Required separation for two, 3-phase, 480 VAC "feeders" in trench

[Headdog]

We have a project that requires us to attach to two separate, existing, 480VAC 3-phase, 75hp motor circuits (at the existing motor control enclosure) and run each circuit through a trench to a new location about 350 feet away. I believe I have correctly calculated the required minimum wire size(s) assuming I use two individual PVC conduit runs for each separate "circuit" (3 phase wires and a neutral or grounded neutral).

The conduit runs will be encased in concrete, in a trench that is 30" deep (conductors are 24+ inches below grade).

My questions are:

1). How do I know (or calculate) the required separation distance to maintain between the two conduit runs (with or without the concrete)?

2). I know I must de-rate the current carrying capability if I combine these two circuits (i.e. putting 6 current carrying conductors and two neutrals together). Is that acceptable to do under the NEC, or must I stick with two separate conduit runs in parallel?

 

[charlie b]

First, welcome to the forum.

Secondly, the NEC does not give a required minimum separation distance between conduits. But the details you see in such places as Figure 310.60 and Figure B.310.2 all show a separation of 7.5 inches. I think that is the usual spacing, even if it is not a specific NEC requirement.

Third, the NEC does allow you to put both circuits in the same conduit. It will require derating, as you pointed out. That may influence your selection of conductor size.

Finally, I am concerned about the parenthetical phrase at the end of your first paragraph. What do you mean by, “. . . and a neutral or a grounded neutral”? You will need three phase wires, you may or may not need a neutral, and you will need an equipment grounding conductor.

 

[wawireguy]

7.5 inches seems excessive. If you were to use Carlon duct bank spacers you would end up with about 4 inches between them so.. hard for me to see that you should go 7.5 but I'm just kinda shootin from the hip here.

Since your running 350' I'd seperate the runs into two conduits like you were thinking of doing. You could combine the runs as long as you account for derating and conduit fill but that's a long pull and anything you can do to help yourself such as two seperate runs can't hurt.

 

[Headdog]

Thanks Charlie B (welcome to the forum). The neutral/grounded neutral comment comes from the fact that I don't yet know if it is a 480 delta circuit (with a ground wire coming into the existing enclosure to ground the enclosure) or a 480 wye (with a grounded neutral coming into the enclosure).

I am assuming that if it is a wye circuit I must run it through the conduit with the three phase wires since it is the neutral and would need to conduct the unbalanced currents. If it is a delta circuit I don't believe I need to run it through the conduit. Of course I know in either case I need a new local grounding circuit at the other end of the conduit run (the new installation). I just wasn't sure of the separation distance, but 7.5 inches (maximum) would be OK.

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Wawireguy, thank you as well. I agree with both of your comments (4" and use two conduits), but I wanted to know if I was missing some separation requirement. It sounds from both of you that it is not given explicitly.

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This is a real pain becuase I don't really know what is "upstream" from these control boxes. I think it is should be a simple overload protection circuit for the existing 75 hp pumps (one on each circuit). The existing enclosures have the motor controller and what look to be fuses. I am trying to figure out if I should put some kind of overload protection in the existing boxes to protect my cable run or just assume it has been done correctly (upstream) for the existing pumps and install a simple "jumper" to the new cables. Then on the other end of the cable run, after the incoming disconnecting means, I can install the branch circuit protection (I am running a 75 hp motor as well on one of the two circuits). In other words it would be like I am running a long "feeder" circuit. Any thoughts? -- HD

 

[charlie b]

7.5 inches seems excessive. If you were to use Carlon duct bank spacers you would end up with about 4 inches between them so. . . .

Just in case this is a matter of miscommunication, when I said 7.5 inches, I believe that meant center to center. I don't have the figures handy, but I think that is what they show. The distance between the edges of the conduits would depend on the size of the conduits.

 

[don_resqcapt19]

Thanks Charlie B (welcome to the forum). The neutral/grounded neutral comment comes from the fact that I don't yet know if it is a 480 delta circuit (with a ground wire coming into the existing enclosure to ground the enclosure) or a 480 wye (with a grounded neutral coming into the enclosure).

If you are running motor circuits, it is a rare case where you need a neutral no matter what type of system is supplying the circuit. An EGC of some type is always required.

 

[acrwc10]

If you are running motor circuits, it is a rare case where you need a neutral no matter what type of system is supplying the circuit. An EGC of some type is always required.

That just got me thinking, if there was an "ungrounded delta" in PVC. would you need to pull a ground in, or drive a rod at the motor/disconnect ? Just a scenario, I've never needed to work on an ungrounded system.

 

[don_resqcapt19]

That just got me thinking, if there was an "ungrounded delta" in PVC. would you need to pull a ground in, or drive a rod at the motor/disconnect ? Just a scenario, I've never needed to work on an ungrounded system.

