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Old 02-07-2011, 10:41 AM   #1
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Wiring a sub panel to another sub panel


I have a sub panel next to my main panel in my house, now I plan to connect a sub panel in my workshop (50 feet from house) to the sub panel in my house.

What are the codes about connecting a sub panel to another sub panel?

Thanks Bricc, I feel safer connecting the subpanel in my new workshop to the subpanel in my house, rather than connecting the workshop subpanel to my main panel.

So, my concern was ... Is there any NEC code which does not allow a subpanel to feed another subpanel?

(My subpanel in the house is wired for 80 amps (I need only 40 amps max), and my work shop subpanel only needs 40 amps (max).

Details:

200 amp main panel
40 amp two pole breaker (80 amps total) in main is feeding house subpanel, using #6 wire for each pole. ... and it connects to a 2 pole 40 amp breaker (80 amps total) in house subpanel (used as disconnect breaker for house sub panel.)

House subpanel
20 amp two pole breaker (total 40 amps) feeds new work shop. (#10- 3 with ground wire)
House subpanel has a 20 amp single pole breaker for small window AC, and a 20 amp single pole breaker for lights and small refrigerator


Workshop subpanel ........... is protected by a 20 amp 2 pole breaker (40 amps total) (used as a disconnect breaker for work shop subpanel)

Workshop subpanel has a single pole 20 amp breaker for small window AC , and a single pole 20 amp breaker for lights and small power tools.

Workshop subpanel ground bus is wired to two grounding rods spaced 7 feet apart.

Workshop neutral bus connects to house subpanel neutral bus, and workshop ground bus connects to house subpanel ground bus. (House subpanel ground bus is connected to main panel ground/neutral bus - which is connected to house grounding rod via the main neutral cable from meter box - of course)

If any of my wires are overheated, are they protected by appropriate breakers?

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Last edited by Uncle Wayne; 02-07-2011 at 12:24 PM.
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Old 02-07-2011, 10:52 AM   #2
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Wiring a sub panel to another sub panel


If it is a separate structure you must run 4 conductors or three conductors with metallic conduit. Ground rods are also required at the separate structure.

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Old 02-07-2011, 11:33 AM   #3
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Wiring a sub panel to another sub panel


What other loads are being served from the 80 amp subpanel?

No space left in the main?

No code stopping you from wiring panel to panel.

You can reply directly to your thread you don't have to edit your original post.
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Old 02-07-2011, 01:17 PM   #4
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Wiring a sub panel to another sub panel


Dear JunkC, You guessed it . I was out of space in the main panel; so years ago (20 years ago) I added a subpanel right next to main panel in my house. I only needed 40 amps, but used a 125 amp sub panel and wired it for 80 amps. (I was out of space, I knew about using thin breakers, but I felt congested wire-wise, I had no space for adding more wires)

In those days, our inspector allowed us to use only one bus bar if the subpanel was right next to the main panel. So I am planning to add a ground bus to my house subpanel and bring it up to code. (I don't want to do it, because the neutral/ground bus in the subpanel goes straight to the neutral/ground bus in my main panel which is inches away from it) (now to meet codes I will disconnect my equipment grounds from the house subpanel neutral bus, remove screw in neutral bus, wire the ground wires to a new ground bus on the other side of the box, and then connect them (the flow) back to the Neutral/ground bus in the main; which was where they were going in the first place. ..... Can you tell me how the new way is safer when the subpanel is only inches from the main panel?
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Old 02-07-2011, 01:48 PM   #5
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Wiring a sub panel to another sub panel


With the neutrals again bonded to the grounds in the subpanel you could have current flowing on parts that should not carry volatage like the panel enclosure. The distance from the main panel is irrelevant.
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Old 02-07-2011, 02:57 PM   #6
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Wiring a sub panel to another sub panel


"With the neutrals again bonded to the grounds in the subpanel you could have current flowing on parts that should not carry volatage like the panel enclosure."

Why is it that no one seems to care if the same thing is said about a main panel; a panel that always has its neutrals bonded to the grounds?

"With the neutrals bonded to the grounds in the MAIN PANEL you could have current flowing on parts that should not carry voltage like the panel enclosure."

(in both cases, the panel enclosure is grounded to the outside grounding rod via the bus bars/huge main neutral wire/meter box/GEC.)

The NEC does indicate that distance is a factor -- reference "sub panels inside the same structure vs in another structure."

Also: There is (or was in 1990 when I wired my house) a part in the NEC codes where it says "if one can stand in one place and reach both the main panel and the sub panel --- yada, yada, yada.

Thanks, I will add a ground bus to my house sub panel, as indicated in my original post, and bring it up to the current code, but only because it seems safer to most concerned.

I concede, and I will wire my house sub panel according to current code.

As indicated in my original post, my workshop sub panel is already wired to current code.

My most important question is:

Does the NEC allow a sub panel to feed another sub panel?

