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| How to wire up a 240VAC receptacle |
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| Spork Schivago:
--- Quote from: Gregg on May 19, 2018, 10:03:29 pm --- --- Quote from: Spork Schivago on May 19, 2018, 07:00:53 am ---We gotta back up the train here a bit. I understand the purpose of the neutral and ground in the house. If we have a short, we want that current and voltage to go to the path of least resistance, hopefully back to the breaker, to trip it --- End quote --- OK, let’s try to go back to some basics. I assume you know how a transformer works; I suggest you grab some paper and draw the circuits out for yourself as you follow along; visual reinforcement is a great learning aid and along the way you might have the “Ah Ha” moment where this becomes clear. Vin / Vout = Turns in / Turns out and the current capacity is more or less determined by winding size If you have a 120VAC primary transformer with a 24VAC secondary, you get 24VAC output Suppose you put this transformer on a metal chassis like old radios etc. and you plug it into the wall with a standard grounded 3 pin plug so that L1 is fused and connected to either wire of the primary and the neutral is connected to the other lead of the primary with the ground connected to the chassis. All is well with the circuit here; the primary is a completed circuit via the neutral and L1. If one of the primary windings anywhere along the coils faults to the chassis, the ground that previously carried zero current suddenly carries the fault current back to the house breaker panel where it completes the circuit via the ground to neutral tie I showed in the previous post and the fuse should blow. At this point of the discussion, the secondary winding is completely isolated from the primary. Let’s call them X1 and X2. You could connect either lead to the chassis (which is grounded) and nothing would happen; no current would flow. The other lead of the secondary would then be 24VAC to the chassis, often called chassis ground in this case which is also referenced to earth ground via the 3 prong plug. Let’s assume neither X1 nor X2 secondary winding are connected to the chassis for this paragraph. If you take two 12 volt lamps of the same wattage in series and connect them to X1 and X2, they will both light equally. They have the same resistance. If you connect the midpoint between these two lamps to the chassis, nothing bad will happen, it is still isolated and, current will not flow to the chassis. Now, connect a 5 watt and 10 watt both 12volt lamps in series and connect them to X1 and X2. The 5 watt lamp gets very bright in a hurry and goes poof; the 10 watt lamp barely glows until the circuit goes open with the failure of the 5 watt lamp; this happens the same if the midpoint is connected to the chassis because it is isolated. Now let’s change things a bit and connect a wire to the center of the secondary windings, a true center tap I’ll call C. Nothing on the secondary is connected to the chassis at this point. X1 to X2 is still 24VAC X1 to C is 12VAC X2 to C is 12VAC Now, connect a 5 watt and 10 watt both 12volt lamps in series and connect them to X1 and X2 BUT with the midpoint connected to C. They both light with their respective brightness and wattage; the 5 watt one doesn’t go poof! The unbalanced extra 5 watts used by the 10 watt lamp is carried back to the transformer center tap C. The 5 watt lamp and half of the 10 watt lamp’s current are the same as two 5 watt lamps in series without the midpoint connected to the transformer; the imbalanced 5 watts for the 10 watt lamp returns back to the transformer via the center tap. Now if we connect the center tap C to the chassis (which is referenced to earth ground as previously stated) there is no harm, no foul and no current flow between C and the chassis. And when we repeat the experiment, the results are the same. The wire from the midpoint of the lamp back to C still carries the 5 watts; chassis ground is just a reference. --- Quote ---The transformer at the pole though....you said, "For instance, if you had 30 amps load on the 120volt side from L1 to neutral and 20 amps load on the other side from L2 to neutral, the neutral conductor would carry the 10 amp difference back to the transformer." --- End quote --- Yes, that is true as in the lamp scenario above; but to carry it further: It would be very helpful for you to draw this on paper. Suppose you have another transformer connected at the primary exactly as above but with two 12VAC secondary windings, completely isolated from each other. So as not to confuse this with the above the first winding leads will be labeled X3 and X4; the other will be X5 and X6. If you connect your 5 watt 12V lamp to X3 and X4 it will light; circuit completed. If you then connect the 10 watt lamp to X5 and X6 it too will light; a completely separate circuit. With both lamps on it you have 4 wires to the transformer secondary With the above 4 wire scenario in mind, if you connect X4 to X5 there will be 24VAC between X3 and X6; and the X4&X5 connection is the center tap and behaves EXACTLY the same as the single winding with the center tap mentioned above. Only three wires are needed for your two lamps and the center tap only carries the imbalanced current. On a side note, not relevant to this discussion, if you connect X3&X5 as well as X4&X6 both making two