3 pole disconnect for 3 circuits

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First off I'm not advocating this. A building engineer asked me this and actually intrigued me a bit. He has three 30 amp circuits going to a space and wants to have a disconnect for all three circuits at the location(shed type deal) to kill all circuits at once. He has a 3 phase 30 amp disconnect switch at his house and asked if it can be used in place of 3 single pole disconnects, as he wants to use what he has. My first reaction was to laugh, then I said I dont think so, then I realized I wasnt sure. Not talking if this is code, just would this setup burn up the disconnect if he had say all 3 circuits running close to trip point( he has 3 breakers in his house). This is purely out of curiosity as me and my helper were discussing this issue.

Colin
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Should not be an issue. My concern is to make sure there are at least two neutrals, the shared one properly shared, if not three.
 

charlie b

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Location
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I don't understand why you would be concerned.
. . . just would this setup burn up the disconnect if he had say all 3 circuits running close to trip point. . . .
The disconnect is rated to have 30 amps flowing through each of its contacts. This setup would have 30 amps flowing through each of its contacts.
 

Jraef

Moderator, OTD
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San Francisco Bay Area, CA, USA
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The only potential issue I foresee is when someone wants to work on only one of those circuits but is forced to kill all three. I don’t know what they are for, but think of a work shed and one circuit is for a tool, another for another tool and the third is lighting. If a tool trips a breaker, you want to work on that tool and if this is the only disconnect, you will be working by flashlight. So if you then put in another disconnect at the tool, what have you saved?
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
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Sorta retired........
First off I'm not advocating this. A building engineer asked me this and actually intrigued me a bit. He has three 30 amp circuits going to a space and wants to have a disconnect for all three circuits at the location(shed type deal) to kill all circuits at once. He has a 3 phase 30 amp disconnect switch at his house and asked if it can be used in place of 3 single pole disconnects, as he wants to use what he has. My first reaction was to laugh, then I said I dont think so, then I realized I wasnt sure. Not talking if this is code, just would this setup burn up the disconnect if he had say all 3 circuits running close to trip point( he has 3 breakers in his house). This is purely out of curiosity as me and my helper were discussing this issue.

Colin

Is this a seperate building or is it attached to the building the service is in?
If it is detached wouldn't 225.30 come into play?
 

JPinVA

Senior Member
Location
Virginia
What does the spec sheet for the 3 pole disconnect show in regard to the max voltage delta between poles? Is the max potential delta between the poles on the separate circuits bounded to never exceed the spec?
 
It is a seperate building but as this property is a small farm, and from what I gather he has done all electric on his property, even in home renovations, he doesnt seem to care too much about code. I'm most definitely not advising it. Just curious about potential hazards of this setup. I think the point of this setup is merely to act as a switch to kill the lights, a couple of receps and 2 space heaters in one go when he leaves.

And I'm not sure on the specs, havent seen the disconnect even, was just told he has a spare 3 pole disconnect at 30 amp rating, with whats gonna be (3) 120v circuits running through it. I'll ask him if he has a model number as I doubt he has any paperwork with it.

And also this is my first time posting though I have uses this site extensively. Thanks for the fast responses guys.
 

Russs57

Senior Member
Location
Miami, Florida, USA
Occupation
Maintenance Engineer
Leaving aside any code issues........and assuming single phase power.......only thing I could see is someone putting a meter across two poles and assuming power is off because those two poles happened to be the same phase.
 

just the cowboy

Inactive, Email Never Verified
Location
newburgh,ny
six pole disconnect

six pole disconnect

They make six pole disconnects for three phase so you can lock out more than one device. I see this as no different.
I would just mark disconnect " warning fed from more than one circuit"

Cowboy
 

JPinVA

Senior Member
Location
Virginia
And I'm not sure on the specs, havent seen the disconnect even, was just told he has a spare 3 pole disconnect at 30 amp rating, with whats gonna be (3) 120v circuits running through it. I'll ask him if he has a model number as I doubt he has any paperwork with it.

