Estimating KWh electicity billing using clamp-on amp meter

[hubops]

Can you take your 2-of-3 phase and connect directly to a heat element ? like the home 230 volt oven or clothes dryer ? John T.

[gfretwell]

There is single phase, 2 phase (that I doubt anyone here has ever seen) and 3 phase. If you only use 2 legs of a 3 phase circuit, it is single phase. Where you ground that does not change the number of phases. In fact 3 phase center tapped delta is just your regular single phase center tapped transformer connected to a delta primary with another transformer (or two) providing the 3d phase. OTOH if you corner grounded that delta you would have 2 (hot) ungrounded conductors and a neutral with 2 pole breakers but it is clearly still 3 phase. Two phase is a totally different breed of cat with 2 phases (4 wires)

90 degrees out of phase. This will usually be 2 transformers center tapped with the center taps grounded (5 wire). Split phase is a type of motor winding, perhaps the most common for AC compressors and other applications that use a start capacitor and no run capacitor. The "phase" is "split" by a start winding 90 degrees out of phase with the run winding.

[gfretwell]

Sure. If this is 3p Wye you will only have 208v to work with so the watts will be less. If it is 3p delta it will still be 240v.

[mike]

snipped pissing contest...

I'd be interested in learning how to estimate electricity billing using a clamp-on amp meter.

Exactly where do you clamp it?

Clamp meters measure "something" and calculate displayed current based on varying assumptions which may not be specified and may be only loosely related to what the utility bills for.

My house doesn't have exposed single conductors in a space big enough to get a clamp meter.

FWIW, I use a Blue Line unit that reads the blinking LED on the power meter. That should be EXACTLY what they're billing for.

If you only need average readings, just read the numbers off the utility meter.

You can also do some fun things with a Palm III that reads the blinking light and graphs it over time.

Would be interesting to learn how to do it with a garden-variety clamp meter with unknown transfer function for loads with non-unity power factor and two load branches with different loads.

[Ralph Mowery]

You can not do that with simple meters.

I have one meter that reads volts, amps, watts. With resitive loads like light bulbs and heater elements without blowers the volts times amps give the same value as the watt scale. If I hook up something with very much inductance such as a motor or even a heater with a blower, the volts times amps are way off from the watt meter.

[mike]

My point exactly...

[danny burstein]

Eyup. And it'll almost even work.

I've done this many a time.

First, though, to use the "official" voltages:

The standard US voltage is 120VAC from one "hot leg" to neutral.

In the common situation where there are two hot legs that are 180 degrees apart, each one to neutral will be that 120VAC.

However, if you go "hot to hot", you'll get 240VAC.

But.. in many cases, like the one you asked about, where a customer gets "2 out of 3", if you go "hot to hot" you'll get the equivalent of 208V.

This is common in NYC (see below)

(There's a square root of 3 involved there).

And yes, you _can_ run your 240V stove or electric dryer on it. BUT the performance will be significantly lower.

At first glance you'd think that the actual wattage (and, in these cases, heat) would be 208/240 or about 0.86 (86 percent) of desired performance, meaning 14 percent longer for the oven to heat up, etc., but...

... but the reality is that it's 0.86 times 0.86, or about 0.75 (75 percent). ` So yeah, a noticable hit. Oh, and especially so if you're using "240V" incandescent lighting which will not only be dimmer but also different colors... [1]

Now where do we see this? In NYC, where I've got way too much experience, apartment and modest sized office buildings will have three phase coming in.

For the typical line of apartments, the first one will get legs "A", "B" and neutral (plus safety ground).

The next apartment will get legs "B" and "C".

The third will get "A" and "C".

Rinse, lather, repeat.

So each apartment will have outlets with 120VAC and... will have some that are effectively 208VAC

All three phases will run to the elevator motor room and other heavy loads.

Yes, there are ways to tap a transformer so that "hot" to "hot" in these will get you 240VAC. But my head hurts when thinking of them.

