parallel ground upsizing

[mjmike]

When sizing the EGC for a parallel circuit, what do you start with (base conductor size) when determining the upsize required on the ground. For a large feeder, there is a number of different parallel combinations. What is the true base conductor size?

Using a feeder for a 1000A 3-phase main breaker in a switchboard, you can use 3 sets 500mcm at 344A each (derated for 80% as nonlinear) for 1032A total. Or 4 sets of 300 at 256A per set (derated for 80% as nonlinear) for 1024A total. Even though the 3 sets of 500 may make more sense, is it considered an increase in conductor size meaning maybe a larger ground?

What is the base conductor size; starting point per say?

 

[chris kennedy]

Any combination the EGC for each conduit is based on T250.122.

 

[Carultch]

When sizing the EGC for a parallel circuit, what do you start with (base conductor size) when determining the upsize required on the ground. For a large feeder, there is a number of different parallel combinations. What is the true base conductor size?

Using a feeder for a 1000A 3-phase main breaker in a switchboard, you can use 3 sets 500mcm at 344A each (derated for 80% as nonlinear) for 1032A total. Or 4 sets of 300 at 256A per set (derated for 80% as nonlinear) for 1024A total. Even though the 3 sets of 500 may make more sense, is it considered an increase in conductor size meaning maybe a larger ground?

What is the base conductor size; starting point per say?

I've asked this question before, and cannot find a straight answer.

For a conservative assumption, I would use my starting size that has the same number of parallel sets & configuration as the desired end result.

If that ends up being less than 1/0 due to an extreme case, the minimum you are allowed to parallel, use that number of 1/0 as your starting point. Because the NEC says the minimum size that has sufficient ampacity for the intended installation. Even if parallel #1's adds up to the desired ampacity, only #1/0's and larger can be paralleled.

My common example is starting at 600 kcmil, and upsizing to parallel 400s, for a long 400A circuit.

Do I pretend it started at parallel 3/0 (or 4/0 in the same pipe)?
Or can I get less of an upsize ratio, by starting with the single set of 600?

 

[don_resqcapt19]

When sizing the EGC for a parallel circuit, what do you start with (base conductor size) when determining the upsize required on the ground. For a large feeder, there is a number of different parallel combinations. What is the true base conductor size?

Using a feeder for a 1000A 3-phase main breaker in a switchboard, you can use 3 sets 500mcm at 344A each (derated for 80% as nonlinear) for 1032A total. Or 4 sets of 300 at 256A per set (derated for 80% as nonlinear) for 1024A total. Even though the 3 sets of 500 may make more sense, is it considered an increase in conductor size meaning maybe a larger ground?

What is the base conductor size; starting point per say?

I would not consider that an increase in conductor size. I would want to see a 2/0 copper EGC in each raceway.

 

[Carultch]

When sizing the EGC for a parallel circuit, what do you start with (base conductor size) when determining the upsize required on the ground. For a large feeder, there is a number of different parallel combinations. What is the true base conductor size?

Using a feeder for a 1000A 3-phase main breaker in a switchboard, you can use 3 sets 500mcm at 344A each (derated for 80% as nonlinear) for 1032A total. Or 4 sets of 300 at 256A per set (derated for 80% as nonlinear) for 1024A total. Even though the 3 sets of 500 may make more sense, is it considered an increase in conductor size meaning maybe a larger ground?

What is the base conductor size; starting point per say?

Increasing size for derating and other local factors doesn't require an upsized ground. The main intent of this rule is voltage drop.

 

[mjmike]

Increasing size for derating and other local factors doesn't require an upsized ground. The main intent of this rule is voltage drop.

Mentioned derating just so somebody can understand how I got the amperage; please don't get hung up on that point.

 

[Carultch]

Mentioned derating just so somebody can understand how I got the amperage; please don't get hung up on that point.

My point is that, if the reason you upsize is not a function of the length of the circuit, this rule probably doesn't apply.

 

[mjmike]

My point is that, if the reason you upsize is not a function of the length of the circuit, this rule probably doesn't apply.

I understand that; not the point of my post.

My point is, what is the starting conductor size for paralleled conductors if the most practical parallel combination equates to a higher ampacity than a less practical paralled combination.

 

[Carultch]

I understand that; not the point of my post.

My point is, what is the starting conductor size for paralleled conductors if the most practical parallel combination equates to a higher ampacity than a less practical paralled combination.

As long as it is the minimum size (ampacity-wise) for that particular number & conduit configuration of conductors in parallel, you don't need to upsize ground.

 

[luckylerado]

I understand that; not the point of my post.

My point is, what is the starting conductor size for paralleled conductors if the most practical parallel combination equates to a higher ampacity than a less practical parallel combination.

You start with the size required in 250.122 based on the OCP device. You then upsize the EGC from there only if you upsize the ungrounded conductors from "the minimum size that has sufficient ampacity for the intended installation"

Your ratio is that of the min required corrected, and derated size before any voltage drop is applied to the calculated recommended size after voltage drop is applied.

Not to be confused with GEC sized per 250.66, based on conductor cross section and upsize is not required.

