Discussion
I wanted to start a discussion on code compliance of utilizing 3 Pole transfer switches with non switched neutrals on a 480 V system. If a fire pump is included in the design most manufacturers do not include neutrals making the system non separately derived. Another engineer came to me with this problem and I suggested pulling a neutral through the fire pump transfer switch so that there are no mixed transfer switches. However the other issue that I am concerned about in this design ( that’s already installed) is application of ground fault detection as the generator is 480v and has a 1200 A circuit breaker
Are you asking about equipment grounds? Grounding Electrode Conductors? Supply side bonding jumpers? Or main bonding jumpers? Is this strictly a code question or are you asking how to utilize software to review conductor damage curves to verify the equipment ground sizes indicated in 250.122 are actually large enough based on the specific overcurrent protection type?
Sorry not software... you said general formulas
In general i am talking about equipment grounding.
Equipment grounds are sized based on the size of the Overcurrent device, such as a circuit breaker or fuse. While "formulas" do exist and they usually are the basis for values expressed in the NEC we won't discuss them as the NEC is the minimum safety standard and we encourage those values to be utilized.
On which basis we identify the the size of conductor
Again, if you have a branch circuit or feeder and protecting that circuit is a 100 amp circuit breaker (Overcurrent Device) then you just go to Table 250.122 and that gives you the CU or AL equipment grounding conductor required for that circuit.
Below 100A from where we will get the reference of NEC For example MOV is operating on 480V,7A installed cable size is 6mm2 .How we will identify the size of grounding conductor
Now you state 6mm² so you sound like you are European and they don't utilize the NEC. Table 250.122 goes down as low as 15 A OCPD witb 14 AWG. The smallest permitted size conductor for building wiring is 14 AWG. While the NEC references 16 and 18 AWG, that is for signal and control aspects. Section 250.122 states that tbe EGC can't be smaller than whats given in Table 250.122. You sound like you are not dealing with building wiring.....250.122 is fairly clear on the minimum sizes.
Yup i was talking about industrial application like oil and gas industry Your explaination had clear the picture
Is the “MOV” reference “Metal Oxide Varistor”? As in Surge Protective Device?
No motor operated valves
https://www.eng-tips.com/viewthread.cfm?qid=377771 Good article on the subject... there may be other codes in addition to the NEC that apply here ( I’m not familiar with oil and gas applications)
I meant blog post on the subject sorry
Sizing the EGC doesnt change...its based on size of OCPD. End of story...the NEC Table 250.122 are required sizes....