400 amp 3 phase 4w UG service conductor sizing

Steviechia2

Senior Member
Location
Massachusetts
Does this sound right?
I will be running 2 conduits with 250 mcm al
Can I take 90 degree column 230 amp x 80% (for more than 4 current carrying conductors) =184 Amps x 2=368 amps
Or do I have to use the 60 degree column which then I would have to use 300 mcm al
 

kwired

Electron manager
Location
NE Nebraska
Does this sound right?
I will be running 2 conduits with 250 mcm al
Can I take 90 degree column 230 amp x 80% (for more than 4 current carrying conductors) =184 Amps x 2=368 amps
Or do I have to use the 60 degree column which then I would have to use 300 mcm al
The neutral of a 3ph 4 wire circuit is not a current carrying conductor for the purposes of 310.15(B)(3). The net current on the neutral is only going to be the imbalance of the three phase conductors. That means you have only three current carrying conductors and no ampacity adjustment is necessary. Now if you have a lot of non linear load that could change things.

The neutral considered current carrying if there is only two phases plus the neutral of a three phase wye system. In that case if both phases are carrying same current, the neutral is carrying approximately that same current level also.
 

Steviechia2

Senior Member
Location
Massachusetts
The neutral of a 3ph 4 wire circuit is not a current carrying conductor for the purposes of 310.15(B)(3). The net current on the neutral is only going to be the imbalance of the three phase conductors. That means you have only three current carrying conductors and no ampacity adjustment is necessary. Now if you have a lot of non linear load that could change things.

So If I use 2 conduits I could use 4/0 al.
If I use 1 conduit I would derate 80% for my 6 conductors, can I use the 90 degree column for the derated?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
The neutral of a 3ph 4 wire circuit is not a current carrying conductor for the purposes of 310.15(B)(3). The net current on the neutral is only going to be the imbalance of the three phase conductors. That means you have only three current carrying conductors and no ampacity adjustment is necessary. Now if you have a lot of non linear load that could change things.

So If I use 2 conduits I could use 4/0 al.
If I use 1 conduit I would derate 80% for my 6 conductors, can I use the 90 degree column for the derated?
You can use the 90C, if the wire is rated 90C for the given conditions , and de-rate from there. You final ampacity cannot be more than the weakest link which is usually the terminations at 75C.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
If you use 4/0 aluminum with 90C wire then you have 205 amps. Multiply by 2 and you have 410 amps but at 80% you only have 328 amps so it would have to be a 350 amps breaker not 400
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
4/0 AL=180 amps
Two 4/0 AL=360 amps

Am I missing something?
I thought he was asking about the 80% de-rating for 6 conductors (1 conduit). There you would start at the 90C. I see he has 2 conduits-- I saw the 80% but didn't realize he was counting the neutral
 

Steviechia2

Senior Member
Location
Massachusetts
Parallel 4/0's are good for 360 amps, not 400. Is your calculated load less than 360?
No it's not

I thought he was asking about the 80% de-rating for 6 conductors (1 conduit). There you would start at the 90C. I see he has 2 conduits-- I saw the 80% but didn't realize he was counting the neutral
I did have a question in there somewhere about 1 conduit.
I was just making sure that I could use the 90c for derating. I know it can't be more than the 75c once tallied.
I knew this but some days reading through the code the brain just doesn't click.:blink:

Thanks All
 

Steviechia2

Senior Member
Location
Massachusetts
The neutral of a 3ph 4 wire circuit is not a current carrying conductor for the purposes of 310.15(B)(3). The net current on the neutral is only going to be the imbalance of the three phase conductors. That means you have only three current carrying conductors and no ampacity adjustment is necessary. Now if you have a lot of non linear load that could change things.

I always thought that the grounded conductor was to be counted I see that now in 310.15B (5)

So would it be the same with a 110 volt branch circuit situation. example-3 current carrying conductors with 3 neutral wires for each circuit in the same conduit. This would only count as 3 current carrying
conductors ?
 

jumper

Senior Member
So would it be the same with a 110 volt branch circuit situation. example-3 current carrying conductors with 3 neutral wires for each circuit in the same conduit. This would only count as 3 current carrying
conductors ?
Each neutral in this case would be a CCC for a total of 6 CCC. The neutral of each circuit would carry the same amount of current as its corresponding hot.
 

jumper

Senior Member
Rob/Infinity wrote this awhile back, it explains it well.

Here's some examples of when to count the neutral as a CCC:

208Y/120 volt system-different circuit types:

A)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C)- 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B)- In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C)- In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with one exception, *if the current is more than 50% nonlinear then the neutral would count as a CCC.

120/240 volt system-different circuit types:

D)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E)- In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
 

kwired

Electron manager
Location
NE Nebraska
It is all about the heat generated in the conductors. If you have 3 phase conductors evenly loaded and a common neutral to all of them then the neutral is carrying zero and not adding any additional heat.

If the neutral is carrying current then at least one of the phase conductors is carrying less current than the others- net result of heating effects is still similar to that of three conductors.

A large amount of non linear loads messes up this approach, but also makes you treat the neutral as a current carrying conductor for this purpose.
 
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