Neutral Conductors and Continuous Loads

Status
Not open for further replies.
F

finhead

Senior Member
I recently saw the following question based on the 2008 NEC

What size feeder conductors are required for a 200A continuous load if the terminals are rated 75C?
A. 2/0 AWG ungrounded and 1/0 AWG neutral conductors
B. 3/0 AWG ungrounded and 1/0 AWG neutral conductors
C. 4/0 AWG ungrounded and 1/0 AWG neutral conductors
D. 250 kcmil ungrounded and 3/0 AWG neutral conductors
The answer given was "D"
I think all of the above answers are wrong. The neutral, IMO, should be 4/0 AWG given the required size of the OCPD is 250A.
Comments welcome.

Brian Dolan
 
I also don't think there is enough info given. What kind of loads are they. If they are all lighting loads with neutral connections and no MWBC then the neutral would need to be the same size as the ungrounded conductors.
 
finhead said:
I recently saw the following question based on the 2008 NEC

What size feeder conductors are required for a 200A continuous load if the terminals are rated 75C?
A. 2/0 AWG ungrounded and 1/0 AWG neutral conductors
B. 3/0 AWG ungrounded and 1/0 AWG neutral conductors
C. 4/0 AWG ungrounded and 1/0 AWG neutral conductors
D. 250 kcmil ungrounded and 3/0 AWG neutral conductors
The answer given was "D"
I think all of the above answers are wrong. The neutral, IMO, should be 4/0 AWG given the required size of the OCPD is 250A.
Comments welcome.

Brian Dolan
Click to expand...

Forget about the neutral it is there to confuse you. The question is "What size feeder conductors".
 
Cavie said:
Forget about the neutral it is there to confuse you. The question is "What size feeder conductors".
Click to expand...

Are grounded conductors not part of the feeder conductors???? But I do see your point.
 
Dennis

The question I quoted was meant to highlight Section 215.2 (A) (1) Exception 2, which states that ungrounded conductors are required be sized at 125% of the continuous load but the neutral only needs to be sized at 100% of the continuous load.
 
Cavie

I would say that the required ungrounded conductors are 250 kcmil. That is, 125% of the continuous load
 
finhead said:
Dennis

The question I quoted was meant to highlight Section 215.2 (A) (1) Exception 2, which states that ungrounded conductors are required be sized at 125% of the continuous load but the neutral only needs to be sized at 100% of the continuous load.
Click to expand...


Ah yes... enlightenment again.
 
Brian,
I agree that in addition to being sized at 100% of the load the grounded conductor must also be sized so that it is protected by the feeder OCPD. That would require a 4/0 conductor. It appears that was missed in the proposal.
Don
 
this would be a typical 200 a commercial panel that did not have loads established at buildout IMO.

So if you oversize for voltage drop, how big does the grounding conductor have to be?:wink:
 
Brain,
After more thought, does the grounded conductor have to be protected by the OCPD and therefore sized based on the rating of the OCPD?.
Don
 
Don,

In response to your first post, I don't think anything was overlooked in the proposal. The last step in sizing conductors serving continuous loads, especially when adjustment or correction factors have been applied, was always to insure that the selected conductor was protected by the OCPD. The 2008 NEC did not change this requirement.

Regarding your second post, Section 240.4 does not make a distinction between grounded and ungrounded conductors.

While the proposal is no doubt technically correct, it creates the possiblity of a two-wire circuit having two different wire sizes. Even though the neutral conductor terminates on a terminal bar, it seems reasonable to expect that it's smaller size will still contribute additional heat to the enclosure that contains the overcurrent devices.

Happy Thanksgiving
Brian
 
Brian,
Regarding your second post, Section 240.4 does not make a distinction between grounded and ungrounded conductors.
Click to expand...
I understand that is what the rule says, but if that were really the case there would have been no need to add wording that was added to the end of 215.2(A)(1) for the 2005 code.
... The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.
Click to expand...
If the grounded conductor is required to be sized so that it is protected by the feeder OCPD, there would have been no need for this rule.
Even though the neutral conductor terminates on a terminal bar, it seems reasonable to expect that it's smaller size will still contribute additional heat to the enclosure that contains the overcurrent devices.
Click to expand...
Yes, it would add some heat to the enclosure, but not directly to the breaker itself. I have doubts that it would have any real effect on the trip time for the breaker.
Don

Don
 
Don
I'm coming around to your point of view. Slowly and reluctantly!
If I were feeding a range, for example, the relationship between the neutral conductor and the OCPD would not be a factor in conductor selection.

Reading the new rule in the 2008 Code, however, brings to mind a two-wire 120 volt circuit where the neutral is smaller than the ungrounded conductor and not protected by the OCPD. It just doesn't seem right here.

Brian
 
The 2008 NEC adds a second exception to Section 215.2(A)(1)

Exception No. 2: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.
 
Brian,
I am not so sure that you are not correct. There is nothing that says the grounded conductor does not have to be protected at its ampacity, it is just that it is not a common practice and other code rules seem to say that you are not required to protect it at its ampacity. Maybe some 2011 proposals are needed here.
Don
 
Status
Not open for further replies.
Top