3 PHASE – balanced circuits work?
Folks,
We are renovating a +30 yr old warehouse space for a new Habitat for Humanity operation. We have +30 years of legacy wiring an assortment of 3 phase panelboards. One that feeds the office space is a 120/208Y. (We need to hunt up the service equipment. We don’t own the disconnects in our 30K SF.)
Having used balanced circuits in single phase residential kitchens and shops (two hots sitting on both A and B phase and share one neutral), I am wondering if this is done with 3 phase – 3 hots on three phases sharing a lone white wire?
We are laying out the wiring for the work stations and there is never enough power. Every lady will have a heater under her desk, for openers. However this panel is fed with 2/0 Cu feeders, so we have hope.
BTW – what do they do about equipment grounding in these panels? The neutral bus is insulated from the panel, but someone has added a bonded terminal for the lone green wire.
The ToolBear
“Never met a man who couldn’t teach me something.” Anon.
Replies
you keeping the three phase???
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Yes, 3phase works the same...three hots sharing one neutral. When the three hots are on different phases, the neutral only carries the difference in the loads. So, if three 1500W heaters are pluggged into three circuits sharing a N, with no other loads, the N carries no load. Add two 60W bulbs (120W) to one circuit, and the N will carry only about 1 amp.
Sounds simple until we electricians start talking about the problems that computers and flourescent ballasts will cause. Harmonics! Harmonics in a nutshell, puts a load on your neutral that based on elementary electrical theory shouldn't be there. It can be so bad that it will overload the N conductor and/or cause other equipment failures.
This is really Commercial stuff we are talking about here though. You know..acres of cubicles and that warm buzzing overhead glow. God I love houses.
You should consult an electrician who is familiar with office wiring.
Yes, it is a tad commercial. Behind Auto Row in Corona, CA. 30,000 SF of warehouse - because they ran 212' of partition wall from front to back. There is a lot more under the same roof. We have a two story office section. There are going to be 4-5 cubicals and the loos on the first floor and three more offices up topside.
Never a dull moment. 30 years of legacy wiring. 277v fluorescents overhead in the warehouse and they all work. 120/208Y panels in the office section. A former paint booth. Panel labels indicating that the space was a store of some sort at one time, so why does the back panel show 3p for the saw, the planer and other adult toys?
Did I mention the legacy plumbing? Why is there a fairly new CU line that taps the cold water in the sink in one office on the second floor, exits a hole in the wall and runs the length of the building to serve a fixture at the far side? And, where is the water meter? And, where are the main panel boards with the disconnects? Not in our space.
The ToolBear
"Never met a man who couldn't teach me something." Anon.
Most shop tools can be purchased with 3p motors. Commercial operations that use a lot of power, have 3p installed because the cost of installation can be less: smaller wire, smaller pipe, less labor, smaller service equipment etc for a given capacity. If you have to pay an electrician to wire an outlet for a saw thats 250 feet from the panel, its cheeper if its 20A 3p than if it was 30A 1p. Not much, but cheeper. You really start saving when you are talking big loads like ACs and convayers and the like.
The 277v lighting is cheeper to install also. You can put more fixtures per 20A circuit than if you were running 120V lighting.
Another advantage of 3-phase motors is no starter winding. They also tend to be able to develop more starting torque than single-phase.
Could use some of that 3 phase at work. 277v lighting would reduce our VD. Been working on a condo assoc. sump pump circuit, 10A load. They ran 12g CU wire around 390 ft (by measuring wheel) to run this - back when.
I calculate about an 11% VD. The conductors at the far end have melted themselves into the P40 conduit. Add to that advanced galvanic corrosion. Of course, the use of common wire nuts in a location that is under water in the winter does not help much. We hope that these direct burial nuts will do the job. The ToolBear
"Never met a man who couldn't teach me something." Anon.
So long as you're only dealing with 120V circuits, you can treat the 3-phase pretty much like 240V. It's just that a complete 3-phase circuit is three hots and a neutral vs the two hots and a neutral for 240V. And obviously, switches, panels, etc, have to have three hot buses vs two.
Like 240V, if you have a subpanel, you can feed it with 3-phase with all four wires (3 hots and neutral) sized the same and protected by breakers appropriate to that size. There are, however, a couple of caveats:
One caveat is that if you have a lot of older computers on one subpanel (over 8-10 years old, I'm guessing), then you may need to up-size the neutral due to "odd harmonics". (Most easily explained on a blackboard.) But this would only be a problem if more than 50% or so of the panel load was these old computers.
