Help with Cutoff switch

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  #1  
Old 05-21-07, 10:48 AM
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Help with Cutoff switch

I posted yesterday about question with my service entrance. Here is the diagram again
http://www.scophoto.com/home/se_diagram.pdf
I don't have it listed, but the cutoff switch has two 300A fuses on the two hot legs. I spoke with an electrician again about it and he said oops, I should have told you earlier but you can't do that. I need each cable between the breaker panel and the cutoff switch fused at 200A at the cutoff switch. The cable between the breaker panels and cutoff switch is rated at 200A. He told me that I need two 200A cutoff boxes with double-pole 200A breakers. Then I can run cables from both cutoff switches to the meter. Is this right?

And if so do I need a seperate ground loop for each cutoff switch or can I connect both of them to the same ground loop?

Mark
 
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  #2  
Old 05-21-07, 11:54 AM
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Before going down the path of discussing the 'cut off switch', I have a few questions:

1) What is the distance between the two panels and the cut off switch?

2) What is the ampacity of the conductors between the two panels and the cut-off switch?

3) Do the 200A panels have main breakers?

4) What is the calculated load on each panel and combined?

5) Do you already have the 400A 'cut off switch'?

-Jon
 
  #3  
Old 05-21-07, 12:04 PM
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Before going down the path of discussing the 'cut off switch', I have a few questions:

1) What is the distance between the two panels and the cut off switch?

One panel is about 20 ft and the other is about 50ft fromt he cut off switch. The house has been wired so moving them is not an option.

2) What is the ampacity of the conductors between the two panels and the cut-off switch?

The conductors between the cutoff switch and breaker panels is rated at 200A. (4/0 aluminum w/ 2/0 ground)

3) Do the 200A panels have main breakers?

Yes they do, but since they are farther than 4ft from the meter box I am required to have a cutoff switch outside by the meter.

4) What is the calculated load on each panel and combined?

Each breaker panel has continous load of about 120A. The total load is about 240A continous. I'm doing electric radiant heat through out the entire house.

5) Do you already have the 400A 'cut off switch'?

Yes I already have a single phase fused 400A cutoff switch.

Mark
 
  #4  
Old 05-22-07, 07:48 AM
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Another Diagram

Here is another diagram that represents what I was told to change to.
http://www.scophoto.com/home/se_diagram2.pdf
Any help on if this is a better setup is welcome.

Mark
 
  #5  
Old 05-22-07, 09:51 AM
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1) The general rule is that the GEC must be _continuous_ and _unspliced_, unless the splice is made by an 'irreversible compression-type connector listed for the purpose' or by 'exothermic welding'. This is the _general_ rule; but your case provides a slight exception:

250.64(D) explicitly permits 'taps' to be made in the GEC when a service is divided up into several enclosures (exactly as you've drawn). 250.64(D) doesn't say anything about irreversible or welding; it simply says that the original GEC itself must remain without a splice. IMHO, as you've drawn things, the GEC is 'continuous and unspliced' to the enclosure on the right, and then you have a 'tap' that goes from the enclosure on the right to the enclosure on the left. Thus in my opinion your plan for the GEC is acceptable.

However, because other parts of the code on GECs explicitly call for special connection techniques, many inspectors will call for similar connection techniques in this case. Just a forewarning. If you can find a way to keep the GEC continuous and unspliced to both enclosures, then you may save yourself a headache. One technique is to pass the GEC _through_ the lug, and on to the next enclosure.

Another technique is to run the GEC _through_ a suitable compression connector to one enclosure, and run the GEC tap from the lug to the other enclosure, and then rent or borrow the appropriate crimp tool (you need the correct tool with the correct dies to match the connector), to squeeze the splice.

You may also avoid this issue if you use a _single_ enclosure to hold both circuit breakers, rather than one enclosure for each. Basically what you would be looking for is a 400A rated MLO panel with at least 4 breaker spaces (room for two 2 pole breakers). This eliminates the whole issue of splices or taps on the GEC.

2) You have another issue which should have been caught at the planning stage: 4/0 Aluminium _romex_ is only good for 150A.

Table 310.15(B)(6) is used for sizing residential main power feeders. People will argue back and forth on the issue of applying it when you have _two_ separate panels feeding a single home, but it is generally permitted for installations such as yours. Table 310.15(B)(6) lists particular conductor and cable types that it applies to. Type NM cables are _not_ on the list.

Type NM cables must be used at their 60C ampacity, even though they have 90C insulation. So going over to table 310.16 and looking up 4/0 Aluminium, we get an allowed ampacity of 150A.

Note: the conductors inside to type NM cable are generally made to the same specifications as THHN conductors, and THHN is allowed for 310.15(B)(6). The conductors in type NM cable are generally not _labeled_ as being THHN. If you can get the datasheet from the cable manufacturer stating that the conductors are type THHN, then you may be able to get the AHJ (inspector) to permit this use. The inspector may pass it anyway.

-Jon
 
  #6  
Old 05-22-07, 09:59 AM
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P.S. I was asking about the conductor length because of 'tap rules'. See 240.21(B)(2). A cable less than 25 feet long, protected at the 'load' end with an OCPD (breaker or fuse) at its ampacity, and protected at the supply end by an OCPD not more than 3x its rating is allowed. The 50 foot feeder kills that idea, however, though it might let you get away with a _single_ additional fused switch, in the 50 foot feeder.

-Jon
 
  #7  
Old 05-22-07, 10:06 AM
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Thanks

Thanks Jon, both posts were very helpful.

Mark
 
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