Electronics > Beginners
How to wire up a 240VAC receptacle
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Richard Crowley:

--- Quote from: Spork Schivago on May 19, 2018, 07:20:11 am ---Because we don't have an L1 and L2,
--- End quote ---
But we DO have L1 and L2. Those are the names of the two "hot" wires coming into your house.  Between L1 and L2 are 240VAC.

In North America, the center-tap between L1 and L2 is grounded and all the 120V loads are connected between the center tap (called "Neutral") and EITHER L1 or L2, so you get half the incoming 240V  And 240V loads are connected BETWEEN L1 and L2.

In most of the rest of the world, they use L1 and L2 (240V) directly.  In many cases "L1" is grounded and called "Neutral". That is the industrial standard and that is what your PDU is designed for.

In some parts of the 240V world, BOTH sides (L1 and L2) are "above ground" and not even ground-referenced at all. They must treat ALL the wires as "hot" except the green-wire safety ground (which they call "PE" Proective Earth)  Because of this, much (most?) 240 volt equipment is designed with the assumption that BOTH sides are "hot".  That is why I find it so curious that you claim that your loads (servers) won't operate if both L1 and L2 are "hot".


--- Quote ---because the 240VAC is on the same phase, where do the electrons go if they're not hooked to a neutral or ground to complete the circuit?    Somehow they have to make it back to the breaker, correct? 
--- End quote ---
The electrons go BETWEEN L1 and L2.  When you say "because the 240VAC is on the same phase" it makes no sense.  That is like saying that a yard-stick is 36 inches long no matter if you hold it at one end or the other end.  Or if you hold it in the middle ("split-phase")


--- Quote ---I use the word ground, but all I mean is a completed circuit.   I know and understand that a completed circuit doesn't need a ground or a voltage potential of 0V.   I could technically power a circuit where the supply is 24V and the return is 12V.   So long as the return is at a lower voltage potential than the supply, we can have the electrons moving.
--- End quote ---
Yes, for 1/120th of a second. And then 1/120th of a second later, the 240V has changed to the opposite polarity (which is why it is called ALTERNATING current.


--- Quote ---Using the water analogy that everyone seems to like to use, we have our "pump" (the transformer I'd buy for the PDU, for example).   The "water" (electrons) flow out of L1 into the PDU.   But then what?
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The current flows around the loop from L1 through L2 back to the transformer on the pole.  The center-tap is there to split the 240V in half so that our historic North American standard of 120V loads can get 120V and not the full 240V coming into your house.


--- Quote ---They need to make it back to the "pump" (transformer) somehow.    I know you say ground / neutral is irrelevant, but in this case, I cannot imagine how those pesky little buggers would make it back to complete the circuit without using one of those other wires that are at 0V potential.
--- End quote ---
   
Neither L1 nor L2 are  "at 0V potential".  Even "Neutral" wouldn't be "at 0V potential" unless it was (artificially) grounded.  The current flow of a 240V load never sees neutral/ground.


--- Quote ---The components inside the PDU would be the loads
--- End quote ---
No. The things (computers) plugged into the PDU are the loads.  The PDU is just a fancy, expensive power-strip.


--- Quote ---and by the time the current / electricity had passed through the entire circuit and was ready to leave the PDU, it surely wouldn't just be traveling up the L1 wire again, would it? 
--- End quote ---
A 240V load current passes between L1 and L2.  It does not know or care whether either side (or the center-tap) is grounded or not.


--- Quote ---We'd have to use one of the additional neutral / ground wires in that instance. 
--- End quote ---
NO!  240V loads use the current between L1 and L2. Neutral/ground plays no part here.  Neutral is provided only for 120V loads.


--- Quote ---Just like if we're dealing with 120VAC receptacles.   We cannot just hook up the hot wire.   That wouldn't do diddly squat.   We'd still have to hook up the neutral so the electrons could make it back to the panel.
--- End quote ---
Yes. From the hot wire (which is either L1 or L2) to Neutral is 120V.  But between L1 and L2 is 240V.  That is what 240V loads use.


