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11-27-2012, 10:27 AM #1
200 amp branch breaker in a 200 amp sub-panel??
We are converting a dumpy little farmhouse into a dumpy little machine shop. The 200 amp, single phase, residential 3 wire 230V service entrance is connected to a 200 amp, Square D QO, 8 space "feed through" panel such as are sometimes used for mobile homes.
The location is beyond the jurisdiction of any municipality and nobody seems to care what I do or how I do it. Of course this puts the onus on me to do it right.
The service entrance panel is mounted outside the house on the south gable end. The entire 200 amps then passes through the exterior wall and into an identical QO, exterior, 200 amp box, mounted inside the front bedroom closet. To comply with codes, the closet wall has been removed to create a 36" x 72" "alcove" in the front bedroom. Nothing else is in the closet. Hence, this interior panel complies with the NEC "refrigerator rule.
This second box, also a "pass through", passes the entire 200 amps out yet again, through 2" EMT, straight up into the attic, for a run of about 20', then down again, to a 200 amp QO main disconnect box, mounted on the front wall of the recessed front porch of the house, between the front door (on the left) and a large picture window.
From this disconnect box, the 200 amps passes through the exterior wall, straight into a third 200 amp box mounted in the living room directly across from the disconnect and next to the front door. There is also code required clear space around this box. This third box is also a 200 amp Square D QO box. But unlike the three upstream boxes (counting the disconnect), it is NOT an exterior box and it is NOT a "pass thru" box. It is a 30 space residential indoor box.
This third box is intended to supply the living room and the front porch. The living room contains a lathe and a Gorton tracer mill. The living room also contains a mid 80s vintage Miller Syncrowave 300 transformer type square wave TIG welder. We also have a couple of Syncrowave 500 welders, although these are not currently in the house or on the front porch.
But they might end up there some day. All three welders consume a whopping amount of single phase power. And that's the reason for my electrical scheme in this house. It is unlikely I will ever need/want to run any of these welders at full output. But I am trying to arrange things so that by cutting power to everything else in the house, I could shunt the full 200 amps to one of these welders. 200 amps will keep both the Syncrowave 300 AND the Syncrowave 500s well fed.
It is my understanding that Square D offers a 200 amp branch breaker and that it will fit in my box. Apparently it's a big old honkin' thing and it takes up 4 full spaces. My tentative plan is to buy one of these breakers, and to install it at the very top of the box. This breaker will spend most of its life in the "off" position.
For your information the 200 amp box will also feed a few minor 220v loads - this being the mill and its hydraulic power unit, and a lathe or two, all of which will be powered by VFDs. This box also feeds a 20hp rotary phase converter. The RPC is installed and operational but for the moment it amounts to surplus capacity. Some day it might supply power to other equipment on the front porch or on the north end of the house.
The said 200 amp branch breaker is pretty damn expensive. Going from memory, I think it's crowding $400 dollars. An alternative approach, and a cheaper one, would be to power the welder(s) from a 100 amp, 125 amp, or even a 150 amp branch breaker. These breakers are quite a bit cheaper than the big 200 amp breaker although they're still pretty expensive.
Under this alternative scenario, I would simply rely on my self-control not to crank the welder up too high. Under the 200 amp branch breaker scenario, I would conversely rely on my self-control to de-energize the rest of the house before flipping the breaker and energizing the welder.
My question to y'all is not "Is this an optimum arrangement. I know that it is not. However, I am attempting to get maximum energy management versatility out of a small residential service. So my question is: Is there a better way?
The original house wiring, which was conventional residential through the wall romex, is now totally isolated from this electrical scheme. So for purposes of this discussion there are no "creeping, uncontrolled loads". The wiring throughout the 3 boxes is 3/0. The house is completely surrounded by a grounding ring of heavy single strand, zinc plated bare copper wire, connected to industrial quality 10' x 5/8" copper clad grounding rods driven into the ground every 10' around the perimeter of the house.
In summary, I suppose my question is really this: Am I safer relying on the 200 amp breaker? Or should I use a 100 amp, 125 amp, or 150 amp branch breaker in this scheme?
Thanks for any and all replies, suggestions, ideas, and admonitions.
11-30-2012, 08:32 AM #2
Just so you won't feel neglected , I too have purchased an older farm house with less than optimal electrical service. I also have an older 300 amp Miller TIG which is far and away my largest single load . Is a 200 amp service enough to run that welder by itself ? ---Trevor
11-30-2012, 10:12 AM #3
All three welders are Miller Syncrowave square wave TIG machines. Two of them are "Syncrowave 500" models. The third one is a Syncrowave 300.
The 300 amp model is fully fed at 200 amps. The 500 amp machines are fully fed IN BALANCED CONDITION at 200 amps. I have a very imperfect understanding of the term in CAPS but UNBALANCED has to do with a setting that can be made to favor either the positive or negative side of the alternating cycle in order to favor penetration or "cleaning". In the UNBALANCED condition the machines require 300 amps. That's why my plans center around the smaller machine.
However, provided I don't burn the house down, any of these machines do marvelous and useful welding tricks at output amperages down to 25 amps. At those amperages they are drawing far less current than my available 200 amps. That's why, provided I make a safe installation, and don't inadvertently crank a knob too high, I should be okay. However, "okay" is far less than "optimal". And that's why I was requesting some guidance.
