?
The number one reason for bearing failure is heat. The VFD drives actually help the life of the bearings in a motor with the ability to do a soft start, motor heat reduction through reduced amp loads. If you have bearing pitting or deformation, you probably have a lot of heat transfer into the motor or the wrong motor size and gear box for your application.
Power, Control Wiring and Conduit
Posted by Greg from Catlettsburg, KY, US on May 15, 2009
Can you install the power and control wiring to a 30hp, 480 volt motor in the same conduit or is separate conduits required?
Running power and control wiring in the same conduit is not allowed per NEC and common practices. The motor power voltage could induce a voltage in the control wires, causing a false signal to be sent the control system.
