Table 250.102(C)(1)

Status
Not open for further replies.
tkb

tkb

Senior Member
Location
MA
When using Table 250.102(C)(1), and using parallel feeders I am interpreting note 3 to say to add the kcmils of each parallel to get the size to use for sizing the size of the grounded conductor.

3. If multiple sets of service-entrance conductors are used as permitted in 230.40, Exception No. 2, or if
multiple sets of ungrounded supply conductors are installed for a separately derived system, the equivalent
size of the largest ungrounded supply conductor(s) shall be determined by the largest sum of the areas of the
corresponding conductors of each set.

We are having a discussion with an engineer that thinks you only need to use one of the parallels to do the sizing.

Example: Parallel 500kcmil 2 sets would be 1000kcmil and would size the grounded conductor at 2/0 CU or 4/0 AL.
The engineer stated that it should be sized at #2 CU or 1/0 AL.

I know that you should add the parallel sets together, I am looking for confirmation.
 
Assuming you are asking about the grounded conductor for each set of paralleled conductors, that is sized by the size of the largest ungrounded conductor in that set. A set being the conductors in an individual conduit or cable.
 
The get that size, don't you add the parallels together to get the size needed on the table and install that size grounded conductor with each set.

So if you had two sets of parallel 500kcmil conductors, you would need a 2/0 CU or 4/0 AL in each parallel conduit or cable.
 
I'm confused. :?
  • You size the ungrounded conductors for the load.
  • You size the grounded conductor for its load, but you get to take into account that it might have less load than the ungrounded (since it is carrying only the unbalanced current).
  • You size the SSBJ per that table.
  • Why is that table talking about the grounded conductor?
 
tkb said:
The get that size, don't you add the parallels together to get the size needed on the table and install that size grounded conductor with each set.

So if you had two sets of parallel 500kcmil conductors, you would need a 2/0 CU or 4/0 AL in each parallel conduit or cable.
Click to expand...

You would need 1/0 cu minimum in each conduit.
 
charlie b said:
I'm confused. :?
  • You size the ungrounded conductors for the load.
  • You size the grounded conductor for its load, but you get to take into account that it might have less load than the ungrounded (since it is carrying only the unbalanced current).
  • You size the SSBJ per that table.
  • Why is that table talking about the grounded conductor?
Click to expand...
As you know, the issue with the table itself was to regurgitate the information from T250.66 to T250.102(C)(1) which could bring into the fray the 12.5% aspect where applicable that is not afforded in T250.66 and it was simply misleading prior to this creation. To be perfectly honest, we do these things for service conductors already and simply use 250.24(C)(2) to direct us to 250.24(C)(1) which directs us to 250.102(C)...or even 250.30 when the SBJ is not located at the source and a grounded conductor it installed which always now directs us back to T250.102(C)(1).

Now, as for feeders we have 215.2(A)(3) for sizing guidance so we don't need that table as it directs us to 250.122. In branch circuits, we get the benefit of the rules in 210.19(A) to help us size those branch circuits, which the grounded conductor is one of them. So in an attempt to answer your question as to "why is that table talking about the grounded conductor" would be due to the references in 250.24(C)(1) and 250.30(A)(3) which all seem to correlate with no real OCPD ahead of the conductors. I will go out on a limb and say it was low hanging fruit to move the reference from T250.66 everywhere when talking about grounded conductors and move it to a regurgitated table used for bonding jumpers and so on and simply allow for the ability to achieve the 12.5% where applicable.

Sorry.....best I got for ya. I do sit on that CMP but this was long established before I came to the party as I did not join this CMP until after T250.102(C)(1) was introduced. However, my friend I can inquire about it with friends of mine who where at the table and provide their thoughts but I would guess they are similar to my assumptions.
 
MasterTheNEC said:
I concur with David. Why, because they are in paralleled and even if we did SUM them out the grounded conductor still can't be smaller than 1/0 AWG as expressed in 310.10(H)(1) when paralleling.
Click to expand...

I understand that 310.10(H)(1) says that parallels can’t be smaller than 1/0, but in my example of two sets of 500kcmil, shouldn’t the size be 2/0 CU or 4/0 AL?

The criteria of the table is “Size of largest ungrounded conductor or equivalent area of paralleled conductors”. The equivalent area of two 500kcmil ungrounded conductors is 1000kcmil.

I don’t see how a 1/0 would comply with Table 250.102(C)(1).
 
tkb said:
I understand that 310.10(H)(1) says that parallels can’t be smaller than 1/0, but in my example of two sets of 500kcmil, shouldn’t the size be 2/0 CU or 4/0 AL?

The criteria of the table is “Size of largest ungrounded conductor or equivalent area of paralleled conductors”. The equivalent area of two 500kcmil ungrounded conductors is 1000kcmil.

I don’t see how a 1/0 would comply with Table 250.102(C)(1).
Click to expand...

The ungrounded conductors in each conduit are 500kcmil. 1/0 is the min size grounded conductor for 500kcmil.

See 250.24(C)(2)
 
Last edited:
tkb said:
I understand that 310.10(H)(1) says that parallels can’t be smaller than 1/0, but in my example of two sets of 500kcmil, shouldn’t the size be 2/0 CU or 4/0 AL?

The criteria of the table is “Size of largest ungrounded conductor or equivalent area of paralleled conductors”. The equivalent area of two 500kcmil ungrounded conductors is 1000kcmil.

