De-rating the neutral conductor for a panelboard serving only three phase loads

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kgk1961

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We are currently installing a project working at the direction of a plant engineer. The equipment that we are installing only has four (4) connection points; three ungrounded conductors and a grounding conductor (480V, 3-phase). This accounts for 95% of the project. The other 5% consists of mostly 277V lighting, which is fed from a separate panelboard dedicated for lighting. (It is an existing facility and 120V is already in place from another source.) Therefore, majority of the panelboards are dedicated solely to providing power to the equipment that is being installed, all of which is 480V 3-phase (with NO neutral connection point).

The engineer has come to us and requested that we add a neutral conductor from the main switchboard to each of the panelboards "just in case" something comes up in the future and they need 277V for controls. He has gone on to direct us to "de-rate" the neutral conductor to 60% of the ampacity of the ungrounded conductors.
:jawdrop:
Is this premissible per the NEC?
How did he come up with 60%?
 
The neutral is based on the larger of the unbalanced load or the size of the feeder OCPD - similar to sizing a EGC.
However the neutral does not have to be derated for a continuous load and can be sized at 100%.
 
kgk1961 said:
We are currently installing a project working at the direction of a plant engineer. The equipment that we are installing only has four (4) connection points; three ungrounded conductors and a grounding conductor (480V, 3-phase). This accounts for 95% of the project. The other 5% consists of mostly 277V lighting, which is fed from a separate panelboard dedicated for lighting. (It is an existing facility and 120V is already in place from another source.) Therefore, majority of the panelboards are dedicated solely to providing power to the equipment that is being installed, all of which is 480V 3-phase (with NO neutral connection point).

The engineer has come to us and requested that we add a neutral conductor from the main switchboard to each of the panelboards "just in case" something comes up in the future and they need 277V for controls. He has gone on to direct us to "de-rate" the neutral conductor to 60% of the ampacity of the ungrounded conductors.
:jawdrop:
Is this premissible per the NEC?
How did he come up with 60%?
Click to expand...
The neutral can't be smaller than that required by 250.122 for a feeder. There is no "60%" rule.
See 215.2(A)(2)
 
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"TO THE TABLE WE GO!!"

"TO THE TABLE WE GO!!"

texie said:
The neutral can't be smaller than that required by 250.122 for a feeder. There is no "60%" rule.
See 215.2(A)(2)
Click to expand...

Okay, so I go to 250.122 (which is for "grounding"), and (A) says, "....shall not be smaller than shown in Table 250.122,...".
So the "Table" (which is for EGCs) says that if my breaker is 400A, I can use a #3AWG copper conductor as my "grounded" conductor.

:?
 
kgk1961 said:
Okay, so I go to 250.122 (which is for "grounding"), and (A) says, "....shall not be smaller than shown in Table 250.122,...".
So the "Table" (which is for EGCs) says that if my breaker is 400A, I can use a #3AWG copper conductor as my "grounded" conductor.

:?
Click to expand...

You got it.
The reason for this rule is to ensure that the grounded conductor is large enough to carry the short circuit current in order to trip the over current device.
 
But this is the minimum, and if line to neutral loads are installed later it must at that time be large enough to carry that neutral load.

Sent from my XT1080 using Tapatalk
 
kgk1961 said:
Okay, so I go to 250.122 (which is for "grounding"), and (A) says, "....shall not be smaller than shown in Table 250.122,...".
So the "Table" (which is for EGCs) says that if my breaker is 400A, I can use a #3AWG copper conductor as my "grounded" conductor.

:?
Click to expand...

GoldDigger said:
But this is the minimum, and if line to neutral loads are installed later it must at that time be large enough to carry that neutral load.

Sent from my XT1080 using Tapatalk
Click to expand...


A similar situation - I will throw in if it would happen to be a service the grounded conductor can not be any smaller than allowed by T250.66.
 
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