Several circuits in same conduit

[Bea]

I didn't say it was. But you do have three separate circuits (15 amp, 20 amp, and 30 amp) in the same conduit. That is at least six current-carrying conductors.

NEUTRALS ARE NOT CURRENT CARRYING CONDUCTORS UNLESS 3 PHASE 4 WIRE WYE WITH NOLINEAR LOADS

 

[infinity]

NEUTRALS ARE NOT CURRENT CARRYING CONDUCTORS UNLESS 3 PHASE 4 WIRE WYE WITH NOLINEAR LOADS

Bea, there no reason to shout (the capital letters). No one has established if this is a MWBC or not. Charlie may be correct in his assumption that there are 6 conductors in this conduit with one neutral per circuit. And I would add that a neutral that is part of a three wire (two hot legs , one neutral) MWBC of a 208Y/120 volt system is a current carrying conductor even if the load(s) are linear.

 

[roger]

Bea, if the three circuits are all two wire circuits, (as you quoted Charlie) then there are no neutrals and all six condutors are CCC"s.


Roger

 

[ktm400sx]

IF you want to add a supplemental equipment grounding conductor in that metallic conduit, then a single #10 wire should suffice for all circuits, assuming the largest current-carrying conductor is a #10 as well.

Can the EGCC be spliced? Reason is the conduit goes to a junction box and then individual conduits go from the juction boxes to the devices.
All three are single phase loads.
Largest conducter is #10

Thanks for all the help

 

[dana1028]

I believe it is a fact that a properly installed conduit system provides a lower impedance fault path than the EGC normally installed in the raceway.

Bob - I am looking at 'Soares Book on Grounding - 7th Ed.' In Chap. 11 they perform a number of tests and measure fault current on a number of return paths.

Unless I am interpretting their finding incorrectly, they seem to indicate an EGC within the conduit provides the best return path [p. 157, Fault-current study circuit analysis - "The reactance of the circuit include the 'B' conductor (internal EGC) will be the lowest..."].

They go on to say..."Furthermore, unless the conduit was well insulated throughout its entire length [which is usually impossible or impractical in typical commerical installations], there would be a significant number of sparks at stray points to constitute a serious fire hazard."

Earlier comments on the test had indicated: "Some interesting secondary effects were observed in the course of the tests. The fist high-current test produced a shower of sparks from about half of the couplings in the conduit run. From one came a blowtorch stream of sparks which burned out many of the threads. Several small fires set in nearby combustible material would have been serious if not promptly extinguished."

These 'high-current' tests were of a fault magnitude of about 11,000 amps.

Anyway - the way I understand the Soares tests if that an internal EGC provides the best fault clearing path, with the metal conduit being #2....with the potential hazard associated with it.

If I am understanding this incorrectly I would appreciate a clarification. I believe the current Soares edition [8th] makes no changes to this chapter.

 

[bstoin]

back to the original...

back to the original...

Hi -
I am going to be running a 15amp 110 volt, 20 amp 115 volt and 30amp 115 volt circuit in the same conduit.
Iam a little confused as to where in the code I find the requirements for the ground conducter for this scanerio.
Any help would be greatly appreciated.

The conduit is metallic

Look at 250.122 (C)Multiple circuits:
Where a single equipment grounding conductor is run with multiple circuits in the same raceway or cable, it shall be sized for the largest overcurrent device protecting conductors in the raceway or cable.
Since the largest overcurrent protective device is 30 amps, you will need one #10 grounding conductor.
The size of wire doesn't enter into the equasion, the size of the grounding conductor is determined by the overcurrent device (amperage) not the wire size.
There are no two wire circuits in this equasion...see above.

 

[LarryFine]

The size of wire doesn't enter into the equasion, the size of the grounding conductor is determined by the overcurrent device (amperage) not the wire size.

Exception: when upsizing the conductors for voltage-drop, but I know what you mean.

 

[don_resqcapt19]

The size of wire doesn't enter into the equasion, the size of the grounding conductor is determined by the overcurrent device (amperage) not the wire size.

Actually you have to look at both the wire size and the OCPD rating to size the EGC. Anytime that the ungrounded conductor has a higher ampacity than the OCPD rating, the EGC has to be upsized. Previos codes only required that the EGC be upsized when the ungrounded conductor was increased in size to compensate for voltage drop. The code wording was change for the 2002 code.
Don

 

[Smart $]

Can the EGCC be spliced? Reason is the conduit goes to a junction box and then individual conduits go from the juction boxes to the devices.
All three are single phase loads.
Largest conducter is #10

Thanks for all the help

Yes, the EGC can be spliced [at the junction box]. In fact, it has to be spliced, or at least tapped, to bond to the box if any of the circuit conductors are spliced there.