I'm an engineer with little "real life" hands on for situations like this. So I just look for the engineering side of things, as much to educate myself as it is to ask the question in the first place. I suspect a three pole designed for 3 phase (typically 208) would probably have the margin for 240, but I don't have the practical experience to know reality...but enough engineering acumen to ask the question.

If those three 120V are fed from a standard residential single phase split, then the split phase can impress 240 across the poles. In my very limited reality experience I don't know if the typical (all?) disconnects in this situation would cover 240 or if there are three phase disconnects rated at 208. Hence, the question. And looking for an answer for my own "real life" education.

If it's a three phase 480, I guess it's moot.
 
Thats kinda my whole reason for asking, not so much asking from a code standpoint but what issues could crop up safety/engineering wise. Ive personally never seen a 208 rated disconnect, the ones ive installed have been either 240v or 600v rated, but most of my experience has been commercial construction, which doesnt seem to throw too many curveballs in that sense.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
Thats kinda my whole reason for asking, not so much asking from a code standpoint but what issues could crop up safety/engineering wise. Ive personally never seen a 208 rated disconnect, the ones ive installed have been either 240v or 600v rated, but most of my experience has been commercial construction, which doesnt seem to throw too many curveballs in that sense.

I have never seen a disconnect rated for only 208 VAC.
 

Adamjamma

Senior Member
But Tony, over here in UK, we use single phase 240 only... no feeding it so as to make 480 in a home. Yet in the home owners shop he may actually have it so one could create 2 separate 240 circuits in the shop... feeding the center of the three circuits as the 240 neutral... not too sure about it from that standpoint...
However, that is my only objection and I would put a three breaker handle tie on it...
In fact, would look at a quad breaker on the one set and a regular breaker on the other... and thus have three breakers side by side on one phase side... so they could not be made into a 240 circuit...

Again, my only problems with it.
 

Tony S

Senior Member
But Tony, over here in UK, we use single phase 240 only... no feeding it so as to make 480 in a home. Yet in the home owners shop he may actually have it so one could create 2 separate 240 circuits in the shop... feeding the center of the three circuits as the 240 neutral... not too sure about it from that standpoint...

If you go in to rural areas you can find 500V (250/0/250) split phase. I can PM a complete list of UKPN “standard transformers” if you wish.

However, that is my only objection and I would put a three breaker handle tie on it...
In fact, would look at a quad breaker on the one set and a regular breaker on the other... and thus have three breakers side by side on one phase side... so they could not be made into a 240 circuit...

Again, my only problems with it.

OK I should have given more detail. Three 250V single phase loads fed off a 433V three phase MCB.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If the disconnect is rated for the maximum current to be drawn, and is rated for the maximum possible voltage between adjacent poles, then there will be no problem with the disconnect itself.

There _might_ be an issue with maximum current; remember that loads protected by a 30A breaker can draw more than 30A for short periods; you see the same things for switches and contactors where the will have different current specifications for different sorts of load. This disconnect might be rated for '30A resistive' or some such.

But the code issue is the maximum number of circuits feeding a detached structure. You are only permitted a single circuit at a given voltage. The 'proper' way to do what the customer wants is to use a feeder rated for the total load, and to separate the individual 30A circuits in the shed.

-Jon
 

Adamjamma

Senior Member
If you go in to rural areas you can find 500V (250/0/250) split phase. I can PM a complete list of UKPN “standard transformers” if you wish.



OK I should have given more detail. Three 250V single phase loads fed off a 433V three phase MCB.

Would love the list but still new to stuff here in London... Just trying to dig through some of the wording in the BS 7671 is a heady trip...
But.. If you are using three phase UK then you could then create a MWBC style USA circuit where any of the two would make a '500' ... like the USA does with the two 120 to a 240... 0r 208... which is not a true 480 because of the 120 degree sections, right?

So, if sharing a neutral or is splitting the neutral, would still handle tie as any leg could provide 240 and thus need the neutral for fault current... have not dealt with such here but think it would be like wiring actual 3 phase in the USA...
 
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