Common EGC

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mull982

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Is it allowed by code to run a common ECG for multiple devices that are connected to different circuits?

In other words if I have (3) devices each being served from a different circuit, am I allowed to run one EGC, and daisy chain this EGC to each of the devices. All circuits are 120V and in the same conduit.
 
The application that I was questioning refers to a 120/240V power panel that has several circuits fed from 20A breakers. These circuits (lets say theres 10 of them) the are routed in a single conduit with a single #12 EGC to a junction box in the field consisting of terminal strips for further distribution.

From each of the terminal strip the 10 circuits are distributed further to the field, and there is one ground block in the cabinet to which the EGC from the panel is connected to, and for which all the field circuits are connected to. All of the field circuits will use a common ECG between the ground block and field devices and are all routed in the same conduit.

From the responses and from looking at section 250.122(C) it appears there is no problem with the one EGC in the conduit from the Power Panel to the ground block in the junction box because all the circuits are in the same conduit and the #12 EGC is rated properly for the largest circuit OCPD of 20A.

The is also no problem with using a common EGC between the ground block in the junction box and all the field devices because they are all in the same circuit and are fed with a #12 which is sized according to the circuits OCPD of 20A.

Do you guys agreee with the two parts of this installation?
 
The EGC seems to be fine but you might have a derating issue with 10 circuits in one conduit and with #12 conductors protected at 20 amps.
 
infinity said:
The EGC seems to be fine but you might have a derating issue with 10 circuits in one conduit and with #12 conductors protected at 20 amps.
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When you talk about a deratting issue, are you referring to derrating the current carrying capacity of the #12 EGC due to the fact that there are seveal other circuis in the same conduit and can cause additional heating?

Does the NEC reference the derrating of the EGC anywhere or is it treated like any other condutor when derrated?
 
Not the EGC but the other conductors. If the other conductors are increased in size for derating purposes then the EGC must be increased also. Sounds like your #12's on 20 amp OCPD's are too small in one raceway.
 
infinity said:
Not the EGC but the other conductors. If the other conductors are increased in size for derating purposes then the EGC must be increased also. Sounds like your #12's on 20 amp OCPD's are too small in one raceway.
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Basically what you are saying is that although the #12 circuit conductors are good for 25A, because there are multiple circuits in one conduit these conductors all have to be derrated and may then after derration they may be rated for less then 20A and need to be increased in size. Because they would then be increased in size, the EGC would need to increase in size as well. Is this what you are saying?

I thought the EGC only needs to be according to the largest OCPD and not necessarily the wire size? So why would the EGC have to increase if the circuit wires increased and but the OCPD stayed the same?
 
Bob gave you the specific code section that would require an increase in the EGC size when increasing the circuit conductor size.

You say you have ten circuits, how are they wired? MWBC or individual grounded conductor for each circuit? What is the system voltage that's supplying them?
 
infinity said:
Bob gave you the specific code section that would require an increase in the EGC size when increasing the circuit conductor size.

You say you have ten circuits, how are they wired? MWBC or individual grounded conductor for each circuit? What is the system voltage that's supplying them?
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These circuits are all 120V. Basically they are all circuits coming from a power panel, so each circuit has a hot and a nuetral. Therefore 10 circuits each having a hot and nuetral will have 20 wires. All circuit conductors are #12 and there is only one EGC run with all of these circuits. I'm not sure what you mean by MWBC?
 
mull982 said:
These circuits are all 120V. Basically they are all circuits coming from a power panel, so each circuit has a hot and a nuetral. Therefore 10 circuits each having a hot and nuetral will have 20 wires. All circuit conductors are #12 and there is only one EGC run with all of these circuits. I'm not sure what you mean by MWBC?
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So you have 20 current carrying conductors which will require a derating of 50%. Now your #12 conductors can only be protected at 15 amps according to 310.15(B)(2)(a). If you were to use #10 conductors you could use a 20 amp OCPD.

#12 THHN = 30 amps @ 90 degrees C

30 amps * 50% = 15 amps adjusted ampacity.


#10 THHN = 40 amps @ 90 degrees C

40 amps * 50% = 20 amps adjusted ampacity.
 
mull982 said:
These circuits are all 120V. Basically they are all circuits coming from a power panel, so each circuit has a hot and a nuetral. Therefore 10 circuits each having a hot and nuetral will have 20 wires. All circuit conductors are #12 and there is only one EGC run with all of these circuits. I'm not sure what you mean by MWBC?
Click to expand...
You're about to find out. :D

If the panel is single-phase, and unless there's a very good reason not to, pair up hots so your 10 circuits could be run on 5 MWBC's; i.e., 5 groups of 2 hots and a shared neutral (which technically is required in order for the grounded conductors to legitimately be called neutrals.)

Now you'd have the figure of 10 conductors to insert into your derating and conduit-sizing equations instead of 20. The shared neutrals ("Is there another kind?" ~ Col. Jessup in A Few Good Men) need not be counted for CCC count. Conduit fill, they still must be considered.


If the panel is 3-phase, the same applies except for 3 hot wires instead of 2 per neutral. That would give you 3 MWBC's plus a single 2-wire circuit, for a count of 11. Since 9 is the 'magic number' which still allows 20a OCP for #12, would it be convenient to run the 10th circuit separately?

The other options are to run two conduits, or place a sub-panel (Bob: :wink:) central to the loads (like near where you'd place the J-box) if construction permits. Of course, you'd have to price doing it both ways, and decide whether it's even worth considering with the building layout.

There is obviously a reason you're using the J-box rather than running the circuits to the panel. What is that reason? That's where I'd start.
 
As Larry pointed out, regardless of the system these circuits are fed from you cannot get away with these #12's on a 20 amp CB all in one raceway.
 
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