The only real difference between the "grounding" for an ungrounded system is the lack of a main bonding jumper. A grounding electrode system is required for the service just like for a grounded system. EGCs are required just like a grounded system.

 

[Headdog]

OK. I think I am understanding that what I said before was essentially correct. If it is a "wye" system I need to run the neutral cable through the conduit and tie it to the "local" ground on the other end, but if it is a "delta" system I do not need to run the wire through the conduit (but can include the conductor anyway as a "bonding" jumper). In both cases I must have a local "Grounding Electrode" and a local EGC tied to it. I think that closes out my question ("A").

Question "B" (a new one) has to do with the OTHER circuit. It is a "mixed" circuit with one (or maybe two) small 460 VAC 3-phase motors, a 480 to 240 3-phase transformer, some 240 VAC 3-phase loads (fan motor, etc), some 240 VAC 1-phase loads, and some 120 VAC loads.

The transformer is 480 delta to 240 VAC delta with one leg center-tapped. Since I haven't done this before, I am concerned about "grounding" the CT. Is that permissible, and if so, what advice should I hear about it? In other words, a "wye" configuration has a clear neutral that is "centered" around the three phases. For this "delta" secondary, I know there will be a "high leg" that should be identified with orange markings, but is that all there is to it (no unbalanced loading or ground currents on the neutral that heat up the transformer)? What am I missing (or don't understand)?

If it turns out that my supply is really a "wye" configuration, and I have to run the neutral conductor (through the conduit to the transformer), do I need to NOT ground it (only tie it to the neutral of a 480 wye to 240 delta transformer)? That would be so my grounded center tap doesn't burn up the transformer (if the supply neutral were also grounded).

I apologize for being so dense about this but I would rather be laughed at now than at a design review! All help is appreciated! Thank you -- HD

 

[augie47]

Headdog,
I probably can not do as well as Charlie & Don on this, but your comments on "grounding" still concern me. With an exception for feeding one structure from another prior to the 2008 Code, an equipment grounding conductor must be installed with your phase conductors. The installation of a grounded (neutral) conductor depends on your load. From what you describe, I see no need for the grounded conductor. Your 3 phase motors would not require a neutral, nor will your 480/240 transformer (primary).
The neutral on secondary of your 480/240-120 transformer would need a grounding system (see 250.30) including a grounding electrode.

I might note that on a standard 480/240-120 transformer the 120 volt load is limited (I believe to 5% of the kva) and you might want to keep that in mind.

 

[don_resqcapt19]

OK. I think I am understanding that what I said before was essentially correct. If it is a "wye" system I need to run the neutral cable through the conduit and tie it to the "local" ground on the other end, but if it is a "delta" system I do not need to run the wire through the conduit (but can include the conductor anyway as a "bonding" jumper). In both cases I must have a local "Grounding Electrode" and a local EGC tied to it. I think that closes out my question ("A").

The grounding electrode is only installed at the service. The neutral conductor is not permitted to be connected to ground beyond the service equipment. In both cases an EGC that originates at the service equipment must be run with the circuit conductors.

 

[sandsnow]

310.15(B)(2)(b)
Whatever spacing you choose has to be maintained for the length of the run.

 

[Headdog]

Thanks to all

Thanks to all

Sorry I took so long to reply, was working through a CDR (we passed).

To "don_resqcapt19" -- I agree. I don't believe I need the "neutral" either (in the conduit). Even if it is an existing "wye" service, the "neutrals" would be tied together via the earth and EGC (on each end of the "run"), so it doesn't make sense to me why I would need to also run another wire.

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To "acrwc10" -- My guess is I should only ground at the motor (via the new ground rod) and provide no additional ground wire in the conduits (no matter what system is used). Maybe the neutral (if a "wye" system) needs to run through the conduit but even that seems unnecessary.

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To "charlie b" -- Understood.

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To "sandsnow" -- I understand. Thanks.

(I have a follow on message I will post seperately):smile:

 

[Headdog]

New thread is "Grounded C.T. Delta Transformer"

New thread is "Grounded C.T. Delta Transformer"

(see the title)

 

[Headdog]

Headdog,
I probably can not do as well as Charlie & Don on this, but your comments on "grounding" still concern me. With an exception for feeding one structure from another prior to the 2008 Code, an equipment grounding conductor must be installed with your phase conductors. The installation of a grounded (neutral) conductor depends on your load. From what you describe, I see no need for the grounded conductor.

I got to re-reading this post and I am getting more confused. At the existing pump controllers (at least from pictures I have seen) there are the 3 phase circuits and a EGC (green tape) for the existing pumps and the controller enclosure (I understand this circuit so far and the need for the "local" EGC system). As far as I know the source of the power is an "ungrounded" delta secondary.