[1] ignoring the issues of how the resistance will change with different temperatures...

[gfretwell]

These days, with smart meter, you can get detailed usage information right from their web site.

[gfretwell]

It is not complicated, they just use delta instead of wye secondaries The flaw in that plan is you only get two 120v circuits out of your 3 phase service. The 3d (red/wild) leg will be 208 above ground but you can still use all three to run your 3 phase loads (at 240v line to line). This is fairly common in industrial bay installations where most of the load is single phase but they still have a significant 3 phase load to serve. The PoCo likes it because they can do it with 2 transformers.

Typically the red leg transformer will be smaller than the one feeding the single phase loads.

If you see 3 equal size transformers, it is usually 3p wye. (208)

[trader_4]

Of course, absolutely. It's electrically identical to split-phase, they are one and the same thing.

[trader_4]

You can't do it with a "garden variety" clamp on meter. You;d need one that can measure power factor, in which case you'd measure the current flowing in the circuit you're interested in, the voltage and the power factor between the two. For the latter two measurements the meter would have to not only be clamp-on but have direct connection inputs as well. If you can't get to the conductor, you can't measure it.

If it was a split-phase service as was being discussed, the power would be:

(highest voltage reading of two hots - neutral current)* 240V * Power Factor

+

neutral current*120V*Power Factor

I used 240/120 above, you'd of course us the actual voltages measured.

For plug-in loads the simplest way is to use a KillaWatt meter.

[trader_4]

.  Since they're so common, would you provide a link to one at homedep ot.com or lowes.com?

s the proper term should be dual phase or split phase. Does that work for y ou? I really didn't expect I'd be waking anyone up by responding to a 7 yea r old post.

You have to start with the electrical engineering definition of phase. It's simply the relationship between two periodic waveforms. That's how we got the term three phase power to begin with. It's based on the fact that the power is being generated as three separate waveforms which are separated by 120 degrees.

Here is a good discussion going back and forth:

I agree with Wbahn, the group moderator there, he's saying it's about seman tics. And note whenever anyone is challenged as to why it can't be called two pha se, it all falls back to arguments, like "but two phase was 90 deg phase differ ence and it doesn't exist anymore". When challenged along the lines of how you define phase, why if you change 90 to 180, which is just one of the many possibilities, it suddenly can't be called two phases anymore, they have no electrical engineering answer, no definition, it's just that they say it isn't. Change the phase angle from 90 to 95, would it still be two phases? Why when it reaches 180 is it no longer two phases? When it's at 185, would it be two phases again?

The argument Wbahn makes about a single shaft generator, instead of supplying 3 phase, make one that supplies 2 phases, at the 90 deg if you like. Two windings, differing by 90 deg, supplying two hots with a common neutral. Would that be two phases? If not, why not? And if yes, then why isn't it two phases if I change the phase angle to 180? If you agree that there are two phases still there, then that is indistinguishable from the split-phase service that enters your house. It's all about semantics and you have to start with the definition of phase. Phase is simply the relationship between two periodic waveforms.

Here, from MIT, an introduction to power systems, where they start out with the simple case of a power system with two phases. Note it's only about two voltage sources connected to two loads, with two voltage sources that can be at any phase angle. Nothing about where the voltage source originated from or what the phase an gle is. Nothing about magic happening at 180 degrees, where it's no longer a two phase system. It's the completely general electrical engineering case of what defines a two phase power system and how you analyze it.

ch3.pdf

Make those two voltage sources be the two sides of the transformer on the pole at your house and it;s the split-phase service that serves your house.

How you generate it doesn't matter. When this came up last time, I even gave you a presentation made by a power engineer at a power industry conference, about how split-phase service needs to be analyzed as a two phase system, because that is actually what it is. I believe it was also published, because I found it online. I don't think he got laughed out of the room, but at the very least it was a power industry source that said exactly what I'm saying. I just looked for it again, couldn't find it. I'll look again later.

