NEC Conductor Sizing Clarification

[tryingEE]

Howdy y'all, I have to do a cost estimate for all the materials need electrically on a project. There are 4 additional panels being added to the facility. I have ran into a couple of questions on the way for conductor and conduit sizing.
All panels are 208Y/120V, so that makes them three phase (I believe... in the one line diagram, only one panel states it is three phase, the others do not have any markings about phase). From my understanding, that would require three conductors, one for each phase. Also, I question the necessity for a grounding conductor, I have seen that panels don't require one (online) but for some reason that does not feel right. So now, each control panel has at least 4 wires in its conduit, 3 for each phase and 1 for ground.
At this point, I did not consider derating, even though there are 4 or more wires ( NEC Table 310.15(B)(3)(a) ). I did this because I believe one of the wires is not technically a current carrying conductor. Now, was time for sizing the conductors with table 310.15(B)(16). The calculation I feel most confident was for 400A panel. I sized it with 600 KCMIL because they handle 420A (75 degrees column because circuit is above 100A). I then used table 250.66 for ground sizing, and got #1/0 cable. Further along, I moved to chapter 9 table 5, and calculated the area. 600 KCMIL at 0.9729 in^2 and #1/0 at 0.2223 in^2. The total area calculated to 3.141 in^2 (3*0.9729+0.2223). This fits under chapter 9 table 4's 3" EMT size for over 2 wires (3.538 in^2).
Now for some more confusion I have... there is a control panel at 1200A. Under table 310.15(B)16, there is no conductor rated purely for 1200A. So, I decided to go with 4 350 KCMIL in parallel. 350 KCMIL is rated for 310A so the 4 in parallel would be at 1240A. Now, for grounding, I used #2 conductor based from table 250.66 (also would need 4 of them I believe). Now here is the wacky stuff, there are 3 phases, each with 4 wires, so there is 12 conductors, then the 4 grounding conductors, so there is 16 total. This leads me to derate, which at that point, don't I have to migrate of the 90 degrees column in table 310.15(B)16? And then, even looking over that fact, if we size the conduit based around these conductors, the area fill is bigger than the 4" EMT in chapter 9 table 4. So do I have to use two conduits for this panel?
I know that was a lot to take in, but this is why I am stumped. I feel like I could be looking way too much into this but it is important and I want to do a good and thorough job (I am a new intern at an MEP firm). So a list form of my questions are:

• Does three phase inherently mean three conductors?
• Do control panels need a grounding conductor?
• Difference between grounding/neutral conductor (which one is NOT a current carrying conductor)?
• When breaking conductors to be in parallel (to carry sufficient current), is there a need to have multiple grounding conductors in parallel now?
• When you go over the 4 conductor threshold and need to derate, do you have to go to the 90 degrees column in table 310.15(B)16?
• What do you do when your area of more than 2 conductors exceeds the area for the max area of a conduit? (Ex: EMT has a max 4" conduit in chapter 9 table 4, with an area of 5.901 in^2, what should I do if my area of conductors is 6.7 in^2?)

 

[LarryFine]

Welcome to the forum.

• Does three phase inherently mean three conductors?

Actually, four with the neutral.

• Do control panels need a grounding conductor?

Yes, but a metallic conduit qualifies.

• Difference between grounding/neutral conductor (which one is NOT a current carrying conductor)?

Grounding is an EGC for bonding, grounded is a current-carrying neutral. Generally, a neutral is not a CCC for derating.

• When breaking conductors to be in parallel (to carry sufficient current), is there a need to have multiple grounding conductors in parallel now?

Yes. Each separate cable (or conduit if you choose to include an EGC) must contain a grounding conductor sized as if it was the only one.

• When you go over the 4 conductor threshold and need to derate, do you have to go to the 90 degrees column in table 310.15(B)16?

No, you're allowed to, as long as your final number doesn't exceed the 75* rating.

• What do you do when your area of more than 2 conductors exceeds the area for the max area of a conduit? (Ex: EMT has a max 4" conduit in chapter 9 table 4, with an area of 5.901 in^2, what should I do if my area of conductors is 6.7 in^2?)

Add another conduit.

 

[augie47]

My answers are similar to Larrys but without details there are no specific answers.
Judging by the nature of your questions I would strongly suggest you have someone familiar with electrical systems assist you.
The generic answers we can give may well lead to a costly mistake. Questions such as defining a current carrying conductor and sizing conductors have a number of variables and need to be determined by folks knowledgeable with the NEC

• Does three phase inherently mean three conductors? a 3 phase load or panel would be a minimum of 3 copnductors (you might also have a grounded and/or grounding conductor but a single phase panel or laod could be supplied by a 3 phase system thus not needing the 3rd phase)
• Do control panels need a grounding conductor? I think what you are calling "control; panel" is a service panel and, if so, the supply grounded conductor is all that's needed.
• Difference between grounding/neutral conductor (which one is NOT a current carrying conductor)? "ing" is not a CCC the "ed" conductor can or can not be depending.
• When breaking conductors to be in parallel (to carry sufficient current), is there a need to have multiple grounding conductors in parallel now? Yes.. all sized by the OCP device
• When you go over the 4 conductor threshold and need to derate, do you have to go to the 90 degrees column in table 310.15(B)16? You would want to IF you are using 90° conductors
• What do you do when your area of more than 2 conductors exceeds the area for the max area of a conduit? (Ex: EMT has a max 4" conduit in chapter 9 table 4, with an area of 5.901 in^2, what should I do if my area of conductors is 6.7 in^2?)

 

[suemarkp]

You are also in the wrong table for grounding, maybe. Equipment grounds in a circuit or panel feeder are sized by table 250.122. If there is a ground electrode conductor say to building steel or water piping, that uses table 250.66.

The equipment ground can be eliminated by using non-flexible metallic conduit or tubing, but some customers specify a wire type ground regardless of the conduit type.

 

[winnie]

I think I see a couple of errors in the OP's question that need to be emphasized.

250.66 is used to size 'grounding electrode conductors', meaning the wires that go from the service equipment to things such as ground rods. This is not the proper table used to size 'equipment grounding conductors'. The 'ground' conductors that go along with branch circuits or feeders are 'equipment grounding conductors', and these need to be sized per table 250.122.

Per the NEC, metallic conduit containing the circuit conductors is sufficient to serve as the EGC, however many job specifications call for a wire EGC, in which case a conductor sized from table 250.122 rating of the circuit OCPD is needed. For a 1200A circuit you need a 3/0 Cu EGC.

When you have parallel conductor installations, you must have a single full sized EGC present in each conduit or cable. If all of your circuit conductors are in 1 conduit, you only need 1 EGC. If all your circuit conductors are in 4 conduits, then you need 4 EGCs.

Usually when you have parallel conductor installations, you will have 1 or at most 2 of your parallel sets in a single conduit, otherwise derating kills the benefit of using parallel conductors.

-Jon