--- or, do all sub-panels have to be fed by the main panel?
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Old 02-07-2011, 03:01 PM   #7
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Wiring a sub panel to another sub panel


Yes a sub can feed a sub.

Quote:
The NEC does indicate that distance is a factor -- reference "sub panels inside the same structure vs in another structure."
Not sure where this is from since the NEC does not use the word subpanel. Are you referring to the pre-2008 exception that would allow a 3 wire feeder to a subpanel?
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Old 02-07-2011, 03:03 PM   #8
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Wiring a sub panel to another sub panel


ou can have as many subs off of subs as you want provided the calculated load is not too much.
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Old 02-07-2011, 03:48 PM   #9
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Wiring a sub panel to another sub panel


Thanks folks.

Jim, you are probably right about the term used by the NEC, but whatever term was used it meant sub-panel, which we spell subpanel, but my spell check insists that "subpanel" is an incorrect spelling of a sub panel. -- go figure

I have been scratching my head about what you said and I am thinking of neutral wires headed for the big service neutral wire at the main panel. I am now agreeing that it would be safer to separate the whites and bare wires in the house subpanel.

Last edited by Uncle Wayne; 02-08-2011 at 10:40 AM.
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Old 02-07-2011, 03:53 PM   #10
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Wiring a sub panel to another sub panel


Quote:
Originally Posted by Uncle Wayne View Post
Dear JunkC, You guessed it . I was out of space in the main panel; so years ago (20 years ago) I added a subpanel right next to main panel in my house. I only needed 40 amps, but used a 125 amp sub panel and wired it for 80 amps. (I was out of space, I knew about using thin breakers, but I felt congested wire-wise, I had no space for adding more wires)

In those days, our inspector allowed us to use only one bus bar if the subpanel was right next to the main panel. So I am planning to add a ground bus to my house subpanel and bring it up to code. (I don't want to do it, because the neutral/ground bus in the subpanel goes straight to the neutral/ground bus in my main panel which is inches away from it) (now to meet codes I will disconnect my equipment grounds from the house subpanel neutral bus, remove screw in neutral bus, wire the ground wires to a new ground bus on the other side of the box, and then connect them (the flow) back to the Neutral/ground bus in the main; which was where they were going in the first place. ..... Can you tell me how the new way is safer when the subpanel is only inches from the main panel?
If your not using metal conduit between your service equipment (main panel with main disconnect) and the sub-panel then you have a 3 wire feeder (H-H-N) supplying your sub-panel. Now think about what dangers might exist if the feeder neutral opens or becomes a very poor connection at one of the termination lugs. The neutral and ground bond at the service equipment for a good reason and that has nothing to do with what happens to neutral and ground load side of the service equipment. 4 wire feeders have been required to any panel (or sub if you want) load side of the service equipment when that sub is in the same building with the service equipment for as long as I can remember. Unless your feeding a detached building from the service equipment there is no 3 wire option.

Also a 40 amp double pole breaker does not make your sub-panel an 80 sub-panel. It's a 40 amp sub-panel.
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Last edited by Stubbie; 02-07-2011 at 06:46 PM. Reason: added double pole
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Old 02-07-2011, 04:00 PM   #11
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Wiring a sub panel to another sub panel


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Originally Posted by Uncle Wayne View Post
Thanks folks.

Jim, you are probably right about the term used by the NEC, but whatever term was used it meant sub-panel, which we spell subpanel, but my spell check insists that "subpanel" is an incorrect spelling of a sub panel. -- go figure

I have been scratching my head about what you said and I am thinking of neutral wires headed for the big service neutral wire at the main panel. I am now agreeing that it would be safer to separate the whites and bare wires in the house subpanel, and I am also thinking it would be safer to do the same in the main panel --- even though NEC allows us to bond them at the main panel single bus. ***Whoops ... I am wrong about that main panel separation. Why?? The big neutral service wire (cable) is both a neutral and a ground wire to the grounding rod outside the house via the meter box. In the main panel I want my equipment grounds to be attached to and sucked toward that big neutral/ground wire. If we separated the grounds and neutrals at the main panel an individual ground wire from an outlet in the house would miss being grounded except by my wet feet doing the jig dance as I touched the panel enclosure.

Thanks everyone, it has been fun.

signed Uncle Wayne (age 67)


(Correct me if I am wrong. It won't be the first time I have been wrong about things. proof: 3 marriages)
Your wrong to consider the ground rod for the equipment ground fault path for human protection. It has nothing to do with your bare grounds and those bonding in the main panel with the service neutral ...forget the ground rod it serves another purpose.
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Old 02-07-2011, 04:22 PM   #12
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Wiring a sub panel to another sub panel


Terminology after 21 years is getting me down:

I have a 200 amps ( @110 volts) main service

This is what is feeding my sub-panel.

a two pole (double breaker) with 40 written on each breaker and a #6 wire in each breaker. It is two 40 amp breakers which are attached. Each breaker is connected to a different leg of the main panel and separate legs of the sub-panel via the same type of double breaker on the sub-panel.