parallel windings (they have to be wound the same direction or phasing if you prefer) you will get 12VAC at double the current capacity. --- Quote ---Where are these loads coming from? Some place in the transformer, or in the house? Why would there ever be an excess of current? A 60 watt bulb will always draw 60 watt, even if you have it hooked to a 200-amp breaker. So how could you ever have an excess of current? Even if you were to directly wire hot to neutral or hot to ground, if neutral and ground are at 0V, wouldn't there be an instant volt drop where they connected and a lot of heat generated? --- End quote --- The loads mentioned are hypothetical. Every time you turn on a light or 120 volt appliance the situation changes. Think of the transformer as the source of your power however you want to think about it; volts, amps, watts etc. If you have a 60 watt bulb connected from L1 to neutral and you turn on your 1200 watt 120V toaster connected from L2 to neutral, there will be an imbalance. The 200 amp breaker doesn’t care unless it sees an overload on one or both legs L1 and/or L2. The breaker doesn’t see the neutral, nor does it have to as the neutral amps can never be greater than either L1 or L2 in normal circumstances. If you directly wire L1 or L2 to neutral or to a properly connected ground it will go boom. If you can stand them, Electroboom’s YouTube videos are happy to demonstrate this so you don’t have to. And to answer your question, there would be heat generated but it would be governed by the available voltage and current as well as the time before something (like the breaker or wire vaporizing) breaks the circuit. If you turn off the breaker and connect line to neutral, the breaker should pop back off before any real damage is done and you won’t experience any real heating. You would be better not thinking of ground and neutral as zero volts but as a reference point. Zero volts is like absolute zero temperature, unattainable in real life but a good reference point. Calling it ground or neutral is a lot more specific, purposeful and doesn’t mess with your head as much. --- Quote ---I had an old camaro and the ignition went bad. I had to hot wire it. I'd use the fuse panel. I'd hook to a 12V source, then use a wire to tie into the fuse for the Ignition. Then I'd hook into the 12V source and tie into the starter, but just long enough to get running. One day, I was a little careless and grounded that 12V source. The wire evaporated in my hand and left a white indent. At first it didn't hurt, then it hurt real bad. But there wasn't no excess current that just traveled safely back to the cells in the battery. Wouldn't an excess current be called a short? --- End quote --- I guess you found out the hard way why it is called “hot wiring” a vehicle. FYI a “short” is basically any unwanted return to source. In your case the wire you were holding made a circuit from the +12VDC source (basically the battery or unfused path to the battery + terminal) to the engine block or chassis which are very well bonded to the – terminal of the battery. A good car battery is easily capable of producing over 1000 amps at about 10 volts for a short time. That is 10 kilowatts available but as your wire that is the weak part of the circuit heats up, its resistance goes up significantly which probably saved you from being burned a lot worse and / or welding something you didn’t want welded. In the Camaro case the current wasn’t excess, it was just available from the source. Your wire was the same as a filament in a headlamp as far as the battery was concerned; it was connected between the battery poles and the current took the path of least resistance in the newly made circuit. The wire you used did have resistance, all wire does. If you had used a really big wire, you could have blown up the battery. --- Quote ---Also, how come our neutrals and grounds in the house are tied together? Why can we not have it like the transformers on the poles where neutral is seperate from ground? Actually, I probably could, couldn't I? Outside, there's a grounding wire that's buried in the earth that hooks to the panel. If I separated the ground and neutral buss-bars in the breaker panel, and made sure my ground buss-bar went to the grounding wire outside, and the neutral went back to the pole, I'd essentially have the same setup as the pole. Someone in this thread mentioned in their house they'd never have the ground tied to the neutral. But in every home I've seen, this is how it's always setup. Is there any dangers to doing what I just described? --- End quote --- Neutrals and grounds are tied together for safety. I think we have already discussed this. In many cases water lines go through the ground, houses are built on slabs of concrete that when damp are pretty good conductors in terms of life safety. If the neutral were not grounded, and wire does have some resistance as do wiring connections, there can be potential on the neutral conductor. Grounding the neutral mitigates the possible severity of a person coming into contact with a neutral while grounded. The transformer on the pole for residential services in the USA almost always has the secondary neutral bonded to earth ground as near to the transformer as possible. It is usually a fairly small (#8) bare wire that goes down the pole to the bottom. The bare wire on the 3 wire drop from the pole to your house is the