The current service to the house IS the upgraded service. 200 amps is as good as they will give for residential, which is what we were when I upgraded the service more than 5 years ago. The old service has been completely disconnected and isolated. I may tie in to the old service when I'm finished with the machine installations - or perhaps not.
11-30-2012, 06:49 PM #4
Vern, can you use a 200amp pass through- the type that plugs into a breaker slot and are dirt cheap, and wire it to a fused disconnect with 200 amp fuses?
11-30-2012, 07:36 PM #5
I don't recall ever seeing "200 amp" and "dirt cheap" used in the same sentence. But I like it! However, I'm not sure what you're talking about generally, and when you say "fused disconnect" specifically. This however is due to my own inexperience no doubt.
The only context in which I understand "pass through" is as used with a certain kind of Square D breaker box - often used to bring power to mobile homes. In those, there are a few breaker spaces (four 240 amp spaces or eight 120 amp spaces). Then, there is a set of additional lugs that enables you to connect heavy wire and take the entire 200 amps out the box. This is different from a main disconnect although it serves the same purpose, with the added benefit of a few additional "free" circuits. Often these are used on poles to provide circuits for new construction.
I already have two of these pass-through boxes connected back to back on opposite sides of an exterior wall. I suppose there is no reason I could not have used yet a third box but instead, I used a regular residential 200 amp, 30 (120v) spaces, indoor box. This box is mounted on the inside of an exterior wall and is fed by a regular 200 amp disconnect breaker.
My objective is to channel the full 200 amps to a large square wave TIG welder on the rare occasion. Unless I go to another type and/or brand of box I am unaware of any option other than:
1) buy an expensive 200 amp branch breaker (about $400 dollars) or,
2) buy a smaller breaker (100, 125, or 150 amps) and simply not run the welder at full power.
The reason for posting this thread is to seek opinions on what is the safer thing to do. Apparently, there are some safety concerns in mounting a branch breaker that is of the same ampacity as the box in which it's installed.
I hope this makes sense. I have been intending to draw a schematic or even take some pictures and post them so that my layout is more comprehensible. The building in question is a tiny frame house built on cement slab. We have converted it into a small family machine shop. We do not live in the house anymore. All of the original and conventional residential wiring (plugs, ceiling fans, switches, etc.) has been completely isolated and is dead. My power distribution scheme allows for this wiring to be reconnected but I haven't decided whether to do so.
11-30-2012, 08:03 PM #6
Not sure about that, I'd have to look up a couple things... never ran into that system before.
One issue is that everything up to the main breakers is the "service", is essentially totally unfused, and can supply pretty much whatever current the load or short circuit draws.
Because of that, you would have to use service entrance wire for all portions of the "through connected" wiring, since it appears to all be on the "service drop" side of every main breaker.
There also are some limits to the length of a 'service entrance" wire, and may be added requirements concerning how, and where, the type SE cable is run.
11-30-2012, 11:37 PM #7
A fused disconnect is a metal box with an on/off lever on the side and fuses inside of it. Used for disconnecting power to equip.
This one has a breaker in it: Cutler Hammer New Disconnect ECC225R w 200A MCB Loaded N3R | eBay
Seems that safety switch is a more common name for fused disconect, here is 200A: ITE JN 424 240V 200A Enclosed Safety Switch Fused Disconnect with Fuses | eBay
I will have to get the box out that has my pass troughs in it to get the part # and exact name, I found these pass throug kits though that I think would be better than what I was thinking, these don't use any breaker spaces. : http://static.schneider-electric.us/...trilingual.pdf With this I got to thinking you may be able to get by with a standard disconect or safety switch without fuses, like this one : GE Safety Switch 200A 600VAC 250VDC Type 1 THN3364 | eBay the main 200a breaker will give you protection.
I would probably put 175A or so fuses in the fused disconnect so the main would "never" trip for added safety margin. Or just so the lights stay on when you run to much juice to the welder at night
I will try to find the proper name tomorrow.
12-01-2012, 02:44 AM #8
If you intend to run those 500s to there potential you need to get shed of all that residential stuff and have only a fused disconect between service and welder.(you have way to many conections to give prolems)
You may be money ahead getting a seperate 480 service just for the welding machines.
Rob F. liked this post
12-03-2012, 01:44 PM #9
As far as putting a branch (or feeder breaker) into a box of the same maximum rating, I don't think that's a safety issue at all if the breaker is approved by the manufacture for use with the box. If there's no fine print in the breaker datasheet that says "For QO boxes larger than X; Not for use in ...", not a problem.
One other issue you should be aware of: NEC wants continuous loads to be no more than 80% of the device ratings. So, only 160A continuous through a 200A breaker.
Sounds like you may not need code compliance (your insurance company may have something to say about that, even if your county or municipality doesn't), and "continuous" can be arguable with regard to welding duty cycles. Still, the NEC intent would be a good thing to keep in mind.
Vernon Tuck liked this post
12-03-2012, 07:42 PM #10
TDJC prison unit down the road from you going to get upset with you hogging the juce from the electric line for those welders. They got to be able to charge "Old Smokey".
Last edited by 4GSR; 12-04-2012 at 04:13 PM.
t.jones liked this post