machine service rating questions

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j_braddock

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while i was in engineering today i learned that one line will have a 600 amp service and i am trying to figure out the system. any help would be greatly appreciated.

this system is a 480 volt 3 phase continuous operation powering (3)75 hp motors, (1)25 hp, (2)120 volt motors, and 2 transformers-(1)1.5kva and (1)3kva, and other small power components.

#1 if the system is rated at 214kw,244kva, and 310.5 full load amps, how do you get to 600 amps? 310.5 amps x 125% = 388.6 amps. so why can't we use a 400 amp busplug?

#2 if the system does require a 600 amp busplug, do i figure the service wire this way.
600 amp bus plug will require wire to the machine rated 125% above the 600 amps (600 x 1.25=750amps). if i run 2 sets of 500 mcm wire (3 1/2" rigid conduit per set),
430amps x .91 temp x .8 = 313 amps. 313 x 2 = 626 amps.

#3 how do you figure the service cable to the machine based upon bus plug rating?

thanks in advance
 
Re: machine service rating questions

First answer: The system is ?rated? in accordance with its ability to handle load, not on the amount of load that is installed at any one time. If you now have 310 amps worth of load (I did not check your math), then it is not unreasonable to make the service capable of handling 600. You could go with a 400 amp rating, but a 600 amp leaves more room for growth.

Second answer: The 600 amp service need only be served with 600 amps worth of cable. You don?t need to install 125% of 600 amps. A pair of 350 MCM would suffice, if there were no de-ratings needed for temperature or for more than three current-carrying conductors. Why do you think you need the 91% temperature de-rate? Also, why do you think you need another 80% derate? Are you planning on putting all conductors into a single conduit? If so, why are you not using two separate conduits?

Third answer: I don?t understand the question. What machine? From the bus plug to the service panel, you need only supply 600 amps worth of cable, as discussed above.
 
Re: machine service rating questions

If all the motors are running at the same time you
have 1 x 96a(75 hp)x1.25 + 2 x 96a(75 hp)+ 34a(25 hp)= 346a + the other load. I do not understand your figures in #1. As Charlie asked why the derating? The 600 amps figures looks good.

[ October 22, 2003, 07:48 PM: Message edited by: bob ]
 
Re: machine service rating questions

first off,
thanks for your answers.

charlie b.
this is how i derived my figures.
the plant is air conditioned, but during the summer, the temperature of the plant around the bus bar area has been recorded at 100 degrees, thus the .91 temperature derating.
i used the .8 factor because the ground wire will carry the unbalanced current load. i did plan to use 2 conduit runs for the 600 amp service.
i thought i read that the wire from the busplug to the control panel had to be rated at 125% of the continuous load, i have been looking for this reference but for the life of me i cannot find it in the service of feeder sections of the book. i guess i used the motor protection rule for the 125%.

thanks again,
james
 
Re: machine service rating questions

Originally posted by j_braddock:I used the .8 factor because the ground wire will carry the unbalanced current load.
Click to expand...
Forgive a bit of nit-pickiness, but the ?ground? wire should never carry current. It?s only there as a safety measure, for when a fault occurs in the equipment. In such cases, it is intended to carry current only long enough for the circuit breaker to open, and to thereby terminate the fault. I think you meant to say the ?grounded? wire, or to use the common term ?neutral.?

But if that wire only carries the unbalanced load from the three phase wires, it does not count as a ?current carrying conductor.? See 310.15(B)(4).
 
Re: machine service rating questions

charlie b,

i'm sorry. this is a 5 wire system - 3 phases, neutral, and ground. i'm not used to the 5 wire system. i guess my question now is, will the neutral wire be condidered a current carrying wire? i would say yes due to the unbalanced loads.

thanks for your answers.
 
Re: machine service rating questions

I would still say "No."

The set of three 75 hp motors and the single 25 hp motor comprise the majority of the load. Each of these four motors will draw a balanced load from all three phases. Even if you go out of your way to place all other loads on one or two phases, I don?t think you could create a sufficient overall imbalance to draw a significant neutral current. And anyone would advise you to make at least a marginal effort to spread the load as evenly as you reasonably can over all three phases. Please allow me to repeat: A wire that only carries the unbalanced load from the three phase wires does not count as a ?current carrying conductor.?
 
Re: machine service rating questions

charlie b,

if i may ask another question, the unbalanced load current would have been 10 amps tops.
is there a rule of thumb, calculation, or some rule that dictates when the neutral current carrying wire for unbalenced loads becomes a current carrying wire?

thanks again to all who post on this website.
thanks again charlie b.
 
Re: machine service rating questions

I am not aware of any published rule. I would say that if a reasonable effort is made to spread the load evenly, then the question of balance need not be raised. In your case, you have four motors that would collectively draw over 300 amps, so a 10 amp neutral current (I don?t know how you derived that figure, but I?ll buy it anyway) is clearly insignificant.
 
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