--- Quote ---With the 120-0-120, I understand how it works now.   I also understand why if I were to not use a double pole breaker but tried wiring up a NEMA L6-30R using a 30-amp single pole breaker in slot 1 and slot 2 (opposites sides of the panel), it wouldn't work.   I'd have 120VAC, not 240VAC.
--- End quote ---
Huh?  There is no such thing as a "single pole breaker in slot 1 and slot 2". Each "slot" is a pole. Between each "slot" and Neutral is 120V. But between two adjacent "slots" is 240V because all the odd number slots are connected to L1 and all the even number slots are connected to L2.  Do not be confused thinking that the left side of the breaker panel is L1 and the right side is L2. The "slots" alternate between L1 and L2 so that you can use a double breaker to provide a 240V branch circuit from two adjacent slots.

Zoom in on this photo to see the details.  L1 and L2 come in at the top from the transformer out on the pole (or buried under the sidewalk). The Neutral comes in to that big terminal at the upper right.  And it is connected to the light-colored screw terminal strip on the right side.  If you look very carefully, you will see that one of the screws in the neutral strip is colored green.  That is where the Neutral is connected to Ground in the North American standard.

Then EVERY OTHER slot is connected to either L1 or L2.  So if you put in a single-pole breaker, you connect to EITHER L1 or L2 and the return path comes back through the Neutral.  But if you install a double-pole breaker one pole connects to L1 and the other pole connects to L2. Because between L1 and L2 is 240V and the load current comes from L1, passes through the load, and returns to L2.  At least for 1/120th of a second.  And in the next 1/120th of a second the current flows from L2, through the load and back to L1.

Notice that during the discussion of a double-pole, 240V circuit there was no mention of Neutral.  And I didn't have one up my sleeve, either.

IanB:

--- Quote from: Spork Schivago on May 19, 2018, 07:32:38 am ---I'm sorry, I haven't gotten a lot of sleep lately, but yes, you are correct.   The NEMA L6-30P / L6-30R only has 3 wires, not four.    So in Europe and other parts of the world that use 240VAC, it's just like ours, where 120V goes down one wire, 120V goes down another?
--- End quote ---

No, there's no 120 V anywhere. Recall that voltage is measured as the difference between two wires. There are only two power conductors in the L6-30 and they carry 240 V. There is simply 240 V between the two power conductors. That's it. End of story. There is no 120 V in a correctly wired L6-30 plug or receptacle. Only 240 V. (Ignore the ground for this discussion, it is there for safety, not for power delivery.)


--- Quote ---This is what my European friend was talking about I think when he said it was a language barrier and that Neutral was meant to be hot.   If that's the case though, and they have two wires carrying the 240V load, then my PDU wouldn't work over there either, would it?   Because we still have single pole breakers.   They'd need 1-phase 240VAC, not two-phase.
--- End quote ---

Breakers are about safety and system protection. If both power conductors are "hot" then each power conductor needs a breaker (a "2-pole breaker"). In North America the 208 V AC or 240 V AC supply typically has two hot conductors, so it needs the 2-pole breaker. This is what the NA/Japan models of PDU provide.

In other parts of the world only one power conductor may be "hot", in which case only the hot conductor needs a breaker (a "single pole breaker"). This is what the INTL or WW models of PDU provide. They are suitable for parts of the world like Europe, but they are not code compliant for use in North America.

Again, this is all about safety and system protection and code compliance. It is not about voltages or power requirements.
james_s:

--- Quote from: Spork Schivago on May 19, 2018, 06:05:16 am ---
That's what I've been missing.   I wasn't talking about the transformer here, I was talking about 120-0-120 split-phase that comes into the house, and trying to see how it's equivalent to 240VAC over in Europe.   Because over there, they'll have L1 and then Neutral hooked to the heating elements, which would still give the 240VAC.   I think I understand now fully how the AC works.

Now, why would connecting earth ground to a 240VAC source that comes from 120-0-120VAC USA split-phase or whatever you want to call it cause a short there?   It's because L1 and L2 are reciprocals of each other, right?  When one is 120VAC, the other is -120VAC.   When one is 75VAC, the other is -75VAC.   When one is zero, the other is zero.   If we added a ground, when either where anything other than 0VAC, we'd have our direct connection to ground, which would be horrible.   Am I finally understanding this?

