table 250.66 or 250.122

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sokkerdude

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WE are in a disagreement here and I need some help. I have a motor control center with a 2000 amp main, that is feed from from a 4160 t0 480 volt transformer. With 6- 500mcm per phaze underground in PVC conduit. And the transformer is feed from a 4160 volt switch here... Which of these tables should I use in determining the ground wire (250.66 or 250.122). What is the difference between a "grounding elctrode conductor" and "equipement grounding conductor" Thank you
 
sokkerdude said:
What is the difference between a "grounding elctrode conductor" and "equipement grounding conductor" Thank you
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Grounding Conductor, Equipment. The conductor used to connect the non–currentcarrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system. See 250.118 for types of equipment grounding conductors. Proper sizing of equipment grounding conductors is found in 250.122 and Table 250.122.

Grounding Electrode. A device that establishes an electrical connection to the earth.
The definition of grounding electrode is new for the 2005 Code.

Grounding Electrode Conductor. The conductor used to connect the grounding
electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at the source of a separately derived system.
View attachment 2158
 
Is it a grounde system?? The secondary side of the 4160/480 volt transformer is Y, but there is no XO on the transformer and no 277 volt used anywhere. There is a significant ground grid system in the plant. with the transformer , building and MCC tied to it.... The 2000 amp main on the MCC is the only device from the transformer.... Unless they are protecting the secondary side of 4160/480 transformer with the OCPD on the primary. I pray that give's enogh information. thank you all
 
sokkerdude said:
Is it a grounde system?? The secondary side of the 4160/480 volt transformer is Y, but there is no XO on the transformer and no 277 volt used anywhere.
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It's important to know if it's a grounded Y system and a grounded conductor (neutral) wasn't run, or if it's a 3-wire Y. That would mean the grounded conductor is internaly grounded. If it is a grounded Y system and a grounded conductor is not present, then it's important to know if the transformer is grounded. In either case, the equipment grounding conductor or system bonding conductor is sized from table 250.66. Also good to know if the transformer is service equipment (utility)or fed from a feeder.
Rick
 
sokkerdude said:
?I have a motor control center with a 2000 amp main, that is feed from from a 4160 t0 480 volt transformer. With 6- 500mcm per phaze underground in PVC conduit. And the transformer is feed from a 4160 volt switch here... Which of these tables should I use in determining the ground wire (250.66 or 250.122). What is the difference between a "grounding elctrode conductor" and "equipement grounding conductor" Thank you
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sokkerdude said:
Is it a grounde system?? The secondary side of the 4160/480 volt transformer is Y, but there is no XO on the transformer and no 277 volt used anywhere..
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If there is no XO bonding connection to any of the transformers then you have an ?ungrounded? system, if there is XO bonding from a transformer then it?s a ?grounded? system, yours is ungrounded.

Your transformer GEC is sized from Table 250.66, see 250.30(B). Your GEC for the MCC is sized from the same Table 250.66, see 250.66.

The GEC connects the non current carrying items and neutral to earth, the grounds purpose is spelled out in 250.4(A)(1). The equipment grounding conductor connects all non current carrying items to form a path to source (effective ground fault path) to allow the operation of the overcurrent device in the event of a short.
 
If its a 277/480 secondary, section 250.20 requires this system to be grounded, IE a system bonding jumper, grounding electrodes, and GEC.

The system bonding jumper makes it a grounded system. Sometimes we have ungrounded delta systems by design. The secondary metal parts are still bonded together, but there is no system bonding jumper.
 
tom baker said:
If its a 277/480 secondary, section 250.20 requires this system to be grounded, IE a system bonding jumper, grounding electrodes, and GEC.
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I don't follow you here. According to the author:

...but there is no XO on the transformer and no 277 volt used anywhere.

So we're looking at an ungrounded system. Sounds like a delta secondary even though he states it as a wye. Do they make wye windings with no XO tap?

If it was a XFMR with an XO, then I don't see why we would have to create a grounded system. It cannot be grounded so that the voltage to ground does not exceed 150 volts and the neutral is not used as a circuit conductor. What did I miss?
 
tom baker said:
If its a 277/480 secondary, section 250.20 requires this system to be grounded, IE a system bonding jumper, grounding electrodes, and GEC.
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Tom,
If there are no 277 volt loads, the code not require a 277/480Y system to be a grounded system.
 
sandsnow said:
...but there is no XO on the transformer and no 277 volt used anywhere.

So we're looking at an ungrounded system...

If it was a XFMR with an XO, then I don't see why we would have to create a grounded system. It cannot be grounded so that the voltage to ground does not exceed 150 volts and the neutral is not used as a circuit conductor. What did I miss?
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Larry, his is an ungrounded system and I agree with you it's most likely delta. He's equipment grounding not grounding the XFMR winding.
 
tryinghard said:
Larry, his is an ungrounded system and I agree with you it's most likely delta. He's equipment grounding not grounding the XFMR winding.
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This could be a Y transformer too. Sounds like motor feed. Three phase conductors and an EGC. No grounded conductor needed.
 
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