I don’t see how a 1/0 would comply with Table 250.102(C)(1).
Click to expand...
The 1/0 AWG was just in terms of the minimums size if the application of paralleling the grounded (neutral) conductor was in play and so on and really just as reference. I think if the application was indeed a condition where paralleling was taking place the grounded conductor is required to also be paralleled.

So I believe in your case if you had (2) 500 kcmils (sets) then you would indeed have 1000 kcmil which based on 250.102(C)(1) would be 2/0 CU or 4/0 AL. However, and I am only assuming this is what your engineer is speaking to, if you parallel that as permitted in 310.10(H) you would have a 2/0 at 133,100 CM / 2 = 66,550 CM and so the paralleled grounded conductor in each raceway has to be no less than 66, 550 CM....which would be 1 AWG at 83,690 CM. However, due to the rules in 310.10(H) the smallest they can be are 1/0 AWG....so that is where the 1/0 AWG in Davids response more than likely came from.

Please correct me if I am wrong David as I do not want to put words in your mouth my friend.

Edit : the reason I do not like to explain this by referencing 250.24(C)(1) or (2) is that is service conductors and this could apply to more than just those conductors.
 
I was looking at a similar install a week ago tkb and trying to wrap my brain around it.

You are allowed to size the grounded conductor as mentioned above based on what is in the particular conduit or cable.

Using your example tkb, if we did parallel 500's in ONE conduit, then yes, it would be based on 1000mcm.
 
david luchini said:
The ungrounded conductors in each conduit are 500kcmil. 1/0 is the min size grounded conductor for 500kcmil.

See 250.24(C)(2)
Click to expand...

250.24(C)(2) refers to 250.24(C)(1) which refers to Table 250.102(C)(1),

Table 250.102(C)(1) sizing criteria is “Size of largest ungrounded conductor or EQUIVALENT AREA FOR PARALLEL CONDUCTORS” the equivalent size of two 500kcmil is 1000kcmil and according to the table the size is to be 2/0 CU or 4/0 AL.

I just can’t get to the 1/0 size that you keep stating.
Can you please make this clearer?
 
tkb said:
250.24(C)(2) refers to 250.24(C)(1) which refers to Table 250.102(C)(1),

Table 250.102(C)(1) sizing criteria is “Size of largest ungrounded conductor or EQUIVALENT AREA FOR PARALLEL CONDUCTORS” the equivalent size of two 500kcmil is 1000kcmil and according to the table the size is to be 2/0 CU or 4/0 AL.

I just can’t get to the 1/0 size that you keep stating.
Can you please make this clearer?
Click to expand...

250.24(C)(2) says the size of the grounded conductor in each raceway shall be based on the size of the ungrounded conductors in the raceway.
 
david luchini said:
250.24(C)(2) says the size of the grounded conductor in each raceway shall be based on the size of the ungrounded conductors in the raceway.
Click to expand...

I agree, no reason to do anything beyond looking at the ungrounded conductor size in each raceway and then go to the table. Same applies to SSBJ's.
 
david luchini said:
250.24(C)(2) says the size of the grounded conductor in each raceway shall be based on the size of the ungrounded conductors in the raceway.
Click to expand...

infinity said:
I agree, no reason to do anything beyond looking at the ungrounded conductor size in each raceway and then go to the table. Same applies to SSBJ's.
Click to expand...

250.24(C)(2) says,

(1) Sizing for a Single Raceway or Cable. The grounded con-ductor
shall not be smaller than specified in Table 250.102(C)(1).


(2) Parallel Conductors in Two or More Raceways or
Cables. If the ungrounded service-entrance conductors are
installed in parallel in two or more raceways or cables, the
grounded conductor shall also be installed in parallel. The size
of the grounded conductor in each raceway or cable shall be based
on the total circular mil area of the parallel ungrounded conductors
in the raceway or cable, as indicated in 250.24(C)(1), but not
smaller than 1/0 AWG.


Informational Note: See 310.10(H) for grounded conductors
connected in parallel.

You need to add the kcmils to get the size per table 250.102(C)(1).
 
tkb said:
250.24(C)(2) says,

(1) Sizing for a Single Raceway or Cable. The grounded con-ductor
shall not be smaller than specified in Table 250.102(C)(1).


(2) Parallel Conductors in Two or More Raceways or
Cables. If the ungrounded service-entrance conductors are
installed in parallel in two or more raceways or cables, the
grounded conductor shall also be installed in parallel. The size
of the grounded conductor in each raceway or cable shall be based
on the total circular mil area of the parallel ungrounded conductors
in the raceway or cable, as indicated in 250.24(C)(1), but not
smaller than 1/0 AWG.


Informational Note: See 310.10(H) for grounded conductors
connected in parallel.

You need to add the kcmils to get the size per table 250.102(C)(1).
Click to expand...

There is nothing to add. You have two conduits with 500kcmil ungrounded conductors in each. 500kcmil in the first conduit requires a 1/0 min grounded conductor. 500kcmil in the second conduit requires the same.
 
It clearly says in the raceway.

The size of the grounded conductor in each raceway or cable shall be based on the total circular mil area of the parallel ungrounded conductors in the raceway or cable, as indicated in 250.24(C)(1), but not smaller than 1/0 AWG.
Click to expand...

If all of the parallel conductors where in a single wireway then you would add them, if they were in several separate raceways then you only need to use the size of the underground conductor in each raceway.
 
Status
Not open for further replies.
Top