--- End quote ---

Look at the schematic of the center tapped transformer winding, you have 3 wires coming out of that transformer, L1 and L2 are the ends and CT from the middle. Now in the North American setup CT is connected to ground, so now what happens when you also connect either L1 or L2 to ground? Draw it on paper if you have to but you should see a direct short across half of the secondary.
Spork Schivago:

--- Quote from: IanB on May 19, 2018, 07:43:31 am ---
--- Quote from: Spork Schivago on May 18, 2018, 09:19:20 pm ---With that in mind, if you were me, would you take the chance and send the PDU back and order the other two, plus the additional equipment to get it running?  Or would you just purchase the transformer and new breaker panel?

--- End quote ---

I would send the PDU back and order the other two PDUs. The reason being that that other PDUs are designed and specified for installation in North America and they comply with the electrical and safety requirements of the North American electrical code.

Your existing PDU is not code compliant and is not suitable for connection to a North American electrical supply. It is intended for another market elsewhere in the world. This is why it has a strange plug on it and why electricans don't want to touch it.

You could, theoretically, install the transformer and the sub-panel, but it's going to be really complicated getting it to pass inspection because it will be a strange and abnormal setup.

If you don't want to take the responsibility of ordering the other PDUs just on the advice of this forum, you should find someone in your line of business to consult who can offer appropriate expert advice. I do not think you have been well advised by the HPE sales consultants in your journey so far.

--- End quote ---

Yeah, I agree with you on the HPE sales consultants.   We had this issue with Microsoft as well.   They've gotten so big, they can't keep track of all the licenses and agreements.   For example, one sales person said we could just use Windows Home edition to run our business, another said Pro or higher, another said we needed VL editions, finally, a sales rep said we needed the CSP (Cloud Service Provider) agreement, which we signed up for.   Then, on Spice Works, a Senior Project Manager who works directly for Microsoft and has an account on Spiceworks, helping people with licenses and everything, says no, CSP is not meant for on-premise servers.   If we hosted our servers off-premise or in the cloud, we'd be okay.   There was some hassle between getting my one VAR who sold us the licenses to talk to this guy, but I don't think they realized how much pull he had.   And he made contact with them, letting them know he wasn't just some idiot on the net surfing the forums.    I guess still, him and the guy who sold us the CSP are arguing over it.

Hopefully, we get to keep the CSP stuff, because I like it and it's a hell of a lot cheaper than purchasing Windows Server edition, now that  it's Core-Based, especially with the number of VMs we need.

I think maybe the same happened with HPE, or perhaps, the technician saw a chance to up-sell me on the PSU and said hey, you need this 1400 watt PSU.    The only reason this PDU was picked was because it was supposedly compatible with the PSUs.    I hate having to have a whole bunch of PDUs in the rack, but if that's the way I gotta do it, that's the way I gotta do it.   I just really hope you guys are right about this being able to run off 120-0-120VAC and they (HPE) are wrong.   The only thing I worry about on the PSU is that intelligent connector for the intelligent PDUs.   The cables aren't normal PSU cables.   The input on the PSU, it's got inside of it, three pins to power it, and then on the bottom, underneath those three pins, it has four pads.   It's those four pads I worry about.   The 500 watt ones that were in it, before the upgrades, they ain't got those pads there but those were made for not the intelligent PDUs.

I'll have to wait until Monday, but I'll see if I can send the PDU back.   I've had it a while now, because I was working on getting the transformer money around and all that jazz.
Spork Schivago:

--- Quote from: IanB on May 19, 2018, 07:45:27 am ---
--- Quote from: Spork Schivago on May 19, 2018, 07:39:55 am ---slot 1 and slot 2 are on the same phase, from the panels perspective
--- End quote ---

No, slot 1 and slot 2 are on the opposite phase, from the panel's perspective. That is why slot 1 and slot 2 have 240 V between them.

--- End quote ---

On my panel, double pole breakers go every other, slot 1 and slot 3, or slot 2 and slot 4.   So I'd think 1 and 2 would both be on L1, where 3 and 4 would be on L2.
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