 

[mjmike]

You start with the size required in 250.122 based on the OCP device. You then upsize the EGC from there only if you upsize the ungrounded conductors from "the minimum size that has sufficient ampacity for the intended installation"

Your ratio is that of the min required corrected, and derated size before any voltage drop is applied to the calculated recommended size after voltage drop is applied.

Not to be confused with GEC sized per 250.66, based on conductor cross section and upsize is not required.

Where are you getting "the minimum size that has sufficient ampacity for the intended installation"?

Lets look at a feed to an 800a breaker. 6-1/ 0 = 816A but a more practical parallel set is 3-300 for 840A. (4CCC in both cases; not a factor) Now, would the ground be 1/0 in both cases or would the ground with the 300 be 4/0? 4/0 as an upsized ratio because 300's are used instead of 1/0. (1,050,000cm/633,600cm) X 105,600cm = 175000cm minimum.

 

[david luchini]

Where are you getting "the minimum size that has sufficient ampacity for the intended installation"?

That language is directly from 250.122(B).

Lets look at a feed to an 800a breaker. 6-1/ 0 = 816A but a more practical parallel set is 3-300 for 840A. (4CCC in both cases; not a factor) Now, would the ground be 1/0 in both cases or would the ground with the 300 be 4/0? 4/0 as an upsized ratio because 300's are used instead of 1/0. (1,050,000cm/633,600cm) X 105,600cm = 175000cm minimum.

IMO, 1/0 would be correct for both cases. (I think you mean 3 sets of 350, not 300.)

 

[mjmike]

That language is directly from 250.122(B).



IMO, 1/0 would be correct for both cases. (I think you mean 3 sets of 350, not 300.)

Thanks, I see the wording. I also did mean 350.

 

[luckylerado]

Where are you getting "the minimum size that has sufficient ampacity for the intended installation"?

250.122(B) 2014

Lets look at a feed to an 800a breaker. 6-1/ 0 = 816A but a more practical parallel set is 3-300 for 840A. (4CCC in both cases; not a factor) Now, would the ground be 1/0 in both cases or would the ground with the 300 be 4/0?

1/0 for both cases.

4/0 as an upsized ratio because 300's are used instead of 1/0. (1,050,000cm/633,600cm) X 105,600cm = 175000cm minimum.

This is flawed because the min in not 1/0 it is the sum of the cross sectional area of 6 1/0. Not 300 but the sum of 3 300s. Not sure what table you are getting those ampacities from but they do not seem correct to me.

Don't forget about 240.4 and 240.6 when determining minimum size

 

[mjmike]

250.122(B) 2014



1/0 for both cases.




This is flawed because the min in not 1/0 it is the sum of the cross sectional area of 6 1/0. Not 300 but the sum of 3 300s. Not sure what table you are getting those ampacities from but they do not seem correct to me.

Don't forget about 240.4 and 240.6 when determining minimum size

I meant 350's not 300's. The ampacities are based on the 90 degree column derated by 80% for 4CCC due to nonlinear loads.

 

[Smart $]

FWIW, 250.122(B) is up for change in 2017 to read:

(B) Increased in Size. If ungrounded conductors are increased in size for any reason from the minimum size that has sufficient ampacity for the intended installation before the application of any adjustment or correction factor(s), wire-type equipment grounding conductors shall be increased in size .The increase in size shall be in the same proportion as the increase in the size of the ungrounded conductors using their circular mil area.

Not sure exactly how to implement that. Will 90°C wire be based on the 75°C column value or the 90°C column???

Speculating, may mean in future that for 3 sets, the minimum size will be 250kcmil 90°C Cu @ 290A each... and having to run 350kcmil would require an EGC upsize of 350/250×(1/0@105600kcmil)=147840 or 3/0 Cu minimum.

Anyway, that's what's currently up for comment. Get involved if you don't like it... :happyyes:

 

[luckylerado]

Ha. That is how I always read 250.122(B) until this forum convinced me to drink the "For voltage drop only" kool aid.

How do I get involved as I do not like it.

 

[mjmike]

Thanks for all the help. For the 800a example, I am in agreement it is 1/0 ground. I guess what makes the most sense to me is for any partuclar parallel set in question, if it is the first appropriate set quantity for that conductor size, then it gets the base ground. Only increase the ground size if you add another set of the same conductor size. Basically treat each individual parallel combination as it's own and not compare it to another when it involves a different conductor size.

 

[Carultch]

FWIW, 250.122(B) is up for change in 2017 to read:

What's the substantiation for this change?

 

[Smart $]

Ha. That is how I always read 250.122(B) until this forum convinced me to drink the "For voltage drop only" kool aid.

How do I get involved as I do not like it.

Create an account on NFPA website if you don't already have one.

Then go to:
(direct) http://submittals.nfpa.org/TerraViewWeb/ViewerPage.jsp?id=70-2014.ditamap&pubStatus=FDR
(indirect) http://www.nfpa.org/codes-and-standards/document-information-pages?mode=code&code=70&tab=nextedition then click on "First Draft Report" link.

There's some "instruction" links at the top right of the TerraView window, but I don't know if the contents help....