mall feeder wire and neutral

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Dominic Pipia

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Hello,

We do electrical maintainence at a large mall. We have incounter an intresting problem. We have been called out several times for a main breaker in the mdp tripping, seimens 150amp 3phase 480v. It feeds a store that has an I line breaker panel with 1 150amps 3 phase 480volt main breaker and 2 smaller ac loads. The distance is approx. 30ft some which is underground. This panel also carries a breaker for a step down transformer to 120/208, and carries various lighting loads in the store.
The 480 volt panel does not have a neutral feeding the store since there isn't any 277 load.
We believe that the mdp for the mall is grounded and bonded together at that point.
Do we still need a neutral at the store's panel? If yes, would this contribute to the main in the store not tripping?
Another option would be the aic is faster at the mdp main instead of the stores main.
any ideas?

Ampman



ampman
 
mall feeder wire and neutral

Is the 480 volt service located within the building and does it have a neutral conductor that is grounded? If not it violates 250.24(C). If the I line panel is being fed from a grounded service panel no grounded (neutral) conductor is required, but it must have an equipment grounding conductor.
As for the main breaker tripping I think you on the right track, you may have coordination problem.
 
Dominic Pipia said:
Do we still need a neutral at the store's panel? If yes, would this contribute to the main in the store not tripping?
Another option would be the aic is faster at the mdp main instead of the stores main.
Click to expand...

The presence or absence of a neutral will not affect the tripping of the breakers.

The AIC rating of a breaker has nothing to do with how fast it trips on 'overloads'. It is possible that a higher rated AIC breaker may trip faster on a very high short circuit (i.e. one that probably results in equipment/conductor damage).

With two, standard, 150A thermal magnetic breakers in series it is all but impossible to coordinate them.
 
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