megloff11x
Senior Member
I have an old building with three-phase 208 and its neutral coming in from the power company. The neutral is bonded to the building's metal structure, water pipes, etc.
We are adding disconnects to provide power for new machines. Some may require isolated grounds for noise reasons. Some won't.
NEC 250.24 describes adding a disconnect where the 3 phases and neutral wires (4 total) come in through the conduit to the box. The neutral bus is bonded to the box metal. The conduit is bonded to the service box and to the disconnect box. I refer to the 2002 handbook exhibits 250.10, 11, and 12, the last of which probably fits best.
There are cases in 250.96 where an isolated ground is needed, and it appears that in these cases, the neutral and ground wires are bonded together further "upstream" from the machine than in other cases. And they run a fifth "ground" wire.
Should I pull a separate, fifth green ground wire from the main service to each disconnect, or will using a four-wire system (3 phases & a neutral) and taking the fifth "ground" wire from the disconnect box's neutral bus suffice?
I am not so much concerned with noise issues as I am with different ground levels. I have seen computer signal wires throw sparks when plugged in, only to later find that the machine ground was tens of Volts different than the computer's ground (from the nearby 115V outlet), and the cable's shield was tied to chassis ground on both ends. This was in a particularly badly wired building.
I'm also concerned for the operator. For example in the 2002 NEC handbook exhibit 250.42, they show a subpanel system for isolated ground. How is that subpanel bonded to protect the user? Is it implied that the metal conduit pictured between it and the main service panel alone will do the trick? Some kinds of conduit do need an extra jumper, but they don't say what kind is used in their example. I see the subpanel box bonded to the ground bus inside the subpanel, but that ground bus is bonded to nothing else.
Bottom line, is it safe for machine and user to use four wires, five wires, or either?
Matt
We are adding disconnects to provide power for new machines. Some may require isolated grounds for noise reasons. Some won't.
NEC 250.24 describes adding a disconnect where the 3 phases and neutral wires (4 total) come in through the conduit to the box. The neutral bus is bonded to the box metal. The conduit is bonded to the service box and to the disconnect box. I refer to the 2002 handbook exhibits 250.10, 11, and 12, the last of which probably fits best.
There are cases in 250.96 where an isolated ground is needed, and it appears that in these cases, the neutral and ground wires are bonded together further "upstream" from the machine than in other cases. And they run a fifth "ground" wire.
Should I pull a separate, fifth green ground wire from the main service to each disconnect, or will using a four-wire system (3 phases & a neutral) and taking the fifth "ground" wire from the disconnect box's neutral bus suffice?
I am not so much concerned with noise issues as I am with different ground levels. I have seen computer signal wires throw sparks when plugged in, only to later find that the machine ground was tens of Volts different than the computer's ground (from the nearby 115V outlet), and the cable's shield was tied to chassis ground on both ends. This was in a particularly badly wired building.
I'm also concerned for the operator. For example in the 2002 NEC handbook exhibit 250.42, they show a subpanel system for isolated ground. How is that subpanel bonded to protect the user? Is it implied that the metal conduit pictured between it and the main service panel alone will do the trick? Some kinds of conduit do need an extra jumper, but they don't say what kind is used in their example. I see the subpanel box bonded to the ground bus inside the subpanel, but that ground bus is bonded to nothing else.
Bottom line, is it safe for machine and user to use four wires, five wires, or either?
Matt
