Isolated ground ?

Status
Not open for further replies.
J

Jerseydaze

Senior Member
I need to run a circuit to a server in an office it currently is on a 30A 120 v isolated ground twist Lock. The new Recept is about 90 ft from panels I am thinking I will run 10-3MC to keep an isolated ground. This seams waistful to me I am basically adding a conductor to ground a box. And the building is mostly Rx. Is this the way you would do it ?Opinions wanted
 
If the requirement is for an IG device than your installation is fine. Do you plan to re-identify the red conductor as an EGC? If so you'll need to comply with 250.119(B).
 
I understand its legal my question is if theres a better way I thought about RX but would I run 10-2 or 10-3 .
 
I'm not sure why you're going to run 10/3 MC. 10/2 MC is the correct wire to run ( the green wire doesn't count as a conductor). If you're using an isolated ground receptacle the green wire is terminated to the ground lug of that receptacle and is isolated from the metal JB. The sheath of the MC acts as the EGC
 
This is what ive always been confused on when ive run mc ive always bonded the ground wire to the box I thought only Ac/BX used the sheathing as a egc
 
goldstar said:
I'm not sure why you're going to run 10/3 MC. 10/2 MC is the correct wire to run ( the green wire doesn't count as a conductor). If you're using an isolated ground receptacle the green wire is terminated to the ground lug of that receptacle and is isolated from the metal JB. The sheath of the MC acts as the EGC
Click to expand...

i always thought that the armor of MC didn't qualify as a ground because there is no bonding strip (a la BX)....am I wrong?
 
jerseydaze said:
This is what ive always been confused on when ive run mc ive always bonded the ground wire to the box I thought only Ac/BX used the sheathing as a egc
Click to expand...
I don't believe you're required to do that but I suppose you can do that if you're not using or have a need for the isolated ground feature. Isolated grounding is required on many data processing pieces of equipment to reduce harmonics induced on the power line. The next time you strip back some MC cable take notice of the slight twist in the conductors. They'll turn slghtly one way for approx. 3-4' and then turn the opposite way for 3-4' and so on. Also manufactured that way to reduce harmonics.
 
emahler said:
i always thought that the armor of MC didn't qualify as a ground because there is no bonding strip (a la BX)....am I wrong?
Click to expand...
You're not wrong. I mis-spoke when saying that the sheathing serves as the EGC. However, both the green wire and the sheath are both bonded to the breaker panel enclosure. If you running a dedicated line for data processing equipment (for example) then the green wire in the MC cable serves as the EGC for that cable assembly and piece of equipment. The run is usually direct from one point to another so the sheath of the MC cable is grounded via the MC connector and the cabinet. If you're doing hospital or patient care work then double grounding and HCF-MC is required as nakulak pointed out.
 
A true IG MC has a hot-neut-green ground-and green ground w/yellow stripe. A true IG device will have the green as a bond to the box and the green/stripe to the ground screw on the device. A true IG device's ground is isolated from the yoke.
 
cdslotz said:
A true IG MC has a hot-neut-green ground-and green ground w/yellow stripe. A true IG device will have the green as a bond to the box and the green/stripe to the ground screw on the device. A true IG device's ground is isolated from the yoke.
Click to expand...

How about the return to ground unity, I have see many tied to the panel, not a true isolated ground. The Nec Handbook has a nice graphic of the Isolated ground.
 
Ok my question was more whats the right way to do this .Now I looking to know how you run an isolated ground in a cable type install.
 
goldstar said:
If you running a dedicated line for data processing equipment (for example) then the green wire in the MC cable serves as the EGC for that cable assembly and piece of equipment.
Click to expand...

Phil, there is no code compliant way to deliver an IG to a metal box with standard 2 wire MC. Just can't be done.

It can be done with the new AP-MC or hospital grade MC or IG MC or maybe 3 wire MC.

However in most cases using 3 wire MC will end up violating 250.119
 
Thanks Bob.

Point well taken. I realized I was wrong after I made the statement but decided to "eat crow" and admit my mistake rather than retract or erase the statement. (Wasn't there a receipe for crow somewhere in the archives ???) After your explaination it seems obvious now that a cable assembly like HCF-MC with the shunt wire built in and an effective ground path is the only way to be compliant.

Regards,

Phl
 
iwire said:
Phil, there is no code compliant way to deliver an IG to a metal box with standard 2 wire MC. Just can't be done.

It can be done with the new AP-MC or hospital grade MC or IG MC or maybe 3 wire MC.

However in most cases using 3 wire MC will end up violating 250.119
Click to expand...


I agree and that's why I said that the 10/3 would work. I didn't realize you were contemplating using 10-2. We always used 2 wire HCFC for these circuits in the past. Now we'll just use 3 wire MC-AP.
 
emahler said:
or....you can use the stuff that's designed for it

IG MC Cable
Click to expand...


That would work too, but given the cost of copper these days I would opt for something that has the cable jacket that also serves as an EGC. Not only that it's one less conductor to terminate on each end. That can be a significant labor difference on a large job.
 
Status
Not open for further replies.
Top