Article 250.122(C) Multiple Circuits

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Dale Hayes

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NEC Article 250.12

(C) Multiple Circuits

If a single equipment grounding jumper is run with multiple circuits within the same raceway, cable, or cable tray, it shall be sized for the largest overcurrent device protecting conductors in the raceway, cable, or cable tray. Equipment grounding conductors installed in cable trays shall meet the minimum requirements of 392.3(B)(1)(c).

Question:

Can (1) equipment grounding jumper be used if there is more than one conduit run for the multiple circuits?

What we have is (8) 3-phase circuits in a electric heater unit - each protected by a 60 amp overcurrent device.

Because of the (24) 6Ga wires we have two conduit runs from the electric heater terminal enclosure going into an Industrial Control Panel. The EGC will originate at the electric heater and end up on the grounding device in the Industrial Control Panel.

All of the (8) 3-phase circuits originate from the same electric heater terminal enclosure. With two conduit runs there will be (4) 3-phase circuits in each conduit run.

Will (1) 10Ga EGC thru one of the "two conduits" be OK?
 

raider1

Senior Member
Staff member
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Logan, Utah
300.3(B) requires all conductors of the same circuit and all equipment grounding conductors to be run in the same raceway.

So this would require an EGC in each raceway.

P.S. what type of conduit are you running?

Chris
 

infinity

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New Jersey
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With 4-3 phase runs in each conduit you'll have a derating problem if these are #6 AWG conductors with a 60 amp OCPD. That would give you 12 CCC in each raceway which require a derated ampacity of 50%.
 

augie47

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Good catch, Rob. also I believe if you increase the size of your phase conductors to meet the derating, you will need to increase the equipment ground also per 250.122(B)
 

infinity

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Good catch, Rob. also I believe if you increase the size of your phase conductors to meet the derating, you will need to increase the equipment ground also per 250.122(B)


Yes Augie's correct, he'll need a larger EGC in each raceway too.
 

charlie b

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Lockport, IL
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I am a bit confused by the description. See if I have this right:
  • In "box #1," you have 8 circuit breakers.
  • In "box #2," you have 8 loads.
  • Each of the 8 loads is independent of all the others, so it really is 8 separate loads, despite the fact that they are all in the same box.
  • You will supply each of the 8 loads by its own circuit.
If that is right, then there is no concern about having to put all the conductors for a single circuit in the same conduit. Each "circuit" consists of only 3 phase conductors, so the existence of other phase conductors that serve other circuits is not relevant.

But each of the 8 circuits must have an EGC. If you want to put 4 circuits in one conduit, then a single EGC will serve all 4 circuits. If you want to put the other 4 circuits in a second conduit, then a single EGC can serve all 4 of those circuits. But you will wind up needing an EGC in each conduit, because each conduit has its own independent circuits.
 

Dale Hayes

Senior Member
Summing it up

Summing it up

The circuits have already been derated based on the number of wires in each conduit. We have also derated the wire for ambient temperatures.

We use metal conduit for running wire. On explosion proof applications we run rigid metal conduit and use special thread sealing compounds (if the explosion proof fittings need to be weatherproof.)

Another question: There is only one ground connection - a single 1/4" ground screw mounted to a 90 lug mounted inside the terminal enclosure. Is one ground screw sufficient for multiple circuits? The ground wires would have to be stacked on the lug/ground screw and sometimes there are more than (two) conduit runs.

So, from what I gather, for multiple circuits in conduit, run a separate ground wire for each conduit ie, (1) conduit = (1) ground wire, (2) conduits = (2) ground wires, (3) conduits = (3) ground wires, etc.
 
Dale
If you are installing the EGCs, then you are correct, install one EGC in each conduit.

Typically a lug is listed as suitable for 1 conductor. It may be suitable for more than 1 conductor, you will find that info either stamped on the lug or on the box it is sold with.
 

infinity

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New Jersey
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Journeyman Electrician
The circuits have already been derated based on the number of wires in each conduit. We have also derated the wire for ambient temperatures.


Care to share that calulation where you end up with #6 condcutors on a 60 amp heater circuit with 50% derating?
 

Dale Hayes

Senior Member
Calculations

Calculations

Heater Unit: 480V, 125KW(+5, -10%), 3-phase, (8 circuits), using (2) 1 1/2"NPT Conduits. Note: we add in the standard 25% derating and the extra +5% derating (5% because the standard heater can be +5% above the rated KW. Example. 125KW +5% = 13.125KW, then add the standard +25% derating = 164.063KW for the calculation purposes.