The other caveat is that if you can in practice have an unbalanced load on a panel (near full load on two legs and essentially no load on the 3rd), then the neutral needs to be upsized. [Actually, after thinking about this point, I suspect I'm wrong. The load on the neutral will never go to zero, but it will never exceed the load on either of the other two wires, provided there's no harmonic problem.]
For 240V loads that aren't especially voltage sensitive, (for things like a range) you can use two of the three phases. Here, though, some up-sizing of the neutral may be required if the device internally consists of multiple 120V loads. (I'd have to do the math to tell you how much.) Most ranges and ovens, however, draw very little neutral current.
Motors, of course, should be 120V or 208V single-phase or 3-phase. It's not wise to run a 240V motor (or air conditioner or whatever) on 208V.
As far as I know, the grounding rules would be the same as for 240V subpanels -- ground and neutral separate except at the service entrance or main panel. But there may be some exception in the code that the code lawyers know about.
Edited 3/28/2005 10:04 pm ET by DanH
UPSIZE THE NEUTRAL
Dan,
How much upsize? I have no problem running a 10g N with the 12g hots just to be on the safe side.
The ToolBear
"Never met a man who couldn't teach me something." Anon.
For odd harmonic problems it's hard to estimate how much upsizing you need without knowing the characteristics of the devices. But generally one "size" should be sufficient, given that modern "listed" devices are required to limit odd harmonics. And note that upsizing isn't required for a single 120V circuit, just a 3-phase feed to multiple 120V circuits.I think some buildings got into trouble with this mostly because they had downsized neutrals -- the neutral actually smaller than the hots, on the theory that there was always a certain degree of "balance" between phases. This undersizing is legal under certain circumstances, but is a real gamble in a relatively small office like yours, and should be avoided, even if technically OK.
@@ And note that upsizing isn't required for a single 120V circuit, just a 3-phase feed to multiple 120V circuits.
@@@
The feeders in the panels in question are all the same - 2/0 CU. Which beats the 10g. stuff at the HFH offices in the O.C. In a Zinsco panel.
So, if I understand, I should be good to go with 12g B,W,G for the workstation outlets. Short run. Panel is in the same room.
The ToolBear
"Never met a man who couldn't teach me something." Anon.
Yeah, the 120V wiring is totally standard -- just make sure you don't get confused by the third rail in the panel when making panel connections.And note that it's a good idea to try to spread the load across all three hots, vs, say, putting all of one room on phase A, all of the next on phase B, and the shop on C. Probably the breakers in the panel alternate A-B-C, and if you just keep taking the next one you'll "round robin" automatically (so relatively little thought/planning required). But figure out the breaker sequence before you start, to make sure this is the case.
Spread the load...
Dan,
This panel looks like the breakers sit on phases A, B, C, A, B, C, etc. in order as you work downward.
Any reason not to use an existing 3p breaker for some of my workstation circuits? That should insure the load is spread about the phases. Got a few of them left in and Home Depot is not the place to find 3 phase stuff.
Hope these are not legacy panels. I am used to Cutler-Hammer, Square D, Seimens for the most part. These panel names are new to me. ITC? Think that was the name on the 277 panel.
The ToolBear
"Never met a man who couldn't teach me something." Anon.
The problem with using a 3-pole breaker for 120V circuits is that (assuming you wire to all 3 poles) you have to disable all the circuits to work on any one of them. This may be a code issue ...Perfectly OK from a pure electrical standpoint, though, AFAIK.(Well, actually, there may be an electrical reason to not do this. A 3-pole breaker will trip most readily if all the phases are overloaded. In theory it should trip if only a single phase is overloaded, but likely in practice there will be less margin of safety.)
Three phase, three pole breakers have only one operating toggle or if it has three, they must be tied together mechanically. Same thing for a two ploe breaker. (not to be confused with a mini-single pole)
An over current on any leg should trip the entire breaker.
Dave
"Should". I wouldn't be really confident until I read the mgfr's specs on the breaker.
3 phase breakers...
Then I had best sent Jason over to Consolidated Elec. to see what they have.The ToolBear
"Never met a man who couldn't teach me something." Anon.
Would UPS's for the computers help with the odd harmonics problem?
-- J.S.
Not especially. The UPS likely uses a switcher supply similar to the PC. It's the switcher supply that causes the problem -- two peaks in current draw, about 120 degrees apart.
hey
ive read this this whole thread and it doesn't seem that anyone has answered your equipment grounding question am i correct
Grounding...
I think the discussion has been focused on the balanced circuit aspects with a shared neutral.
Have a go at the grounding issues.
