NEC and isolation transformers

[macgyver2010]

No, all of the ground rods need to be connected into a single system.

Effectively, you cannot create an 'isolated from all other grounds' 120V system unless you are a hospital operating room.

Thanks Jim. I'm finally getting the feel for this install. One more question. Is there a section of code that states what you said in your last sentence here or is that just how it all pans out from the various sections of code?

 

[macgyver2010]

Thanks Jim. I'm finally getting the feel for this install. One more question. Is there a section of code that states what you said in your last sentence here or is that just how it all pans out from the various sections of code?

Oh, wait. The other guys answered that question. Thanks guys.

 

[Cold Fusion]

Right, but gnd would be all that is not isolated. Neutral from the main panel would not go to the primary side. Neutral on the secondary side would be isolated but gnd would not, but this would still eliminate ground loop condition (of power anyways).

But if the isolation transformer is considered a separately derived power then wouldn't the sub panel now become a main panel which would then require the neutral and gnd to be bonded?

You seeing my dilemma?

No I am not seeing your dilemma. The NEC is quite clear. The sub-panel ground and neutral are tied right solid together. The bond happens at the main panel. So the sub-panel neutral is tied right solid to the house grounding electrode system and to the utility MGN.

Adding an isolation transformer does not change that. That's what I mean by:

See (2008) 250.20.B.1. There is no provision for isolation transformers to actually isolate the secondary neutral from the house neutral. ...

(edit) As plenty of others have said: Also see (2008) 250.50. All grounding electrodes that are present shall be bounded together. So you are not going to put in a separate grounding electrode for the SDS panel and not tie the SDS neutral to the house main grounding electrode system.

(My responses are in red)The reason is to prevent any noise that has been introduced into the main house conductors by various means from being able to get into the audio/video devices in this home theater setup.
That was my guess as to why your customer wanted to do this

I'm trying so see just what codes would apply for the install. Or if code would even allow it in residential.
SDS transformers are allowed in residential. You just have to bond the neutral to the house grounding electrode system

They are allowed in healthcare and somewhat in commercial but that has vague description also.

I cant seem to find much on separately derived power in residential either.
That's because for single phase 120/240, the rules are the same - residential or other.

There are noise mittigation methods that do not depend on having an isolated electrical system. There are lots here that can advise you (not my area of expertise).

cf

 

[macgyver2010]

Ok, before I decide not to proceed with this install I want to pose this question again.

If I were to go ahead with the install and knowing the neutral must also be grounded, would it be better to feed this panel with paralleled 120v or series 240v.

And if it was 120v and then having to place a jumper from one hot lug to the other hot lug to power both busses, are 2 conductors allowed in 1 main lug?

 

[macgyver2010]

So I guess it wasn't just me that couldn't find this. The best I could find is 230.82 which indicates what is allowed. SO I would assume you cant jump 1 lug to the other even in a SDS.

 

[macgyver2010]

Ok. I'm being questioned on the outcome of our discussion. I indicated that 250.26 requires the neutral on the secondary side of the SDS and the comment I got back was that 250.26 was for the main panel and that 250.30 covers grounding for SDS and there is not a requirement in there that says the neutral of an SDS must be connected to gnd.

Can someone clarify?

 

[Cold Fusion]

Ok. I'm being questioned on the outcome of our discussion. I indicated that 250.26 requires the neutral on the secondary side of the SDS and the comment I got back was that 250.26 was for the main panel and that 250.30 covers grounding for SDS and there is not a requirement in there that says the neutral of an SDS must be connected to gnd.

Can someone clarify?

The clarification is all in previous posts. As has already been said:

230.30 tells you how to ground an sds, IF the sds is to be grounded.

250.20.B.1 Tells you that the 120/240 system you are discussing shall be grounded.

250.50 and 250.53c tells you all grounding electrodes in a building must be tied together.

cf

 

[jim dungar]

...the comment I got back was that 250.26 was for the main panel and that 250.30 covers grounding for SDS and there is not a requirement in there that says the neutral of an SDS must be connected to gnd.

Who gave you this comment, the audio-visual guy or someone trained in the NEC?

 

[macgyver2010]

Right, thats for grounding an SDS. But where does it say that the neutral on the secondary side of an SDS must be tied to ground?

 

[macgyver2010]

Who gave you this comment, the audio-visual guy or someone trained in the NEC?

Actually someone who supposedly knows NEC, but I've been questioning that knowledge. I would never take a customers word for something.

 

[raider1]

Right, thats for grounding an SDS. But where does it say that the neutral on the secondary side of an SDS must be tied to ground?

250.30(A)(1).

This section requires a system bonding jumper installed to connect the equipment grounding conductors of the separately derived system to the grounded conductor.

Chris

 

[macgyver2010]

250.30(A)(1).

This section requires a system bonding jumper installed to connect the equipment grounding conductors of the separately derived system to the grounded conductor.

Chris

Well I have reread that section another 20 times and I interpret that 250.30(A)(1) only applies IF you are going to ground the neutral on the secondary side. The highlighted part says that the neutral shall not be connected to ground on the load side of the SDS.

In another effort so solve this issue I have researched manufacturers of isolated power supplies and how they are to be connected. Here is one interesting document I found and I've read some others that are similar.

http://www.apcmedia.com/salestools/SADE-5TNQYQ_R0_EN.pdf


Believe me guys. I'm not here to disagree with any of you. I came here in hopes of getting the combined knowledge of professionals to get an accurate answer. I personally was not sure and I'm happy with the discussions so far. I value each and every answer you give. And even if its only an opinion I would still like to hear it.

Please comment on the paper I attached. Thanks, Ron.

 

[don_resqcapt19]

Well I have reread that section another 20 times and I interpret that 250.30(A)(1) only applies IF you are going to ground the neutral on the secondary side. ...

250.20(B) applies to the secondary of your "isolation" transformer and requires it to be a grounded system. Now that you have applied 250.20(B), you have to apply the rules in 250.30(A)