non seperatly derived 312.5kva 3ph 277/480v vehicle mounted/portable genset system

[ctaylo360860]

Hoping to get some more help on this genset install.... this would have to be with sizing my conductors from my genset to my Automatic transfer Switch...

nameplate on genset motor is:
kw-276 volts-208/416
kva-364 amps 958/479

nameplate on genset alternator:
volts-277/480
amps-381.6
kva-312.5


So my genset has a 300a breaker, I planned on sizing my conductors big enough for 400A between genset and Service entrance rated ats. I am doing this incase in the future it is needed to install a larger 400a breaker. I am looking at running 5 wires in a piece of liquid tight flexible metal conduit between ATS and GENSET. if I do this I will have to derate those conductors to 80% since I'm over four current carrying conductors? so if I were planning to upsize to 400amps instead of 300amps id actually have to run 250kcmill copper paralleled to get 400amps. 255A x 80% = 204amps... 2 paralleled conductors = 408amps... with that said Ill have to have two pieces of 3" liquid tight flexible metal conduit with five conductors in each raceway... my lugs are 1=600kcmill or 2-250kcmill. I'm wondering if I should just size the conductors to the 300A breaker because its going to be unlikely they will add more load to the generator, and the 300A breaker is sufficient to carry the load they have right now.... Also I'm wondering if anyone has any ideas that i can easily install and uninstall the genset. the genset is used to keep tarps for corn bunks on during a power outage, but once the bunks are empty I will be uninstalling and storing generator. I was thinking something like a powertite 400amp pin and sleeve cable connectors and plug and some 600mcm so cord with 5 conductors ... the only problem is I cant find any cord that big nor can I find a pin and sleeve that will take 3 grounded conductors, 1 grounded conductor, and a e.g.c. . I need to run 5 conductors in each raceway since my genset is a non separately derived system, and I need isolate grounded conductor from equipment groundings conductors and frame of genset... any and all help is greatly appreciated!!!! THANKS IN ADVANCE!

 

[ctaylo360860]

this is the existing 400a load center. I plan on setting my 400Amp service entrance rated ATS before the 400 amp load center. Im going to take the Service entrance conductors out of the 400a circuit breaker on the load center, and land them on the 400a breaker inside of my service entrance rated ATS. I will bond my grounding and grounded conductors in the ATS. Ill then run conductors to the 400a circuit breaker in the load center from the load side of my ATS. I'm planning on 2 parallel runs so that they are easier to work with.. I will isolate grounded and grounding conductors inside of my 400A load center. I will also be running conductors between 300A circuit breaker on generator and the generator side of ATS. I will isolate the grounding conductor form the grounded conductor and frame of the genset..... I planned on up sizing my conductors to 400amps, incase in future we ad more load on the 400a load center. I will be paralleling these as well so that they are easier to work with.... As i understand, i would have to run 250kcmill copper in each parallel run to get 400amps since i will have to derate to 80% as I have 4 current carrying conductors in each raceway. i can get away with 3/0 copper between load side of ATS and 400A circuit breaker on load center because my raceways are less than 24 inches...

 

[zbang]

If you must connectorize the install, look at using single-conductor flexible cord (prob. SC type), protected from damage, and Camlok connectors. For a seasonal install/remove, I probably wouldn't bother, not worth the added cost.

In a 3-phase wye connected system, in most cases the neutral isn't considered a current-carrying conductor and the EGC isn't considered one, either, so you only have three.

 

[LarryFine]

I am looking at running 5 wires in a piece of liquid tight flexible metal conduit between ATS and GENSET. if I do this I will have to derate those conductors to 80% since I'm over four current carrying conductors?

No. Because the neutral carries unbalance current, it need not be counted as a CCC under most conditions.

 

[ctaylo360860]

If you must connectorize the install, look at using single-conductor flexible cord (prob. SC type), protected from damage, and Camlok connectors. For a seasonal install/remove, I probably wouldn't bother, not worth the added cost.

In a 3-phase wye connected system, in most cases the neutral isn't considered a current-carrying conductor and the EGC isn't considered one, either, so you only have three.

Thanks for the info on the camlocks and cord. I haven't got a chance to look into them yet, but I'm hoping to do so later today... is the neautral not considered a current carrying conductor because most 3-phase wye are motor load,s and don't utilize the neutral? do you have an example of when a neutral is considered a current-carrying conductor? I'm glad to hear I only have 3 ccc, I did not want to try and wrestle paralleled 250kcmil copper into the ats and genset... THANKS AGAIN!

 

[ctaylo360860]

No. Because the neutral carries unbalance current, it need not be counted as a CCC under most conditions.

Thank you for that info! I'm glad to hear I don't have to wrestle paralleled 250kcmil into my genset and ats! do you have an example of when a neutral would be considered a CCC? THANKS AGAIN!

 

[synchro]

do you have an example of when a neutral would be considered a CCC? THANKS AGAIN!

If you had all L-N loads with a lot of harmonic content then that could cause significant current in the neutral conductor. For example, rectifiers can draw narrow pulses of current which will add independently in an RMS manner in the common neutral, instead of cancelling. In the worst case of zero overlap between equal magnitude current pulses on the 3 phases, the RMS neutrail current could be 1.73 times the phase current.

 

[ctaylo360860]

okay so say I had a 3-277v lighting loads leaving my panel in a raceway. I would have 3 ungrounded conductors and 3 grounded conductors and 1 grounding conductor. I would then have 6 current carrying conductors in this raceway, and would have to derate them to 60%. or could you just run a multi wire branch circuit? 3 ungrounded conductors and 1 grounded conductor and a grounding conductors making 4 current carrying conductors in your raceway? If i understand what you stated above, a mwbc would be dangerous because of induced harmonic currents in the system neutral conductor resulting in total harmonic distortion, and current would exceed he load current of the devices themselves?

 

[zbang]

Three ungrounded conductors + grounded in a MBWC count as only three CCC because the grounded conductor can only carry any unbalance current that isn't going back on a phase conductor- if each ungrounded is carrying exactly 10 amps, no current flows on the grounded at all; if only one ungrounded is carrying 10 amps and the other two carry nothing, the grounded gets that 10 amps.

If, and it's a big if, you have "substantial harmonics", then the grounded becomes "current carrying". For most purposes, it isn't. About the only place I've really seen that is in stage lighting systems using phase-control dimmers. It could also happen in some office settings with a lot of (electrically) badly-behaved computers with with some VFDs (as mentioned above). Even then, the total load is the same, it's the harmonics increasing the instantaneous current on the neutral/center-point/grounded-conductor that causes the trouble. Look up "triplen harmonics" for a bunch of info on that.

Side note-
Much of the code in this area is really about heat (it started as a fire-prevention code). Each conductor has resistance and thus produces heat. If you have (3) 2-wire circuits running at 10 amps each, that's (6) wires each carrying 10 amps (and that much heat produced). Fold that into a MWBC, you now have (3) wires carrying 10 amps and (1) carrying nothing (no imbalance) so only (3) dumping heat into the conduit. (And if you look at the ampacity tables, you'll see rather-different numbers for "in conduit" and "in free air", not because the resistance is different but because with the latter the heat can dissipate.)

 

[synchro]

I agree with zbang that the worst case theoretical current in the neutral from harmonics will not occur in practice. If you size the neutral conductor the same as the phase conductors you will have plenty of margin.