I expect I need to mount a secondary terminal strip on the panel for the grounds. The neutrals are insulated and unbonded - just like a subpanel should. The one ground present is hooked to a large lug.
Any reason why all the circuits in one conduit cannot share one ground?
(For some reason, this question doesn't come over in Romex World.)
The ToolBear
"Never met a man who couldn't teach me something." Anon.
hi your system should have 5 wires 3 hot 1 neutral and 1equip. ground
if there is no equip. ground wire the conduit [metal-emt, imc, rigid]
is an acceptable equip. ground as long as it is mechanicaly and electrically sound in which case all that needs to be done is to mount a ground bar in the panel boxand yes all the circuts in the conduit can use that one ground as long as it is sized to the highest amperage circut in the pipe
@@@
hi your system should have 5 wires 3 hot 1 neutral and 1equip. ground
That's my intent.
if there is no equip. ground wire the conduit [metal-emt, imc, rigid]is an acceptable equip. ground as long as it is mechanicaly and electrically sound in which case all that needs to be done is to mount a ground bar in the panel box
I will pull ground wires for all these new circuits. The panel needs a ground bar. I think they were using the conduit for the ground back when.
and yes all the circuts in the conduit can use that one ground as long as it is sized to the highest amperage circut in the pipe
Will do. Thanks for the input.The ToolBear
"Never met a man who couldn't teach me something." Anon.
Berg,
Not to high jack a thread...but all three phase machinery I've ever seen, and I've seen a lot, that is NOT hardwired has a four conductor (three hot's and a ?neutral?) plug. So, how is it grounded? Through the neutral? or not at all?
Jon
The 4th wire is a ground.Pure 3 phase loads don't need a neutral. Some thing might like 120 or so for some auxcillary prupose. But you can only get that on a 208 system anyway. So they will use a local transformer to generated the needed voltage.http://www.networkcable.com/pages/components/nema_nonlocking.htmlLook at the;11-xxx it is a older 3 phase without neutral or ground.
15-xxxx is 3 phase with a ground.
18-xxxx is 3 phase with a neutral, but no ground.If you look at all of these, the grounds are ones with the round/U-shaped pins. Power pins are alwasy flat (or L shape for keying).I don't know if there are any standardized connectors for the higher voltages.
Bill,
Thanks for the clarification.
Never knew if my shop set-up was wired right (rotary phase generator) given I wired that last wire to ground on the sub-panel (vs. the unbonded neutral bar).
Jon
Edit: Come to think of it, one of my 3 phase machines has an electromagnetic switch that is energized by only one leg.......Umm?
Edited 4/5/2005 3:48 pm ET by WorkshopJon
Edited 4/5/2005 3:48 pm ET by WorkshopJon
ToolBear,
If yours is a tenant space in a large commercial structure then your electrical panel is a subpanel and should have the neutral bar isolated from ground.Your incoming feeders would terminate on either the main breaker or main lugs,the feeder neutral to the neutral bar and if an equipment ground was pulled it would terminate on the ground bar,which is screwed to the panel enclosure.If they didn't pull a ground in and are just using the metallic conduit as a ground(which we try not to do)the ground bar is mounted to the enclosure and your green branch equipment grounds would just terminate on it.I'm not sure what you mean by a "bonded terminal".There shouldn't be a connection in your sub panel between the neutral bar and the ground bar,it isn't a service panel.Back at the main electrical service there is a grounding electrode conductor between usually a metallic water line and the neutral bar.Then there would be conductors going from ground rods/structural columns/reinforcing rods in the concrete, depending on the E.E.'s design, going to the ground bar/bus in the service panel.All of the equipment grounds going out to feeder panels would attach to this ground bar.There is a main bonding jumper between the neutral bar and the case of the panel.
Using single phase circuits on three phase breakers isn't a good idea.Dave is correct that an overcurrent on one leg will trip the other two circuits.Not a good idea if you are composing a document on a computer and the cleaning lady gets a rug caught up in the vacuum cleaner.So it's bad design,nobody does it on new construction,and if you read Article 110 I think you'll see that to be approved, electrical equipment has to be installed and used for the purpose for which it is intended and according to the manufacturer's instructions.
@@@ If yours is a tenant space in a large commercial structure then your electrical panel is a subpanel and should have the neutral bar isolated from ground.
All our panels are sub panels. There is doghouse out back with the main panel boards. This place was variously a department store, an RV builder, a body shop and more. We think we have 50 yrs of legacywiring, not 30.
The neutrals are isolated from ground.
