three phase load

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hhsting

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I have one motor 50 HP, 480V three phase fed by service disconnect. Three phase load does not require neutral however NEC is saying their has to be main bonding jumper at the service disconnect. Main bonding jumper requires bonding neutral, ground, and service disconnect enclosure to together. However, I do not need neutral at all one three phase load all balanced. The question is do I need to bring in neutral to the service disconnect even though I do not need neutral?
 
hhsting said:
I have one motor 50 HP, 480V three phase fed by service disconnect. Three phase load does not require neutral however NEC is saying their has to be main bonding jumper at the service disconnect. Main bonding jumper requires bonding neutral, ground, and service disconnect enclosure to together. However, I do not need neutral at all one three phase load all balanced. The question is do I need to bring in neutral to the service disconnect even though I do not need neutral?
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As already stated-250.24(C). But you need to realize the importance of this. Without the neutral brought to the service disconnect there is no fault return path.
 
texie said:
As already stated-250.24(C). But you need to realize the importance of this. Without the neutral brought to the service disconnect there is no fault return path.
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Which would not make any difference if the service were ungrounded. In that case a single fault would not cause any hazardous voltage anywhere.
But since the wye point of the service is grounded at the utility transformer secondary, a fault to the motor case or any other exposed metal in your facility will bring the voltage at that point close to the utility line to neutral voltage. This will continue indefinitely because a typical earth ground (via GES only) at your service panel will not conduct enough current to trip OCPD and terminate the fault condition.
 
hhsting said:
I have one motor 50 HP, 480V three phase fed by service disconnect. Three phase load does not require neutral however NEC is saying their has to be main bonding jumper at the service disconnect. Main bonding jumper requires bonding neutral, ground, and service disconnect enclosure to together. However, I do not need neutral at all one three phase load all balanced. The question is do I need to bring in neutral to the service disconnect even though I do not need neutral?
Click to expand...

Yes. After that point, you need and use an equipment ground.
 
The NEC calls this a 'grounded conductor'. Between the utility and the service disconnect the grounded conductor serves as both a circuit conductor _and_ a grounding/bonding conductor.

Your load doesn't need the neutral circuit conductor. Just think of it as the grounding/bonding wire and you will see why it remains necessary.

-Jon
 
I have balanced three phase load. I do not need to bring neutral i.e grounded conductor however code requires grounded conductor brought to the service disco. So what size should the grounded conductors be? What table should be used?

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hhsting said:
I have balanced three phase load. I do not need to bring neutral i.e grounded conductor however code requires grounded conductor brought to the service disco. So what size should the grounded conductors be? What table should be used?

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See 250.24(C)
 
ptonsparky said:
Yes. After that point, you need and use an equipment ground.
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Ground and neutral are not the same thing.
Sure, the neutral may be grounded but why would you neutral rather than ground at the motor?
 
Besoeker3 said:
Ground and neutral are not the same thing.
Sure, the neutral may be grounded but why would you neutral rather than ground at the motor?
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He needs a return fault current path at the motor.

If a neutral is not needed at the load, and, If he's using a wire type conductor for the fault return path, the conductor between the service disconnect and the motor is the Equipment Grounding Conductor, not a Neutral Conductor.

The conductor between the service disconnect and the utility transformer is the Neutral or Grounded Conductor, not the Equipment Grounding Conductor.


JAP>
 
Besoeker3 said:
Ground and neutral are not the same thing.
Sure, the neutral may be grounded but why would you neutral rather than ground at the motor?
Click to expand...

Under US utility standards (NESC) the service _neutral_ is used as the fault current path back to the transformer. So up to the service point (where NEC rules take over) 'ground' (meaning fault current path) and 'neutral' (meaning grounded circuit conductor) are in fact the same thing. After the service point they must be separate.

The OP does not need to bring the neutral past the service point; they do need to bring the grounding/bonding conductor past the service point.

-Jon
 
winnie said:
Under US utility standards (NESC) the service _neutral_ is used as the fault current path back to the transformer. So up to the service point (where NEC rules take over) 'ground' (meaning fault current path) and 'neutral' (meaning grounded circuit conductor) are in fact the same thing. After the service point they must be separate.

The OP does not need to bring the neutral past the service point; they do need to bring the grounding/bonding conductor past the service point.

-Jon
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So whats grounded conductor nean in NEC if not neutral? How does grounded conductor tie to service point? Service point phase tie to phase conductor, neutral service point ties to where?

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hhsting said:
So whats grounded conductor nean in NEC if not neutral? How does grounded conductor tie to service point? Service point phase tie to phase conductor, neutral service point ties to where?

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That is what the whole point of the main bonding jumper is.

As the service point you go from a grounded conductor (serving as both the circuit conductor and the fault current path) (on the NESC side) to separate 'neutral' and 'ground' conductors (on the NEC side).

-Jon
 
jap said:
He needs a return fault current path at the motor.

If a neutral is not needed at the load, and, If he's using a wire type conductor for the fault return path, the conductor between the service disconnect and the motor is the Equipment Grounding Conductor, not a Neutral Conductor.

The conductor between the service disconnect and the utility transformer is the Neutral or Grounded Conductor, not the Equipment Grounding Conductor.


JAP>
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Perhaps I expressed myself poorly.
A line to line fault would result in the lines and trip the overcurrent device.
A line to ground fault would be detected by the ground fault device.
I don't see what part a neutral would play in this application.
We don't run a neutral for a three phase motor
Sometimes there are auxiliary circuits like motor heaters but these are normally on an isolated 110V supply.
 
Besoeker3 said:
Perhaps I expressed myself poorly.
A line to line fault would result in the lines and trip the overcurrent device.
A line to ground fault would be detected by the ground fault device.We do not regularly have ground fault devices that trip in the milliamp range
I don't see what part a neutral would play in this application. The neutral is the fault return path to the transformer from the Service Equipment.
We don't run a neutral for a three phase motor. We don't either. From the Service on it is an Equipment Ground.
Sometimes there are auxiliary circuits like motor heaters but these are normally on an isolated 110V supply.
Click to expand...

A neutral can be or is considered a normal current carrying conductor. An equipment ground is not. Yes, it can be with low level faults, but that is exactly what we want it to do. Without it and the neutral return, equipment potential rises.
 
In simple terms.

He needs a conductor at the Service disconnect to get the return fault current back to it's origin at the transformer.
This most generally is a "Neutral" or "Grounded Conductor" that is bonded to the Service Disconnect.

He then needs some way to clear a fault should there happen to be one on the load side of the Service Disconnect.
Not all installations have ground fault monitoring systems as was mentioned.

This is most commonly achieved by an "Equipment Grounding Conductor" installed from the load back to the Service disconnect.

There are several different types of 3 ph service configurations and different ways to achieve shut down of a piece of equipment due to a ground fault.

I'm simply describing one that I'm most familiar with.


JAP>
 
ptonsparky said:
A neutral can be or is considered a normal current carrying conductor. An equipment ground is not. Yes, it can be with low level faults, but that is exactly what we want it to do. Without it and the neutral return, equipment potential rises.
Click to expand...
Where would you connect a neutral on a standard 3-phase cage induction motor?
 
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