I call it a 40 amp double breaker -- we used to call it a a 40 amp 220 volt breaker.

If it supplies 40 amps at 220 volts, How many amps does it supply at 110 volts?

I figure that is an 80 amp service feed @ 110 volts for the sub panel.

I am using 40 amps (@110 volts) two 20 amp breakers to feed my workshop and 40 amps @110 to feed my (in one room of my house) small AC on one circuit and small refrig and lights on the other circuit .

In short 110 volt-wise: a 200 amp service is feeding an 80 amp sub panel. And, the 80 amp sub panel is feeding a 40 amp sub panel.

Please correct me if I have overlooked something.
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Old 02-07-2011, 04:56 PM   #13
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Wiring a sub panel to another sub panel


Stubbie, in reference to the ground rod(s), I know what they are for.I just added the flow of ground wires (when needed) for other things. During an electric storm the metal weather head and the meter box and all the ground wires in our houses serve as a lightening rod network to ground lightening to that ground rod outside our house. Ergo -- The full available path of ground wires is worth mentioning.

A ground rod or a ground field’s only purpose in life is to have a designed electrical path to dissipate a static discharge voltage (such as Lightning) to earth.



Current will always return to its voltage source if given a choice.

AC will seek a path to the AC source, and a Static Discharge (Lightning) will seek a path back to its source; the earth.

****

If one gets a loose (or burned) hot wire in an electric drill, the hot wire will find some other way to get to ground/neutral if it touches metal inside the drill (or anything else that conducts electrons) . The drill itself is grounded (via the ground wires) back to the neutral/ground bus in the main panel . Why do you think that is?

That is why I said the ground wires (we always called them equipment grounds) should go back to the service neutral/ground cable via the neutral/ground bus - instead of through the body of the person using the electric drill. Ground wires are great for giving the hots an alternative path away from the user of electric equipment because average circuit breakers don't open for business until the wire gets to the rated temperature,. (GFCIs are the exception of course) The ground wires returning to the neutral/ground bus in the main panel is a GFCI of sorts.

What have I overlooked?

Last edited by Uncle Wayne; 02-07-2011 at 11:54 PM.
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Old 02-07-2011, 05:12 PM   #14
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Wiring a sub panel to another sub panel


Quote:
Originally Posted by Uncle Wayne View Post
Terminology after 21 years is getting me down:

I have a 200 amps ( @110 volts) main service

This is what is feeding my sub-panel.

a two pole (double breaker) with 40 written on each breaker and a #6 wire in each breaker. It is two 40 amp breakers which are attached. Each breaker is connected to a different leg of the main panel and separate legs of the sub-panel via the same type of double breaker on the sub-panel.

I call it a 40 amp double breaker -- we used to call it a a 40 amp 220 volt breaker.

If it supplies 40 amps at 220 volts, How many amps does it supply at 110 volts?

I figure that is an 80 amp service feed @ 110 volts for the sub panel.

I am using 40 amps (@110 volts) two 20 amp breakers to feed my workshop and 40 amps @110 to feed my (in one room of my house) small AC on one circuit and small refrig and lights on the other circuit .

In short 110 volt-wise: a 200 amp service is feeding an 80 amp sub panel. And, the 80 amp sub panel is feeding a 40 amp sub panel.

Please correct me if I have overlooked something.
If you are using a 40A double pole breaker, then you are dealing with 40A service from that breaker, not 80A. The breaker will trip when *either* hot leg draws in excess of 40A, not when the sum of the legs draws more than 80A. As noted earlier, a double-pole breaker is essentially two 120V breakers joined with a common trip handle; the breaker does not care if you are actually applying a 120V or 240V load.

You could theoretically use a 40A double-pole breaker to feed a sub panel with ten 20A breakers. The real question is going to be what simultaneous load are you actually going to put on the sub panel at any given moment.
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Old 02-07-2011, 05:38 PM   #15
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Wiring a sub panel to another sub panel


Dear Clash, roger that. I agree. I have arranged my breakers (loads) to do just that. And, I want both breakers to open if either leg of my house sub-panel exceeds 40 amps.

Also, with that in mind, I plan to add an outside outlet 20 amps on my work shop - just to be available. I will not use it and my small equipment outlet at the same time. So I plan to put them on the same leg of the sub panel in my workshop.

The other leg of my sub panel will be used for a small window AC in summertime and small space heater in the winter. (never both at same time). If I did such a thing the breaker on that leg would open and both sides would open??.

Does this sound like a good plan for sending 20 amps to each leg of a sub panel?

I understand that the double breaker will trip both sides (both 20 amp breakers are attached) if either side exceeds 20 amps in my workshop sub-panel. -- Is that how you see it?

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