grounding subpanels

Status
Not open for further replies.
solaeros

solaeros

Member
Location
Beautiful IDAHO
On a job I'm working on there is a gentleman installing several subpanels off a 1200 amp service. These are 120/208, 3 phase, between 125 & 225 amp ocpd and instead of running a ground wire he's planning to install a bonding jumper off a bonding bushing connected to the feeder conduit to a ground bar. I haven't seen anything in the code against this but I never would have thought this would be a good practice; especially considering the conduit connectors are of the set screw style (emt). I understand the concept of grounding via a raceway but it still seems wrong for the main egc on a subpanel; what are the forums thoughts on this?
 
It's allowed by the NEC.

Assuming the conduit is installed professionally it can provide a lower resistance fault path than a copper EGC inside the raceway.

I have run 1200 amp feeders using EMT as the EGC and I have installed 20 amp circuits using a copper EGC.

What I decide to do depends on job conditions and specifications.
 
I agree with Bob, when properly installed EMT is just fine as the EGC. Even the bonding bushings are unnecessary.
 
iwire said:
Assuming the conduit is installed professionally it can provide a lower resistance fault path than a copper EGC inside the raceway.
Click to expand...

What does "installed professionally" mean?
 
georgestolz said:
Jeez, even Bob spelled "professionally" right the first time. :D
Click to expand...
You know..I was looking at that, "professionally"..I said that ain't right! There's too many damn letters

:)
 
celtic said:
What does "installed professionally" mean?
Click to expand...

celtic said:
That sounds like "properly" as opposed to "professionally".
Click to expand...

I think either work for what I was trying to say, I assume a professional will make sure;

The locknuts are tight

The connectors are tight

The couplings are tight

The supports hold the EMT very securely so flexing will not result in loose fittings.

The EMT is not run in area that it is likely to be subject to damage.

georgestolz said:
Jeez, even Bob spelled "professionally" right the first time. :D
Click to expand...

You don't know that....
icon10.gif
 
Ok this will surely get a laugh... Im a residential journeyman.....I dont know what EGC stands for.... I know GEC, is it the same thing just differently worded? Yes yes I know.....boss tells me Im an idiot all the time, I figure better you guys than the guy who signs my check.
 
EGC = Equipment Grounding Conductor

Here are a few others you may see here at the forum.

GES = Grounding Electrode System

SDS = Separately Derived System

CCC = Current Carrying Conductor

MWBC = Multiwire Branch Circuit

AHJ = Authority Having Jurisdiction

SABC = Small Appliance Branch Circuit
 
RampyElectric said:
I dont know what EGC stands for.... I know GEC, is it the same thing just differently worded?
Click to expand...

Rampy, they perform different tasks. The first Graphic below sums up a Grounding Electrodes function, the Grounding Electrode Conductor is the conductor from the service equipment to the rod in this particular graphic, it is bonded to the Neutral Conductor.

1016665767_2.gif


Now the EGC.

The Equipment Grounding Conductors function is shown as the green fault clearing path in the graphic below. Note that it's purpose is to create a low impedance path back to the source.

The GEC (in gold) which is shown leaving the "Main" would play very little (if any at all) part of clearing the fault.

NOTE: The grounding that is shown at the transformer will not be part (for all intent and conversation purposes) of our NEC required Grounding Electrode System so we won't consider it in this post.

1016666050_2.gif


Roger
 
solaeros said:
On a job I'm working on there is a gentleman installing several subpanels off a 1200 amp service. These are 120/208, 3 phase, between 125 & 225 amp ocpd and instead of running a ground wire he's planning to install a bonding jumper off a bonding bushing connected to the feeder conduit to a ground bar. I haven't seen anything in the code against this but I never would have thought this would be a good practice; especially considering the conduit connectors are of the set screw style (emt). I understand the concept of grounding via a raceway but it still seems wrong for the main egc on a subpanel; what are the forums thoughts on this?
Click to expand...
He's running a neutral to each of the sub panels and only bonding at the main disconnect right? Then there shouldn't be any current flowing through the ECG unless there's a problem. He doesn't even need the bonding bushing for sub panels... the box is grounded by contact with the box connector and locknut.

When you run branch circuits in emt you don't worry about the metal boxes not being grounded do you? They even provide ground screw holes in the boxes to attach your devices.

RampyElectric said:
Ok this will surely get a laugh... Im a residential journeyman.....I dont know what EGC stands for.... I know GEC, is it the same thing just differently worded? Yes yes I know.....boss tells me Im an idiot all the time, I figure better you guys than the guy who signs my check.
Click to expand...
Generalizing a little: EGC...Equipment Grounding Conductor- which includes, wire, metallic conduit, metal boxes, etc. and definetly not to be confused with Grounding Electrode Conductor- which must be a wire (or busbar) attaching your grounding elctrodes(s) to the ground bus for your bonding point to the Grounded Conductor (or neutral)
 
Last edited:
DaveTap said:
When you run branch circuits in emt you don't worry about the metal boxes not being grounded do you? They even provide ground screw holes in the boxes to attach your devices.
Click to expand...
It's a common concern that the raceway may not serve as an EGC as effectively as a conductor in concert with the raceway. Personally, I see it as inexpensive insurance and a better overall design to include a conductor.

Don has posted links to the GEMI software in the past, which calculates the resistance of raceways for determining their effectiveness as EGCs. I tried to use it on my machine and it didn't work.

In general, as I hear it, a metallic raceway alone would have a lower resistance than a nonmetallic wiring method and an NEC-sized minimum conductor. But the sentiment remains, at least for me... :)

DaveTap said:
Grounding Electrode Conductor- which must be a wire (or busbar) attaching your grounding elctrodes(s) to the ground bus for your bonding point to the Grounded Conductor (or neutral)
Click to expand...
It could be argued that the GEC is to be connected directly to the grounded conductor bus, as opposed to landing it on a grounding bus in the service and using the main bonding jumper as the connection. I personally believe that to be a shade on the excessive side of enforcement, IMO. :)
 
RampyElectric said:
Thanks for not making me feel dumb guys. Its nice to be able to learn without riducule. Seriously...thanks
Click to expand...
The gloves come off when you hit 100 posts ;)

Seriously....ask, ask, ask.
For every question YOU ask, there are many others that are afraid to ask for the reasons you stated.
 
Its a little confusing. 250.118 says the following raceways are allowed as equipment grounding conductors.
If you feel that a RMC or EMT raceway needs an EGC, then its a un-professional or un-proper installation. Its not a conduit issue its a installation issue. But I always pull an EGC. One loose setscrew...on a Friday job in a high bay...
 
Status
Not open for further replies.
Top