Now, I want to remove the old pumps, attach to the three phase conductors, and run them through a trench 350' to another area where there will be a NEW (local) EGC system. The existing pump controller enclosure will still be grounded locally using the existing EGC (which will not be removed). The new (350' away) structure and electrical items will be grounded to a new, local, ground rod (a "new" seperate EGC system).

So, why would I also need to run that EGC circuit from the existing pump enclosures for 350' and then tie it anyway to the new "EGC" system? I am not understanding where that is required (in fact it seems that it is no different from the power company sending ungrounded delta service w/o a EGC travelling along on the with the three phase conductors to the point of usage). It seems to me that the EGC system should be a "local" entity (shortest path to local ground), do you disagree? Otherwise, I would be able to use the original EGC circuit throughout my new system and not even have (or need) a local ground rod. To me that is the "less safe" condition due to the length of the conduit wiring. Just to complicate matters further, our new "system" will have lightning protection as well, so there be even more additional grounding electrodes that must be tied into the "local" EGC system. Without an EGC in the conduit, both EGC systems protect their respective areas without any safety consequences that I can "see". Please explain in greater detail (if you still think the EGC must be included in the conduit) because I do not understand.

 

[billsnuff]

an EGC has to be run and is to carry a 'fault' (L-N) back to the source via a 'low impedance path' to operate the OCPD. A high impedance path (earth) will not cause the OCPD to operate and will create a potential hazard.

 

[Headdog]

Billsnuff,

OK I may be seeing the light from afar but I'm not all the way there yet. I feel like I am still "lost in the weeds".

I agree that there has to already be some kind of overcurrent protection "scheme" upstream (of the existing pump controller boxes) to protect the exisiting circuits. But what if it is an ungrounded delta secondary configuration (with the added "local" EGC circuit; but no neutral per se)? Even with the EGC in the new conduit run, how would it "clear a fault" (line to ground) since there is no (return) connection to ground (becuase it is an ungrounded delta)? For a line-to-line fault it would "clear" the fault with or without the EGC would it not?

I can understand if it is a "wye" configuration (since there is a neutral that is grounded back at the secondary). I also understand if it is a grounded delta configuration (one leg grounded or a center tap grounded). But doesn't it matter what kind of power configuration is used, before you can assume the EGC is required (in the conduit) to clear a fault?

I don't know how to express it clearly. I am still confused on why you would intentionally set up a "daisy chained" ground circuit (the current EGC and the new EGC) when it can be avoided. It would seem to me that for an ungrounded delta secondary circuit you could avoid it. It would take two ground faults the trip the OCPD, but why is that an issue. In a grounded delta configuration you intentionally make one "leg" referenced to ground anyway. That is not considered a safety issue so why would the first ground fault be an issue? What am I missing?

 

[billsnuff]

I have a 3 ph wye supplying a bldg. 3 ph wire and center tap referenced to earth (gnd rod). that center tap is also attached to the MDP cabinet. from there 3 ph wires and an EGC are run to all equipment. I use no neutral.

Prior to the latest code, the conduit (metal/not PVC) supplied that connection.

I hope I said this correctly. Can anyone post a pic from grounding and bonding, please?

 

[Headdog]

I have a 3 ph wye supplying a bldg. 3 ph wire and center tap referenced to earth (gnd rod). that center tap is also attached to the MDP cabinet. from there 3 ph wires and an EGC are run to all equipment. I use no neutral.

Prior to the latest code, the conduit (metal/not PVC) supplied that connection.

I hope I said this correctly. Can anyone post a pic from grounding and bonding, please?

:-?
I meant the EGC when I said "neutral". I think of "wye" as 3 phase wires and a neutral but I guess that is not correct?

The thing is, I don't know if it is a "wye" configuration (I'm trying to find out). If it is, I can understand putting the 4-wires in the conduit (3 phase and EGC). Then the EGCs (before and after the long conduit run) will have to tie together.

However if it is an "ungrounded" delta secondary power source, I still don't see why I can't establish the EGC on the far end of the conduit run (the new system). That is where I intend to put in my system's OCPDs. At the beginning of the conduit run (the power original power source) any shorts/faults will be cleared by that OCPD. At the end of the conduit run (the new system), shorts/faults will be cleared by those OCPDs. Is that unacceptable and if so please try again to explain with an example of the posed danger. I'm just having lots of trouble understanding. Thanks!

 

[billsnuff]

definitions

definitions

Equipment Grounding Conductor - The conductive path to connect normally non-current carrying metal parts of equipment together and to the system gounded conductor or to the gounding electrode conductor or both.


Grounding Electrode Conductor - A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.

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