MainBox-MetalicConduit


  #1  
Old 01-20-04, 12:10 PM
Guz
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Arrow MainBox-MetalicConduit

I think my last post was conffusing, lets try again:


This is my 200A Main Box, what do you think?

-200A Main Box
-Wire is UF-B 12/2 with ground.
-17 - 20A breakers and one 30A.

Inside the 2 inches metal conduit there are:
9 - UF-B12/2 with ground in one side and
8 - UF-B12/2 with ground and 1 - 8/2 with ground in the other.
(UF-Bs were a gift, so all the house is wired with it)

(Head, meter, tensor, ground rod and cable between meter and box not yet in place)
(of course Im not intending to put the meter and cable between meter and box)

My concerns are in the conduit with, let say, 9 - UF-B12/2 with ground each side. Some one in
this forum told me that this is not the common way to do it, but has been made. I wanted to send
the picture anyway so youll have a better idea of what Im talking about.

Edge of Box is 21 1/2 from the window (just like the old one), 28" from ground and 21 from
above (soft is the name?).

Am I ready to call the power company?...
... or Should I call 911 or fire fighters instead :-))) ?

Thanks.
-----------------------------------

PLEASE, IN ORDER TO SEE THE PICTURES, DON'T CLICK ON THE LINK, INSERT THE WHOLE ADDRESS IN YOUR ADDRESS BAR INSTEAD OR COPY AND PASTE IT IN THAT ADDRESS BAR.

complete view

http://guz1.tripod.com/box3.jpg

----------

lower closeup

http://guz1.tripod.com/box1.jpg

------------------------------------------------

upper closeup

http://guz1.tripod.com/box2.jpg

--------------------------------------
old box view

http://guz1.tripod.com/box4_old.jpg

(box4_old.jpg is low slashed)
 
  #2  
Old 01-20-04, 02:52 PM
W
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Unhappy UF-B in Metallic Conduit

Guz,

I think that you may have a problem. It depends upon the code requirements in your area.

The maximum current that a conductor can carry is set by _temperature_, the heat generated when current flows through the wire, and the thermal characteristics of the insulation. In some cases, using very high temperature insulation systems, you can push more than 30A through a 12ga wire. On the other side of the coin, if there are other heat sources around, or lots of thermal insulation, then the current capacity of 12ga wire is less than 15A.

In the case of the NEC, rather than having to do an engineering calculation on the wire, there are various rules of thumb and tables that you can use. If the current to the wire is limited via the tables and simplified calculations, then you will end up with a reasonably safe installation.

When you look into the NEC you will see tables for wire size, temperature rating, and wire ampacity, the amount of current that the wire can safely carry on a continuous basis.

The rules for NM cable (including the way that you are using the UF cable) say that the wire should be 90 degree wire, but that you should figure the ampacity as if it were 60 degree wire, with one exception: derating for temperature and raceway fill can be done from the 90 degree values. This last point is important, because when you bundle cables together, each cable heats the others, and you have to derate the wires. But 12ga THHN wire (the sort in NM-B cable) has a nominal ampacity of 27A. Normally you are required to protect this wire with a 20A breaker, under an exception for small wires. But you can use the 27A number for your derating calculations.

In your case you have _18_ current carrying conductors in each raceway. This means that you are required to derate the conductors by _50%_, and your 12ga wires will only be rated at 13.5A. I leave as a question for others on the forum: what circuit breaker would be appropriate for this? Remember that the conductors are feeding general purpose circuits which include receptacles.

I think that you may have to change your installation in order to be code compliant. There are several possibilities that come to mind:
1) put in 4 conduits rather than 2
2) convert some of the circuits to multiwire (you use 12/3 cable and share the neutral between two circuits on opposite poles of the supply, so that 4 current carrying conductors are replaced with 3, and you can get away with counting them as 2 for most home loads)
3) change the breakers to lower value breakers

Good luck!

-Jon
 
  #3  
Old 01-20-04, 07:19 PM
W
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UF-B in metallic conduit

I got home and did some double checking.

1) The table that I used for ampacity was incorrect. Table 310.16 gives the 90C ampacity of 12ga wire as 30A...note that an exception for 10-14 ga wires forces the use of a maximum of a 20A breaker on 12ga wires.

2) Table 310.15(B)(2)(a) states that for 10-20 current carrying conductors in a raceway, you must derate by 50%. This means that you would need to use 15A breakers.

3) The current ratings are based upon an ambient temperature of 30C; if your area is generally warmer than this, then you may need to derate further.

4) If you convert your 9 circuits (18 current carrying conductors) into 4 multiwire circuits plus one regular circuit, then you will still have 10 current carrying conductors, and will still have to derate by 50%

5) Finally, in the article on UF cable, it is explicitly mentioned that it can be used as NM cable, following the NM cable article. But the UF cable article does not have the ampacity derating exception, and simply says that the cable must be used at at 60C rating. Given that NM-B cable is rated at 90C, I doubt that any AHJ would have a problem with using UF-B cable and derating from 90C, but this is a point that is somewhat unclear to me in the code.