As far as running the circuits as a MWBC, TTBOMK there is no code that precludes doing so. For example, if there is no voltage drop or nonlinear considerations for the loads, you could run one each #10, #12, and #14 on a different phase, one #10 neutral, and one #10 EGC from a 3? wye panel to junction boxes in the raceway, and splitting off to the loads as a pair of #10's, #12's, or #14's with equivalent sized EGC's to the individual loads. At any point in the raceway system, the neutral/grounded conductor and EGC must be equal in size to the largest ungrounded conductor in the raceway section.

 

[George Stolz]

Dana, I just wanted to thank you for contributing that post, it's interesting to think about.

 

[thunder15j]

Maintaining conduit sysem

Maintaining conduit sysem

I believe it is a fact that a properly installed conduit system provides a lower impedance fault path than the EGC normally installed in the raceway.

How about properly maintaining a properly installed conduit system that is used as the fault path? That is beyond the original original installers control. Maybe not so much an issue with galv rigid as EMT would be.

Some time ago I remember reading an article(E,C&M or EC) about an electrical worker who was electrocuted while working in an attic. The emt conduit had seperated near where the worker was crawling/working. There was a fault current on the loadside conduit (high impedance, no EGC).The poor guy bridged the gap between the two parts of the conduit run ...... you can figure out the unfortunate results.

Edit UBB by request

 

[thunder15j]

IF you want to add a supplemental equipment grounding conductor in that metallic conduit, then a single #10 wire should suffice for all circuits, assuming the largest current-carrying conductor is a #10 as well.

The EGC is sized to the overcurrent device rather than wire size. A hard starting motor can be at 300 % fusing or even higher. 430.52 (2002)

Edit UBB

 

[S'mise]

250.122

250.122

I disagree with it being considered 3 ccc. The neutrals are ccc. Regardless,if it's 4 or 6 ccc that still puts the derate at 80% (using thhn)in the 90C collum says #10 is good for 40A X .8 = 32A So even if you dont have to derate, # 10 still works for the largest circuit (30 amp) 250.122 says to use a #10 for your ground (30amp circuit) Provided it does not have to be made larger. Right?

Short answer use a #10 if you need a equiptment ground

 

[iwire]

How about properly maintaining a properly installed conduit system that is used as the fault path? That is beyond the original original installers control.

The world is full of what ifs.

What if the circuit breaker fails to trip?

What if the ground pin falls of a cord cap?

I generally do pull grounding conductors, that does not change the fact that the steel conduit can provide a lower impedance path than the EGC inside it.

 

[S'mise]

Pull the green!

Pull the green!

I agree that it is always better to run the equiptment ground in the conduit.
Esp. if you are using emt. The set screw cant be made tight enough for my liking. We have all seen the lack of continuity in 20 year old pipe. Worth the effort to pull the green wire.
:wink:

 

[roger]

This is a little OT but, addressing the metallic conduit/cable sheath verses insulated conductor as the better EGC.

NFPA 99 in addressing the grounding requirements of 517.13 recognizes that the metallic conduit or metallic sheath of a cable listed as an EGC per 250.118, is the primary (better) EGC and the insulated conductor is secondary, NFPA 99 A 4.3.3.1.3

Roger

 

[infinity]

I disagree with it being considered 3 ccc. The neutrals are ccc. Regardless,if it's 4 or 6 ccc that still puts the derate at 80% (using thhn)in the 90C collum says #10 is good for 40A X .8 = 32A So even if you dont have to derate, # 10 still works for the largest circuit (30 amp) 250.122 says to use a #10 for your ground (30amp circuit) Provided it does not have to be made larger. Right?

Short answer use a #10 if you need a equiptment ground

How can you be sure that it's more than 3 CCC's? The OP says nothing about the number of conductors in the conduit only that it contains 3 circuits. It could be 3, 4, 5 or 6 CCC's depending on the system and how the circuits are installed.

 

[S'mise]

I not saying it's more than 3 ccc. just #10 would still be fine for 3.4,5 or 6 ccc.
Derating can be done for >3 ccc from the 90c collum if you use thhn.