neutral conductor. WHY on earth would you want to separate the ground form neutral in your house? It would be a clear violation of the code. It could put you and your family at a greater risk of shock. The National Electric Code is revised every 3 years, is based on a lot of history of mishaps and contains the best reasonable ways to mitigate problems. Look up ebay item 401538337928 it is a 2008 copy of the official NEC handbook for $18. Just buy it, read through it and keep it for future reference. The hardcover handbook has a lot of the code explained in real person English Grounding and specifically proper grounding is a huge field in itself; there are power engineers that have made a career out of nothing but grounding. --- End quote --- Thank you for your explanation but in all honesty, I was so tired from lack of sleep last night, I don't even remember writing the post. I had some stuff to finish and I think when I called it a night, it was over 36 hours of me being awake, it was around 0300 but after getting into bed, I wasn't able to actually fall asleep until 0600. For the Official NEC, you don't need to waste your money on a copy of the book. You can do what I do, just go to https://www.nfpa.org/NEC and click on Free online access to the NEC® and other electrical standards. Then you're not getting the outdated standards, it also lists the changes from the previous edition to the latest (the latest is 2017 I believe), and best of all, it's totally free. You just need to sign up for an account. There are options to download a digital copy (for money) or purchase an actual physical copy (for money), but you don't need to do that to get access. You can browse the entire 2017 NEC for free using that site. When I question something (for example, what wire type do I need in the basement, can I drill holes through the first floor floor joists and run the wire or should I staple it, do I need to use the plastic staples, etc), I just head there, login, and check it up. It's extremely handy. After reading what I wrote, I think maybe what I was thinking was having an isolated ground from neutral. I dunno why in earth I would want that. I do have a need for an isolation transformer for a couple DUTs, so maybe that's where it came from? I dunno. Usually, I try to be in bed by 2100 ~ 2200, but sometimes, there's deadlines I have to meet, and I run behind so I have to stay up longer than I like. I know I could have slept a lot longer today but I had my wife wake me up at 1100 so I could get some work done. Anyway, thank you, and everyone else, for helping me understand everything properly now! I've sent the email to my Account Executive to see where we stand, but because of the price tag actually being more for the North American version (with providing almost half of the version I currently have), I might just still buy the transformer. As IanB said though, that might throw off the inspector. So I'll first see what my Account Executive says and on Monday, while I'm waiting for a response, I'll call the local code enforcer. I got his cell number but I don't want to disturb him on the weekend. We're not buddy buddy kinda thing, you know? I'll see if he'd recommend against the 240:240 transformer. Little bit a sleep and it suddenly makes perfect sense how to wire it up and everything. I don't know why I couldn't grasp it the other day. Thanks though. |
| Spork Schivago:
--- Quote from: james_s on May 19, 2018, 10:39:02 pm ---I do like the UK term "earth" for referring to what we typically refer to as "ground". You could ignore the fact that it's AC all together and think of this in terms of DC. If you take a pair of D batteries and connect them end to end you'll have 3V from one end to another. Now pretend that they're concealed and you can't see the + and - markings, that's fine because that information is irrelevant since your hypothetical load doesn't care about polarity. Now take a wire connected to the joint in the middle between the two batteries and attach that to a stake in the ground, now you can call that point ground or earth of you like. Now between the two remaining wires you'll still have 3V, but if you measure between either one and ground you'll see 1.5V. If you connect either one to ground you'll have a short. In DC you'd call it +1.5V and -1.5V but what really matters is the relative potential between two points and since the polarity is constantly changing with AC it's simpler to refer to it as phase. I don't know how it can get much simpler than this. --- End quote --- Yeah, I don't know how it could get simpler either or why your posting that. I definitely understand it now. Last night, I just didn't have any sleep, so maybe that was why I had a hard time grasping it? I know my wife has mentioned some problems to the docs about my memory. She goes to the neurologist with me and a few other appointments. She says there's times that people that I know real well seem to disappear from my memory. It's just not people, but time as well. Sometimes, I guess I lose a year or two or can't remember my birthday. But what's odd, I can't ever remember not being able to remember stuff. So, it's like some memories are isolated from others and when I cannot remember my best friend, for example, I might remember something that I can't remember now, but when I can remember my best friend, that something is gone and cannot be reached! They're having me do