-Jon
 
  #4  
Old 01-21-04, 11:30 AM
Guz
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I understand, although it took me a while to read, re-read and get some definitions from dispersed
sites about the NEC (which was adopted in my area). Now I know a little more.
I never saw the Tables, just references to them, but those references just reaffirm what you are
talking about. I really appreciate your help and time.

Now I have some new questions:
Metallic conduit is more than 24" long ... but is 2 " wide, and 9 Ufs dont get too tight inside.
1- It doesnt mater the conduit size?
2- Can I reduce the conduit size?
3- What is the proper size of conduit for this job?
LAST AND VERY IMPORTANT
4- How this situation (lets say 18 12/2 NMromex to 200A MainBox) is usually resolved?



(Water heater, stove, dryer and house heater use gas, and the refrigerator is the only appliance in
really continued service)
 
  #5  
Old 01-21-04, 11:39 AM
J
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1. No, it doesn't matter.
4. Do you have to use conduit?
 
  #6  
Old 01-21-04, 11:59 AM
Guz
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No, I just thought that was the right way to go in order to protect the wire in an exterior, although it is UF-B sun resistant. So I asked how this situation (lets say 18 12/2 NMromex to 200A MainBox) is usually resolved?. Because I'm very open to hear how could I do it in a way aproved by NEC. (and avoid to drop 15A breakers "if" posible).

I think you have some answers and I'd love to hear of that.
 
  #7  
Old 01-21-04, 12:22 PM
W
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UF-B in Metallic Conduit

John,

I have to disagree in part with your answer to question 1; clearly it does matter to the reality of the situation (heat dissipation from the conductors) how big the conduit is. Imagine if the conduit were 12" in diameter; the situation would be loads better in terms of heat dissipation than the normal concentration of cables near the panel in a wall.

However, on the important point we agree: in terms of the NEC requirements, the size of the conduit is not considered; put too many wires in a conduit of _any_ size, and you are required to derate. As I stated in my first post, the tables and calculations provided in the NEC are vast simplifications of the engineering calculations required to find the exact ampacity of the cables, and as such ignore many mitigating or aggravating factors; rather the NEC calculations produce a reasonable number with what is usually a big safety cushion. IMHO having 18 UF-Bs in a pair of large conduit is _probably_ safer than having 18UF-Bs buried in an insulated wall...but I am not a professional engineer, could not put a stamp of approved calculation on such a statement, and _code_ says that 9 UF-Bs in a conduit require derating.

Guz,

I am not familiar with areas where the panel is installed outdoors, but I presume that the 'usual method' is to bring all of the cables in through the back of the panel into the stud cavity, and then run them up and down to the attic or the crawl space. This looks like an attempt to get all of the cables up to the attic space, on the _outside_ of the house.

I believe that in this case, with the work already done, the best choice is to change to 15A breakers (which will decrease the capacity of any one circuit, but I suspect you won't notice a change in the utility of your circuits). But this would get you into trouble with any circuit that is required to be 20A. I think that you will need to get more ideas from the professional sparkies here. Also check with your local inspector; there may be a way that you can get approval for a non-compliant installation.

-Jon
 
  #8  
Old 01-21-04, 06:37 PM
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Two thing I would question are the height of the meter socket. Around here the center of the glass needs to be in the 3'6-5' from the ground. Yours looks much higher.

The other item is the conduit elbow for the service entrance. I would not consider this conduit to be a complete system.
 

Last edited by pcboss; 01-21-04 at 07:33 PM.
  #9  
Old 01-21-04, 07:31 PM
texsparky
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Your local power company should provide you with a drawing of their requirements. Ive never seen seen a PoCo complete the service riser. We have to install the conduit, straps, wire, weatherhead, point of attachment, etc. .

When wiring a home with an exterior panel, the wires are ran inside the exterior wall and enter through the back of the panel.
 
  #10  
Old 01-22-04, 04:50 PM
Guz
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I have to thank all of you guys, I allways recive the help I'm looking for.
I think I have this problem covered, using your answer as guides to make a deeper reserch.
This is the way I'm going on:
I'll go through the back of the panel with 10 of the UF-Bs and I'll let 8 of them into the conduit, 4 each side. Because the conduit is already in place and covering the holes, and because I don't want to drill too much the joists.
Throug the joists I have 1 and 1 1/2 holes with no more than 4 UF-Bs each, with no conduit and separates by plastic staples when running along joists. (Most of the holes were already there).

Regarding of your concerns about heigt of meter... that's exactly 6feet from ground to center of the meter hole.

Elbows are widw used in this area, all my neighboors have them, the old MainBox have it, and you can see lots of them (metal and PVC ones) in Lowes and HomeDepot with images showing the place of each part, actually I took the idea from them.
The work is not completed because I didn't go further before to ask some questions. Anyway, its suppused that I don't have to mess with anything above the top of the MainBox. But I have the head, fasteners and tensors ready for them. (they will bring the "fat" wires for me).
 
 

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