The customer has a 480V MDP with no neutral bus bar feeding an 800A panel with no neutral bus bar. The 800A panel is being removed for renovation. The panel has both three phase and single phase loads on it. The lack of a neutral bus bar was discovered when the contractor removed the panel. The ground bus bar was being used as a neutral and the equipment grounds were connected to the metal in the floor - in other words there was no direct fault path. The supplied equipment in the building is ancient.
I have two questions:
1. Do you have documented horror stories about installations like this for me to give to the customer? Remedying the situation is likely to cost $100K (for the whole building) and they are likely to need incentive. I'd like to encourage them to fix this themselves, promptly, without involving authorities.
2. In the case of a ground fault (yes, I have been receiving Mike's newsletter and I should know this, but please help me out.) it seems to me that if the phase conductor shorts against the case, that the current has to flow all the way to the service entrance before the circuit is completed. The local breaker wouldn't trip until the current through the entire path was sufficient to trip it. Do I have this right? The impedance involved would be for the building, not for the local panel.
A secondary issue on this is that the ground wires that are being used as a neutral are #2 aluminum. The panel is fed with three sets of 3-300MCM aluminum#2 G. I have derated the new panel to 700A to accomodate the aluminum feeder ratings, however, the ground wires are severely undersized - should be 3/0. The service entrance is 300' from this panel.
I have two questions:
1. Do you have documented horror stories about installations like this for me to give to the customer? Remedying the situation is likely to cost $100K (for the whole building) and they are likely to need incentive. I'd like to encourage them to fix this themselves, promptly, without involving authorities.
2. In the case of a ground fault (yes, I have been receiving Mike's newsletter and I should know this, but please help me out.) it seems to me that if the phase conductor shorts against the case, that the current has to flow all the way to the service entrance before the circuit is completed. The local breaker wouldn't trip until the current through the entire path was sufficient to trip it. Do I have this right? The impedance involved would be for the building, not for the local panel.
A secondary issue on this is that the ground wires that are being used as a neutral are #2 aluminum. The panel is fed with three sets of 3-300MCM aluminum#2 G. I have derated the new panel to 700A to accomodate the aluminum feeder ratings, however, the ground wires are severely undersized - should be 3/0. The service entrance is 300' from this panel.
