Ground cable size required for MV circuit

[Redskinsdb21]

Ground cable size required for MV circuit

I am currently designing a power system to use 3 1/C 500 kcmil MV cables. I am doing a calculation to determine the size of conduit required as well as grounding cable size required.

I found thread on forum listed here about grounding cable size required:

thread238-502586: Sizing an equipment grounding conductor for high voltage in duct bank

Is this part of that thread correct for sizing ground cable on anything over 1000 Volts?


7anoter4 (Electrical)20 Feb 23 12:26
See NEC
Part X. Grounding of Systems and Circuits of over 1000 Volts
250.190 Grounding of Equipment
(C) Equipment Grounding Conductor
(3) Sizing. Equipment grounding conductors shall be sized in
accordance with Table 250.122 based on the current rating of
the fuse or the overcurrent setting of the protective relay
See also:
IEEE 142/2007 2.7.4.4 Equipment grounding conductor sizing

If correct, why do they call it equipment grounding conductor? Wouldn't it just be grounded conductor? when I think of equipment grounding conductor I think cable to ground Xfmr panels, sgear panels or any other equipment exterior to electrical room or substation ground. What am I missing here?

 

[waross]

Equipment grounding or bonding conductor apart from system grounding conductor.
Different rules, different sizing.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

In the NEC, a grounded conductor carries load current (neutral) while an equipment grounding conductor normally carries no current.

 

[Redskinsdb21]

dpc (Electrical)23 Jan 24 01:36
In the NEC, a grounded conductor carries load current (neutral) while an equipment grounding conductor normally carries no current.

So in this case wouldn't the ground cable carry neutral current back to ground? and also carry current under fault conditions that include ground?

 

[Redskinsdb21]

waross (Electrical)22 Jan 24 23:41
Equipment grounding or bonding conductor apart from system grounding conductor.
Different rules, different sizing.

I think what you are saying to Equipment grounding is same as bonding conductor? and the system grounding conductor is what would be the ground cable ran with the 3 1/C 500 kcmil cables? If that's the case then is guidance below by 7anoter4 incorrect? If so, what is appropriate way to size system grounding conductor?

7anoter4 (Electrical)20 Feb 23 12:26
See NEC
Part X. Grounding of Systems and Circuits of over 1000 Volts
250.190 Grounding of Equipment
(C) Equipment Grounding Conductor
(3) Sizing. Equipment grounding conductors shall be sized in
accordance with Table 250.122 based on the current rating of
the fuse or the overcurrent setting of the protective relay

 

[7anoter4]

All conductor intentionally grounded is a grounded conductor-NEC Definitions.
However, an Equipment Grounding Conductor it is a special grounded conductor which run with or enclosing the circuit conductors [see NEC 250.118]

 

[waross]

The system grounding conductor connects the system ie: the transformer neutral, to the ground electrodes or ground grid.
I understand that in the US, the equipment grounding conductor size is based on the circuit protection ampere rating.
Consider the case of a motor supplied by three nos. 12 AWG conductors and grounded by a #14 conductor.
On a 480 Volt circuit, the voltage to ground will be 277 Amps.
In the event of a short to the motor frame of one of the supply conductors, the supply and grounding conductor will act as a voltage divider and as the impedance of the grounding conductor is greater than the impedance of the supply conductors, more than half of the voltage to ground may appear on the motor frame as a touch voltage.
That is more than 277/2 = 138.5 Volts on the motor frame.
Now consider the case where the motor is fed by oversized conductors to reduce voltage drop, but the grounding conductor remains the same.
The touch voltage on the motor frame will be even higher until the protection trips.
The Canadian code addresses this by sizing the grounding conductor based on the size of the largest ungrounded conductor.
The code is a minimum.
Sometimes good engineering dictates higher standards.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jghrist]

In the NEC, a grounded conductor carries load current (neutral) while an equipment grounding conductor normally carries no current.

So in this case wouldn't the ground cable carry neutral current back to ground? and also carry current under fault conditions that include ground?

The equipment grounding conductor is not connected to the system grounded conductor except at the service equipment enclosure. Neutral load current cannot flow through the equipment grounding conductor, only through the system grounded conductor.

 

[kartracer087]

NEC 250.184 covers grounding of medium voltage systems. As a minimum, equipment grounding conductors, where present, need to be sized at least 20% of the area of the phase conductors (if considered a neutral) or based upon the short circuit withstand of the circuit. Shielded power cables often do not have the capacity in the shields to act as equipment grounding conductors there is typically a maximum 5-cycle short circuit current rating that the shield can withstand before it is melted and it may not be large enough to be suitable. So using a 5-cycle rating on a single grounding conductor will show you if that conductor will be suitable as a ground. Following the rules below will generally satisfy these requirements:

As a better practice, I tend to size medium voltage equipment grounding conductors at 1/3 (33%) of the phase conductor where the system only serves loads connected in line to line arrangements (or delta), in essence what the code would consider a single point grounded system. This usually the case in most industrial power systems. If the system serves loads connected between phase and system neutral (on a solidly grounded or reactor grounded neutral system, under what the code would consider a multi-grounded neutral system), then I will size the equipment ground conductor to be the same size as the phase conductor. In essence this grounding conductor will then carry any unbalanced load current so sizing it larger is necessary to reduce ground potential rise in the circuit. Usually the equipment ground conductor will have 600V insulation (THWN or XHHW-2) but it may also be a bare copper conductor. Bare copper equipment grounding conductors cannot be used in multi-grounded neutral circuits, they must be insulated at 600V. In this arrangement it is expected the grounding conductors will carry load current and as such they need to be properly sized to carry that load current. Using a full size ground is a good insurance policy for these systems.

Medium voltage systems are very different from low voltage systems in the consideration of using grounding conductors as neutrals. You are not permitted to ever do this in a low voltage system but you are in a medium voltage system. Care should be taken to ensure load balancing is done on 3-phase, 4-wire medium voltage systems to ensure the neutral currents are kept as low as possible.