Measuring current at breaker box confusion - Page 1
Forum section: electronics / beginners

author: Fried Chicken
author_rank: Regular Contributor
timestamp: October 25, 2024, 10:08:33 pm
content: The electricity bill in the home I'm in has always been exceedingly high: higher than what I feel is right.Today I installed two GFCI breakers for the outlets underneath the sink. It's my understanding that ground fault protection wasn't required for these circuits at the time as they are each on a dedicated breaker. Regardless, I didn't like the idea, and installing a GFCI outlet wouldn't help if the extensive piping under the sink caused a leak. This I managed no problem.While I was in the breaker box, I decided to start doing an energy audit on all the different breakers with my clamp meter. Just a single HVAC blower draws 6 amps! The confusion (both in theory and in practice) started when looking at the 240 volt outlets (across two poles of the transformer), powering a number of devices, including the HVAC and the pool pump. Single pole breakers were easy, attach the clamp meter on the hot line of the breaker, but how do I measure consumption across the double pole breakers? Do I go over both wires? Do I go over the neutral wire? Do I go over each individually and average all three? Do I go over each and subtract the neutral? The same when doing the big mega wires coming directly from the transformer, and the combined neutral. How do I get meaningful data from this?

author: wasedadoc
author_rank: Super Contributor
timestamp: October 25, 2024, 10:15:30 pm
content: If a device is connected across a pair of out-of-phase hots, you just measure the current in one of the hots. Either (extremely carefully) physically insert an ammeter in series with one or put a clamp meter aroundoneof the hot wires.

author: themadhippy
author_rank: Super Contributor
timestamp: October 25, 2024, 10:18:46 pm
content: Quotejust one of the 2 hot wires,if you put both in the clamp youll get 0 as there 180 degrees apart so cancel each other,same goes with a hot and neutral.

author: TimFox
author_rank: Super Contributor
timestamp: October 25, 2024, 10:19:19 pm
content: And double check: you should get the same current measurement when clamping around each of the two lines from the companion-trip two-pole breaker.If not, you may have a wiring issue or short to ground or neutral somewhere.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: October 25, 2024, 10:30:50 pm
content: That's a lot easier than I thought.I was NOT getting the same measurement across the hot wires, but that was as the pool pump motor had just started so it might have been doing wonky stuff.What about the neutral? Does that contain any valuable information? The double pole I was measuring goes to an additional breaker box on a separate part of the house. Might that explain a discrepancy?For an overall house audit, simply measure from the big giant wires at the top? That doesn't quite make sense since the double breakers are on the same hot line.

author: IanB
author_rank: Super Contributor
timestamp: October 25, 2024, 10:32:17 pm
content: Sometimes a 240 V appliance may take 120 V from one of the hots and the neutral to power things like control boards or lamps.If you want to compute total power in a 240 V circuit, you can do this calculation:Total power = (Hot 1 current x 120 V) + (Hot 2 current x 120 V)You can replace 120 V with the actual voltage if you care to measure it also, but for a rough estimate and for increased safety, you can just assume it is 120 V. That will get you in the ball park.

author: IanB
author_rank: Super Contributor
timestamp: October 25, 2024, 10:34:39 pm
content: Quote from: Fried Chicken on October 25, 2024, 10:30:50 pmYes, do the same calculation as above with the measured current on the two incoming supply wires.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: October 25, 2024, 10:38:03 pm
content: Y'all are so helpful it warms my heart. Genuinely thank you so much.I'm still confused how the double pole breakers work, since they are on a single bus bar?Additionally, measuring at the large supply lines for the bus I get a higher reading than the sum of the individual breakers on that bus.

