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#181845 11/03/08 03:47 PM
Joined: Apr 2006
Posts: 6
MTWire Offline OP
Junior Member
I am curious as to what the best wiring method for providing power to a 16 amplifier rack would be. Each amplifier requires a 208V 20A 2 Pole Circuit with an isolated ground, and currently are cord and plug. Would a plugmold or similar type of method be the best choice, or should I just install 16 4" Squares with industrial raised covers and locking plugs?

Joined: Jan 2005
Posts: 5,316
Cat Servant
I was right there with you ... and then you mentioned 'isolated ground.' Now we have another complication to address.

First, the 4-squares would work fine; I;m not entirely sure that isolated ground versions of the receptacles exist, though.

Your saying "208V" makes me think of three phase systems. If there are to also be 120V circuits, you will want to avoid getting the neutrals combined.

If the receptacles are isolated ground, that leaves open to question just how the cases of the amplifiers are grounded. I doubt that the amplifiers qualigy as 'double insulated.' Therefor, in addition to the power circuits, you need to ensure that the rack, and the cases, are reliably grounded.

Joined: Apr 2002
Posts: 7,297
Likes: 5
Hubbell makes IG twistlock receptacles in a 20 amp, 208 volt config.

The rest is Reno's....

Joined: Jul 2004
Posts: 9,688
Likes: 10
Why would "isolated ground" affect the way the cases are grounded? They will either attach to the IG or the EGC. Since he said 208 I assume the amps do not have a neutral connection so that can of worms is eliminated (except possibly in some input side equipment).

If I was using the computer room model I would bring a 200a sub panel to the rack with IG on the unused neutral rail. and split out each 20a from there. The only question is if the rack is isolated from building ground or from the amps. If the rack is to be IG the sub will be the end of the normal EGC and the rack and all attached equipment would connect to the IG. In any case everything that is supposed to be grounded will hook to one or the other.

Greg Fretwell
Joined: Jan 2005
Posts: 5,316
Cat Servant
Thanks, HotLine. It's nice to see that the stuff you need is actually available.

Greg, I'm not sure what is being attempted here, what the isolated ground is intended to accomplish. As far as I'm concerned, everything ought to be just fine, plugging the units into any old receptacle .... but for one thing: radio noise.

I once did some work for a recording studio. The folks there were most adamant that even a tiny amount of arcing as fault current made the transition between conduit and fitting would be picked up by their gear ... so an awful lot of attention was paid to grounding the equipment. In the OP's application, that would suggest to me that extra attention be paid to the connections between the equipment and the rack, and the bonding of the rack to the conduit as well.

I also get nervous around audio equipment, in no small measure because a lot of it is not UL listed - and much of what is listed gets modified in the field. That's another reason why I say "assume nothing."

Joined: Jul 2004
Posts: 9,688
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If they really want isolation I would not bring the neutral or the EGC to the rack, isolate the rack from any other incidental ground contact and bring your isolated ground to it and all the attached amplifiers. Going back on the line side you want the IG to pass through all intermediate panels without hitting the EGC until you get it back to the service disconnect where it lands on the main grounding bus where the MBJ and GEC lands.
That way your amps are attached directly to the ground electrode with no intermediate ground connection. That was the way we set up computer rooms before IBM figured out IG was 1% fact and 99% snake oil.

Every time I have tracked down ground loop hum in an audio system it was from another interconnected piece of equipment. That usually traced back to unbalanced current on a neutral. (using 2 circuits either with different loadings or different phases and seeing the resolved voltage drop on the neutrals on the DC common)
You usually fix it by bonding everything together using something other than the signal lead commons.

In MTs scenario, I would think if everything was in or bonded to that (IG) rack he should be OK, particularly if there are no L/N 120v loads feeding the amps. At that point the IG is probably wasted effort but old legends die hard. Sell them some wire if that is what they want. The customer is always right

Greg Fretwell

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