(8) circuits at 24.7amps per circuit.

Calculations are made from these tables:

Table

Table 310.15(B)(2) (a), for 10-20 wires, Multiplier = .50

Table 310.16, 6Ga = 75Amp, derated for 46-50 Deg. C Ambient, the Multiplier = .82

Calculation: 60Amp x .50 x .82 = 24.6Amps

Therefore: 6Ga 90C THHN wire with 10Ga EGC.
 

Smart $

Esteemed Member
Location
Ohio
Heater Unit: 480V, 125KW(+5, -10%), 3-phase, (8 circuits), using (2) 1 1/2"NPT Conduits. Note: we add in the standard 25% derating and the extra +5% derating (5% because the standard heater can be +5% above the rated KW. Example. 125KW +5% = 13.125KW, then add the standard +25% derating = 164.063KW for the calculation purposes.

(8) circuits at 24.7amps per circuit.

Calculations are made from these tables:

Table

Table 310.15(B)(2) (a), for 10-20 wires, Multiplier = .50

Table 310.16, 6Ga = 75Amp, derated for 46-50 Deg. C Ambient, the Multiplier = .82

Calculation: 60Amp x .50 x .82 = 24.6Amps

Therefore: 6Ga 90C THHN wire with 10Ga EGC.
When you say heater unit, are you saying 1 load or 8 loads within the unit? Sounds like the latter, but using the singular form of the word "unit" tends to make one think it is a single load rather than a multi-load unit.

If you calculation after applying adjustment factors results in a value less than your ocpd and less than the next lower standard size ocpd, you are in violation (unless qualifying under such articles which permit this, such as 430 and 440)

At 24.6A after adjustment, you'd only be permitted to be on a breaker rated no greater than 25A.
 
Last edited:

Smart $

Esteemed Member
Location
Ohio
PS: you'd need to run #6 egc's too, if you run wire-type. You could get around this by using only the conduit as the EGC(s)
 

Dale Hayes

Senior Member
"Reversed" Calculation for Sizing Wire

"Reversed" Calculation for Sizing Wire

The previous calculation was done in a reverse order for sizing the "wire".

Calculation: 60Amp x .50 x .82 = 24.6Amps

So ...

For sizing the circuit protective device (fuses) for each circuit:

Fusing = 24.6Amps x 1.25 x 1.05 = 32.3Amp (35 Amp Type J fuses.)

So ...

6Ga wire (due to derating) and 35Amp J fuses for each circuit.

PS: There are (8) separate circuits welded into (1) 16" -150# steel R.F Flange.
 

Dale Hayes

Senior Member
Oops!

Oops!

Each circuit amperage before any derating is 18.8Amps.

Fusing should be 25Amps per circuit.

18.8Amps X 1.25 x 1.05 = 24.7Amp


So...

6Ga Wire due to derating with 25Amp type J Fusing as the circuit protection device.
 

Dale Hayes

Senior Member
Egc

Egc

Smart $

Question: Why#6 egc's too, if you run wire-type?

Can 10Ga 90C THHN be used as the EGC per Table 250.122?

There are (2) 1 1/2" conduit runs... so (2) 10Ga EGC's ... (1) per each conduit?
 

Dale Hayes

Senior Member
Nec 250.122(b)

Nec 250.122(b)

To:

Don, Illinois

If you are using #6 circuit conductors and a 25 amp OCPD, 250.122(B) will require that the EGC also be #6.

Sure enough, after doing the math ... 6GA EGC

Calculation of the conductor increase (based upon wire cmil):

6Ga (26,251cmil) / 10Ga (10,383cmil) = 2.53

So...

2.53 x (25,251cmil, 10GA ECG for 25Amp ODC) = 26,251cmil (6GA ECG)

Thanks!!!!
 

don_resqcapt19

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Illinois
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retired electrician
To:

Don, Illinois

If you are using #6 circuit conductors and a 25 amp OCPD, 250.122(B) will require that the EGC also be #6.

Sure enough, after doing the math ... 6GA EGC

Calculation of the conductor increase (based upon wire cmil):

6Ga (26,251cmil) / 10Ga (10,383cmil) = 2.53

So...

2.53 x (25,251cmil, 10GA ECG for 25Amp ODC) = 26,251cmil (6GA ECG)

Thanks!!!!
No real need for any math when the OCPD is 30 amps or less. The EGC and the circuit conductors are required to be the same size when the OCPD is 30 amps or less. However you do need to do the math when the OCPD is larger.
 
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