author: bdunham7
author_rank: Super Contributor
timestamp: October 25, 2024, 10:41:18 pm
content: Quote from: Fried Chicken on October 25, 2024, 10:30:50 pmFor most 240V loads there should not be any current on the neutral wire if there even is one going to the outlet or device. There are exceptions for things like electric ranges that can use a lot of current at 120V and things like furnaces that may have a small control load or inducer blower or something like that at the lower voltage. The current on each hot wire should be of the same magnitude and any difference should show up as a neutral current (not necessarily exactly the same as the difference), if not that difference represents a measurement error.Circuits that use a two-pole breaker to feed two separate 120V circuits using a common neutral won't be balanced of course, so you'll see differences between how wires and you'll likely have a neutral current on those.

author: bdunham7
author_rank: Super Contributor
timestamp: October 25, 2024, 10:43:25 pm
content: Quote from: Fried Chicken on October 25, 2024, 10:38:03 pmThey aren't on the same bus bar, you'd have to look at the geometry of a breaker panel with the breakers out to see how they do that.

author: xrunner
author_rank: Super Contributor
timestamp: October 25, 2024, 10:51:30 pm
content: Quote from: Fried Chicken on October 25, 2024, 10:08:33 pmI installed a Micro Air Easy Start on my outside Lennox system this spring which I can connect to via Bluetooth and monitor the current the compressor is using. Right now it's using 6.9 A. You say your inside HVAC blower is using 7 A? Yes that does seem very high.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 25, 2024, 10:56:51 pm
content: Quote from: Fried Chicken on October 25, 2024, 10:38:03 pmThey are on separate bus bars. In a typical US residential breaker panel the slots alternate between the two phases. That way double pole breakers always contact two phases. If you search you should be able to find photos of the inside of a breaker panel with the cover removed.

author: calzap
author_rank: Frequent Contributor
timestamp: October 25, 2024, 11:56:09 pm
content: I always consider clamp-on ammeters as giving approximate readings, especially if they are used in a noisy AC environment. They are likely calibrated with a single conductor of specific diameter going straight through the middle with no other active AC conductors nearby … not the usual case for field use. Their main advantages are they’re quick, safer than series measurement, and current flow does not need to be interrupted. Direct series measurement should be more accurate but is a hassle for building wiring.Mike

author: IanB
author_rank: Super Contributor
timestamp: October 26, 2024, 12:47:49 am
content: I think you can get an idea of the uncertainty of a clamp-on ammeter reading by moving the clamp around, different angles, different positions around the wire, and see how much the reading varies. It's been my experience that for AC readings, it doesn't vary that much.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 26, 2024, 12:57:25 am
content: Quote from: calzap on October 25, 2024, 11:56:09 pmA "hassle"? I'd say that's quite the understatement; try damn near impossible, at least in practical terms.I mean, you're gonna somehow (safely) wire in a humongous shunt resistor, turn the power back on and measure the voltage drop, then remove the sucker?Do people actually do this? I was an (unlicensed but very very careful) handyman-electrician, and never heard of anyone measuring current this way.Or can you get breakers that have a shunt built in for this purpose?

author: Fried Chicken
author_rank: Regular Contributor
timestamp: October 26, 2024, 01:22:03 am
content: Quote from: xrunner on October 25, 2024, 10:51:30 pmI'm starting to suspect the high current usage comes from the AC always starting up and shutting down, and the power company doing something wonky. Before I call them, I'm going to keep interrogating.I have a legacy pool pump motor but that doesn't account for the ludicrouso bill. 7 amps on the INSIDE blower seems really high to me, but still I Think something funky is going on.