these exercises, brain ones, that's supposed to help my brain find new pathways. In a way, the brain is much like the circuit boards we work on I guess, and what do you do when you have a burned out trace? You can run a jumper wire or essentially, find another way for the electrons to flow. I guess the brain works similarly. If someone has a stroke, they have to sometimes "reteach" their mind how do things that it used to know, like walk. My issue isn't so much with walking. The damage I sustained affects the prefrontal cortex and for some reason, there are holes there, or legions. What is bad, they're spreading and I have to go to some two week hospital thing where they monitor me and try to stress me out to try and figure out why they're spreading. I have the global volume loss of T2 white matter (something along those lines), where she says I'm losing brain function. Anyway, exercising the brain is supposed to help and my wife is supposed to "quiz" me every day. What did you eat for breakfast? What day is it? What TV show were we just watching? What day is it? What did you eat for breakfast? What did you eat for lunch? Stuff like that. |
| Spork Schivago:
--- Quote from: james_s on May 19, 2018, 10:41:06 pm --- --- Quote from: Spork Schivago on May 19, 2018, 10:35:07 pm ---Ah shit. We might actually have to go for the transformer still. The P9S16A's are actually cheaper than the P9S13A's. We would actually be spending more money purchasing just one, let alone two. Cost effectively, I think it'd be a hell of a lot cheaper to just wire up the transformer. --- End quote --- Remember that a transformer that size will be expensive and it will burn up a non-trivial amount of power just sitting there. I'm not familiar with the specific PDU's you're dealing with but I would be tempted to get a bunch of rack mount power strips and plug them into a row of standard 120V receptacles. You could easily install several dedicated 20A 120V circuits. --- End quote --- Transformer is actually cheaper than the PDU I believe, but I haven't gotten the quote, but you do bring up a very important part that I keep on thinking about. How efficient will this transformer be? Someone I think referred to it as a hot piece of metal hanging on the wall. To me, that means it won't be very efficient at all. In the end, maybe the separate North American P9S13A's would be financially cheaper than the transformers power loss that shows up on the electric bill...In the long run, the P9S13A's are probably the less expensive way to go. And the best part, I'd get the redundancy which is something we need eventually. If I can get that now, that's one less thing to worry about later on. |
| IanB:
--- Quote from: IanB on May 19, 2018, 10:57:49 pm ---So you would need to provide a 40 amp circuit using the recommended 8-gauge wire to a 40 amp breaker in the panel. --- End quote --- And you would be running your whole system off one 40 amp circuit, with no redundancy. If that breaker tripped for any reason then everything would shut down. |
| Spork Schivago:
--- Quote from: IanB on May 19, 2018, 10:57:49 pm --- --- Quote from: Spork Schivago on May 19, 2018, 10:35:07 pm ---Ah shit. We might actually have to go for the transformer still. The P9S16A's are actually cheaper than the P9S13A's. We would actually be spending more money purchasing just one, let alone two. Cost effectively, I think it'd be a hell of a lot cheaper to just wire up the transformer. --- End quote --- Remember that the transformer suggested earlier by Richard Crowley is 7.5 kVA. If that were to be fully loaded that would draw 7500 VA / 240 V = 31.25 A. Multiply that by 1.25 to get a continuous load rating (your server farm is a continuous load) and you get 39 A. So you would need to provide a 40 amp circuit using the recommended 8-gauge wire to a 40 amp breaker in the panel. You would not be able to run the transformer off an L6-30R receptacle, you would have to wire it in permanently. Then you would have to wire up the secondary side of the transformer to your PDU. You probably don't want to just put a fly-lead on the secondary with an IEC receptacle on it, so you will need to source an appropriate commercial use breaker panel that supports 240 V line to neutral with single pole breakers, and then install wall mounted IEC receptacle(s) to plug your PDU into. And then you have to get it to pass inspection. If you cost all that out, I think it might be cheaper just to buy the two new PDUs. --- End quote --- Yes, I've actually already thought about all of this and talked to the wife and said if we can send it back, I think we're going to send it back. I told her we'd need the second panel, the wires for it, the breakers for it, we'd still need the receptacle, we'd still have to pay for inspection (which very well could fail, and then what? Stuck with the PDU and still gotta buy two of the smaller units!) Hopefully, they'll accept the return based on that's what HPE said I _needed_ and couldn't power it off my current panel, although, I was stupid, and didn't word it properly like I should have. I called it 120-0-120 split-phase, which I shouldn't have done. That's how I've been referring to it the whole time, even when they did the on-line power analysis. That's my fault. They probably didn't even know what the hell it was and just said in their minds, say what? Hell no! |
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