Heeding Dave's thought, we bought a carton of 20A/1P breakers and won't use the 20A/3P one. The ToolBear
"Never met a man who couldn't teach me something." Anon.
It'd be a good idea to bone up on 3-phase systems before working on anything.
We had a similar situation at a Habitat affiliate (office space in converted industrial bldg), and an unqualified volunteer was working on some fixtures one day. He was working the circuit hot ("didn't want to shut down the office")--stupid. He opened the neutral in a fixture/junction box, and in doing that, created a 277/480v series circuit out of the three 120v circuit sharing the neutral. This blew several surge protectors/backup UPSs on the desktop PCs and two servers. At least it didn't blow up the computers. He didn't have a clue why it happened. He was lucky that he didn't get a serious shock or worse.
I suggest getting John Traister's "Electrical Wiring Commercial" for good info on three-phase systems. Be advised that the systems usually have 480v, 277v, and 120v (and sometimes 208v) present. Lighting is typically 277v to ground; contact with 277v is serious stuff; with 277v, what might result in a tingle with 120v can lock you onto the wire or whever, and kill you.
The 480v feeding the setpdown transformers may be in the same conduits and panels as the 120v. When you've working around 480v, it's a completely different game. The potential for disaster from arc flash and blast is real--if a knockout slug falls and shorts two phases of the 480 circuit, it can be like a small A-bomb.
On old wiring (with dirty and degraded insulation), for making voltage measurements above 277v, I wear 500v-rated lineman's gloves. The 480v can crawl on dirty insulation, and what you can get away with on 120v, you can't with higher voltage systems.
Also, do not trust any color-coding of conductors. In buildings likeyours, I've seen circuits where the polarity was reversed (at a setpdown transformer) between the grounding conductor and hot! My point is, take nothing at face value; for instance, verify all equipment grounding pathways (I've seen plenty of old commercial and industrial buildings where the conduits have been pulled apart at fittings or couplings, and the conduit is the grounding path). The Ideal Suretest is the tool to use for this, for the 120v brach circuits.
Your best bet would be to get a commercial electrical contractor to donate a half-day of time to assess the system and fix the most serious conditions, and maybe give you a little lesson in 3-phase.
Good luck. E-mail me if you have specific questions.
Cliff
CAP: Thanks for your note. I really appreciate your input. I have forwarded it in the hopes that we can scare up a commercial electrician for a consult and tour. Your best bet would be to get a commercial electrical contractor to donate a half-day of time to assess the system and fix the most serious conditions, and maybe give you a little lesson in 3-phase.@@ Amen to that!I suggest getting John Traister's "Electrical Wiring Commercial" for good info on three-phase systems. @@ Thanks for the recommend. Where sold? Copied the 3 phase section from Richter and Swan’s Practical Electrical Wiring, 16th edition. It was a start. (BTW – anyone seen the new edition? Amazon has them. Worth the $50+)Be advised that the systems usually have 480v, 277v, and 120v (and sometimes 208v) present. @@ The 480/277 ITC panels look to be in good shape. They are over by the drive door and in the back. The warehouse overhead lights are all 277 and they all work, so I hope to stay away from those panels. In the office area the panels appear to be 120/208Y. Also, do not trust any color-coding of conductors. @@@ I don’t trust any of it.
I’ve seen very few grounding conductors in the office panels and a number of the outlets have little smoke smudges on their face plates. There is at least 30 yrs of legacy wiring in the offices. To see the upstairs is to shudder. Given the age of the place, using the conduit as the grounding path was then the Best Practice. In the age of computers, we need better – and I need to run clean home run power for the server farm.There are panels that have been decommissioned (and very neatly – all the conduits capped off, etc.), decon panels that now have only two circuits passing through and other stuff I have not discovered yet. In short, it’s interesting.With hot panels and the disconnects a field trip away, I am rather reluctant to start pulling new wire in old conduits with a metal fish tape without opening every box on the line to see what is going on in there. If I did that, I’d still wonder what I missed. I have encountered enough loose wire nuts elsewhere. My approach in these cases is to run surface mounted conduit (a decorative accent) and pull new circuits. (I've seen plenty of old commercial and industrial buildings where the conduits have been pulled apart at fittings or couplings, and the conduit is the grounding path). You must have toured this place. We got ‘em. What I wonder about is what I can’t see in the walls.
The Ideal Suretest is the tool to use for this, for the 120v brach circuits.@@@ The Branch Circuit Analyzer? I have been hoping Santa would bring one. Humm. This building would justify one. The ToolBear
"Never met a man who couldn't teach me something." Anon.