author: calzap
author_rank: Frequent Contributor
timestamp: October 26, 2024, 04:39:13 pm
content: Quote from: Analog Kid on October 26, 2024, 12:57:25 amThe shunt resistor or coil can be inside an ammeter with the appropriate capacity which is inserted in series with the breaker and conductor. But it’s not a big deal to enclose a shunt resistor in heat shrink with stubs sticking out and use that. Turn off the breaker; if at all possible, power down the bus; disconnect the conductor from the breaker; put a stub conductor in the breaker; clip ammeter leads to the stub and original conductor; turn on breaker; power up bus; read amps.I’ve done this conductor to conductor in junction boxes with a regular multimeter when a clamp-on wasn’t handy or when trying to identify a particular circuit. This can be a technique for identifying a particular conduction path when there are a bunch of conductors in a box. Only one has to be active to set-off an inductive detector.For building wiring, a clamp-on ammeter works well enough most of the time.Mike

author: Gregg
author_rank: Super Contributor
timestamp: October 26, 2024, 08:09:13 pm
content: To make sure you are getting the best reading with your clamp meter, make sure the jaws are fully closed; I see from a couple of your pictures they are not. If you can isolate the wire you are testing from the others, the readings will be more consistent. I suggest a gloved hand between the clamped wire and the rest of the conductors if possible; use a genuine leather glove. Your body is a sufficient shield.What type of fan / blower is using 7 amps? Is it a large blower for the whole house? What is the fan motor rating? Maybe the bearings need lubricating. Or does the breaker feed the entire AC system including the outdoor unit with the compressor? It looks like the breaker where you are measuring 10 amps is a 40 or 50 amp breaker by the wire size, it probably was sized properly when installed (meaning the measured 10 amps isn’t outrageous). There is also a neutral and possibly a ground conductor going to the same conduit; that would explain the 0.20 amp difference.Fan curves are a fascinating study. Depending on the type of fan, slowing the fan speed by something like 5% can cause a drop in power requirements by 15 to 20% and many manufacturers are running the fans for maximum volume to meet arbitrary numbers for marketing purposes plus the manufacturer doesn’t know the pressure differential for your particular house. If you have a belt drive fan, you might try slowing it down a little.While you have the breaker panel open, you might want to make sure all of the connections are tight.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 26, 2024, 08:12:11 pm
content: So I'm curious: what size (both resistance and power rating) would that shunt resistor be?I started making an educated guess but realized I don't know how to figure this out:The only quantity we know here is Vtotal. How do we calculate Itotaland therefore Vshuntand Rshunt? Seems to me there are too many unknowns here. (And then we need to calculate the power dissipation of the shunt.)(Probly going to feel pretty stupid when answer is simpler than I thought ...)

author: TimFox
author_rank: Super Contributor
timestamp: October 26, 2024, 08:23:35 pm
content: Ideally, you want the voltage across the shunt to be high enough to measure accurately, so maybe choose 150 mV rms if your most sensitive AC voltage scale on your DMM is 200 mV rms full-scale.Therefore, if you expect 10 A rms, a reasonable resistor would be 150 mV / 10 A = 15 m\$\Omega\$, which would dissipate 1.5 W mean power.Such "shunt" resistors are best built as 4-terminal resistors, for obvious reasons.If you abhor arithmetic, choose 10 m\$\Omega\$.Seehttps://uk.rs-online.com/web/content/discovery/ideas-and-advice/shunt-resistors-guidefor commercial examples.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 26, 2024, 08:27:21 pm
content: Thanks. In the meantime I figured out that the way to go was to make some initial assumptions like you did, then work out the values from there. Makes total sense now. (As in sense resistor ...)