[Ed's Electric]

No, professionals call it single-phase.  Unless you want people to think you voted for Hillary, you should use the proper terminology.

[trader_4]

  Since they're so common, would you provide a link to one at homedepo t.com or lowes.com?

the proper term should be dual phase or split phase. Does that work for yo u?

hink you voted for Hillary, you should use the proper terminology.

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Let me guess, you voted for Trump?

[Bubba]

Grab a dual-trace oscilloscope.

Connect the Scope Common lead to transformer L1

Connect Scope Trace 1 lead to the transformer center tap

Connect Scope Trace 2 lead to transformer L2

You'll observe two screen traces in perfect sync that vary only in amplitude.  Only a Hillary voter would mistake it for two-phase.

[trader_4]

ude.  Only a Hillary voter would mistake it for two-phase.

Did you read the link I posted to the discussion on that other forum?

The group moderator, Wbahn posted this:

Take two generators built on a single shaft (the same way that three-phase generators are built on a single shaft) so that you get two 120VAC outputs that are 180 degrees out of phase. Would it be reasonable to call this two- phase power?

Bubba, what's your answer?

Now run them to a house and tie them together so that you have 120VAC betw een the common and either of the other lines and 240VAC between the two lin es. Would it still be reasonable to call this two-phase power?

Bubba?

Could you tell the difference between that situation and the one in which t he wires coming to the house are from a center-tapped transformer?

Bubba?

If not, then wouldn't it be reasonable to use the same name for either? (that was Wbahns final question)

Bubba?

And what he described would still show exactly the same thing on the scope as the test you described. In all the above, if we changed the phase difference to 90, would it then be two phase? How about to 179 deg? In engineering we don't make it up case by case, we analyze the general case. 180 is just one special case, where it's easy to generate two voltage sources that are 180 deg out of phase with each other. Or go take a look at the MIT electrical engineering course in power systems where they show the mathematics of what they call two phase power. There are simply two voltage sources driving two loads. They do the analysis for a 90 deg phase difference, but you could just as well do the same analysis for a difference of 70 degrees or 180 degrees. The same analysis, the same math applies, because to the system, it is the same thing.

It's like Wbahn says, it's a matter of semantics and convention.

If you feel differently, then give us your definition of two phase,

3 phase, and N phase power and show us why split-phase is not also two phases? The only way I see it's not would be to rule that special case out by *definition*, which is back to the semantics. In that case, if you can site the electrical body, authority, organization that has done that, I'm sure we'd all like to see it.

[Bubba]

The transformer for my house has a single phase input.  The output of the transformer is single phase as well.  I don't have a two-phase generator on my utility pole.

[trader_4]

ase generators are built on a single shaft) so that you get two 120VAC outp uts that are 180 degrees out of phase. Would it be reasonable to call this two-phase power?

f the transformer is single phase as well.  I don't have a two-phase g enerator on my utility pole.

There you have it, typical. Instead of responding to the simple questions that go to the heart of the matter, just diversion. Others can note that I responded to your oscilloscope connection question. Note that I also gave you an opportunity to define for us what makes a system one phase, two phase, N phase and then explain why residential doesn't meet the definition of two phase. If you can't define it, then it becomes arbitrary. Which is kind of what I and the moderator at the forum said, it's just semantics.

[Sam E]

[snip]

That sounds like how you get 208V.

208V is the sum of two 120V that are 120deg out of phase.

240V is the sum of two 120V that are 180deg out of phase.

480V is the sum of two 277V that are 120deg out of phase.

[gfretwell]

It is single phase. There is only one secondary winding derived from a single phase primary. The fact that you center tap that winding and ground that tap does not magically add a phase. I suppose the unknowing can call it anything they want but professionals call your typical home service 120/240 single phase.

If I used your logic, a string of Christmas tree lights is "100 split phases" because I would see a separate little 1.2v wave form across each of the 100 bulbs on a scope and each one would be 180 degrees out of phase from the bulb on the other side of the common lead on the scope.