NEC and isolation transformers

[macgyver2010]

Hi everyone. New guy here. Just found the site and it looks great.

Anyways, I'm in conversation on installing a 5kva and 10kva isolation transformers in a residential location and have been scouring NEC code and cant find what code says. Now I think I know why but I wanted to ask you guys who have more knowledge of the code book then myself.

I'm not necessarily asking how to install but more so where in the code does it cover residential isolation transformers, if it does.

Thanks in advance.

 

[roger]

What exactly are you referring to in "residential isolation transformer"?

Roger

 

[macgyver2010]

This would be a 10kva isolation transformer powered from the main panel and feeding a sub panel. To be used for audio/video devices.

 

[roger]

See article 647.

Roger

 

[macgyver2010]

Thats one section that I have already been trying to define. My assumption of 647.3 (1)would indicate that you cant install in residential, however this transformer is not 60v to ground and 647 only seems to apply to that.

The secondary on this transformer has 2 winding, 120v each or if bonded in series 240v pulling neutral off the bond. Seems like a different animal then 647.

 

[roger]

Thats one section that I have already been trying to define. My assumption of 647.3 (1)would indicate that you cant install in residential, however this transformer is not 60v to ground and 647 only seems to apply to that.

And I agree which was the reason I pointed you to it, although I didn't know if you might of been asking about an article 517.160 type system which would also be illegal in a residential installation.

The secondary on this transformer has 2 winding, 120v each or if bonded in series 240v pulling neutral off the bond. Seems like a different animal then 647.

Can you provide some name plate data off of the transformers.

The way you are describing it I don't think there is a problem.

Roger

 

[macgyver2010]

Here are some images of the label.

 

[macgyver2010]

Yes I had also read through 517 but as you said it doesn't apply to residential. I've only been able to find bits and pieces on separately derived power in residential but nothing clear.

 

[jim dungar]

You have a standard isolation transfromer with a 120x240 primary and a 120/240 secondary.

You will not be able to create a completely isolated from ground system per 250.20(B).

The NEC reguires you to ground the midpoint (neutral) of the secondary if you are using a 3-wire output, or to ground one leg if you are only using 2-wires (250.26).

 

[macgyver2010]

Ok, I read through that. So you believe that indicates that NEC allows installation of an isolation transformer in a residential location? Or does it mean that if installing in an approved location such as commercial or healthcare then that code would apply.

Let me also add that in this particular install there is currently a sub panel in which neutral is isolated from ground.

If this was going to be installed (A) would running 240v to feed panel be better or 120v paralleled off both windings. However the second would required 2 conductors in a main lug to run a jumper to the other hot lug. I didn't think 2 conductors in 1 main lug was allowed.

 

[raider1]

May I ask what you hope to gain from installing an isolation transformer in this residential application?

Chris

 

[Cold Fusion]

This would be a 10kva isolation transformer powered from the main panel and feeding a sub panel. To be used for audio/video devices.

...installing a 5kva and 10kva isolation transformers in a residential location and have been scouring NEC code and cant find what code says. ...

In the infinite wisdom of code panel 5, with their one-size-fits-all mentality, a 120/240V residential premisis wiring system can't get there. See (2008) 250.20.B.1. There is no provision for isolation transformers to actually isolate the secondary neutral from the house neutral.

You can't get the house system neutral separated from the house grounding electrode system. But you can get the house neutral separated from the utility MGN - look into a Ronk Blocker:
http://www.ronkelectrical.com/blocker.html

However, here is a method I have used:

Since the AHJ has no jurisdiction over what gets plugged into a residential receptacle, make your isolation transformer, panel, and receptacles, into a portable cord and plug device and don't bond the transformer secondary neutral to the house grounding electrod system.

However, since you would be choosing to build a non-standard piece of equipment, one would have to insure the safety - RCDs, gfci's, careful bonding back to the house grounding electrode system. edit - Maybe even add ground detectors.

And you might not be able to sell the thing - that could be getting into the AHJ area or in my case state law.

Just a thought

cf

 

[Cold Fusion]

...Let me also add that in this particular install there is currently a sub panel in which neutral is isolated from ground. ...

That's good the sub-panel installer separated the ground and the neutral at the sub-panel - that's the right thing to do. But the ground and neutral are not isolated. They are tied together back at the main panel.

cf

 

[macgyver2010]

May I ask what you hope to gain from installing an isolation transformer in this residential application?

Chris

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. I'm trying so see just what codes would apply for the install. Or if code would even allow it in residential. 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.

 

[macgyver2010]

That's good the sub-panel installer separated the ground and the neutral at the sub-panel - that's the right thing to do. But the ground and neutral are not isolated. They are tied together back at the main panel.

cf

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?

 

[raider1]

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?

The isolation transformer is a separatly derived system and must be bonded in accordance with 250.30.

Chris

 

[macgyver2010]

The isolation transformer is a separatly derived system and must be bonded in accordance with 250.30.

Chris

Ah, see. I knew you guys would come through for me. Thats what I was thinking. I'm going to read that section now but I want to ask this question while I read that.

If this panel now becomes a main panel for the separate derived power, then could this panel have its own grounding system to its own ground rod and not be tied to the house ground?

 

[jim dungar]

Ah, see. I knew you guys would come through for me. Thats what I was thinking. I'm going to read that section now but I want to ask this question while I read that.

If this panel now becomes a main panel for the separate derived power, then could this panel have its own grounding system to its own ground rod and not be tied to the house ground?

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.

 

[raider1]

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.

Agreed,

That was why I was questioning the purpose for an isolation transformer in a residential application such as what the OPer was suggesting.

Chris

 

[ibew501ed]

no because all grounding electrodes in a building must be tied together 250.53c and 250.50
sorry for the late post
just dont type fast enough