author: IanB
author_rank: Super Contributor
timestamp: October 26, 2024, 08:47:41 pm
content: If you are really interested in where your power is going, you would be better to make cumulative readings over a whole day. This would be of much more benefit than trying to make more accurate readings with a sense resistor.Depending on your skills and interest, the kind of thing you could do is to use a Raspberry Pi as a data logger, and use current transformers around various line conductors to measure the current. Your Pi can then just sit in the background logging for a long period, and you can analyze the data afterwards. Depending on how many CTs you hook up, you can log more than one thing in parallel. Or with only one CT, you can log each suspicious circuit one after the other on different days.Or, you could log total incoming current on the feed to the panel, and they try to correlate current peaks with what devices are operating in the house at the time. At a minimum, you could do a rough check with the power meter, and make sure they are in approximate agreement.The reason for this is that instantaneous power has little meaning, in and of itself. What counts is accumulated power consumption over a long period.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: October 27, 2024, 05:02:34 am
content: Quote from: Gregg on October 26, 2024, 08:09:13 pmI just noticed that, but I made sure when doing my readings for the most part the clamp meter was closed.I really wish I had a competent AC person that could look at and really dial in my HVAC setup. I would pay good money for a proper in-depth tune up, including pulling a vacuum for a full 24 hours, getting the pressures exactly right, dialing in the fan speeds, or lubricating bearings (This never even occurred to me).Nobody does that kind of work here, but it could easily save hundreds over the course of a year.I think I might slow the fan speeds down.Quote from: IanB on October 26, 2024, 08:47:41 pmThis doesn't make sense to me.I have some big and consistent draw devices (HVAC, pool pump, fridges), so unless something really weird is going on, the instantaneous reading should suffice, no?To people suggesting I read current directly: no way, no sir, no how. What you're suggesting is dangerous and stupid; Please be less cavalier about making dangerous suggestions like that. I don't need 6-digits of accuracy here, +/- 2 amps will suffice.

author: IanB
author_rank: Super Contributor
timestamp: October 27, 2024, 06:37:45 am
content: Quote from: Fried Chicken on October 27, 2024, 05:02:34 amNo. There are two important pieces of information, the power draw, and the time it is running.Unless you are prepared to sit down next to each device with a pen and paper for 24 hours, and write down the time it switches on, and the time it switches off, how will you know the energy usage in kWh?The most common reason for a higher-than-expected energy bill is a device running for longer than it is supposed to, not a device pulling more current than it is supposed to.

author: J-R
author_rank: Super Contributor
timestamp: October 27, 2024, 06:53:31 am
content: An AC clamp meter is a good solution here and accuracy will be more than adequate for measuring current at that moment. Orientation really doesn't matter, only that what you want to measure is inside the clamp vs. outside.Equipment that cycles will require logging or some educated guessing, as mentioned.Here is a graph of one of my refrigerators, and you can even see the defrost cycle.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: November 05, 2024, 08:06:13 pm
content: Quote from: IanB on October 26, 2024, 08:47:41 pmI initially dismissed this idea, but I'm turning around on the idea.I'm not inclined to build a custom logging setup, but I will see if there's a device that does it.So far I've found an Exatech D160

author: Fried Chicken
author_rank: Regular Contributor
timestamp: November 11, 2024, 05:46:25 pm
content: I got a kill-a-watt lolWhen that gets in I can test the appliances directly

author: Lstkartagi
author_rank: Contributor
timestamp: November 11, 2024, 06:46:27 pm
content: You might consider a whole home energy monitor, something like the Sense system. You just connect the two voltage wires (L1, L2) and clip on two current CTs. Their claim to fame is automatic identification of individual appliances, which takes some time and is not always that great. However, much over looked is the real-time power graph which is excellent, both on the web and phone interface. It is easy to turn something on then off and see the change. Sense.com. Occasionally they go on sale at Amazon.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: November 11, 2024, 11:25:41 pm
content: Quote from: Lstkartagi on November 11, 2024, 06:46:27 pmWOW.I just checked sense, and I know the utility recently installed a smart energy monitor. This might legit be exactly what I'm looking for!Thank you! Thank you for bringing this up, I had no idea!

author: Fried Chicken
author_rank: Regular Contributor
timestamp: November 14, 2024, 08:13:33 pm
content: So the pool motor indeed draws 10 amps or 2,400 watts. It's rated for 2 hp, so according tothis siteit's almost under full load at 10 amps.It might be smarter and worth considering putting a smaller motor in, lowering the flow on the pipes, and decreasing the current by more than just the rating would suggest, no?

author: IanB
author_rank: Super Contributor
timestamp: November 14, 2024, 08:19:29 pm
content: Quote from: Fried Chicken on November 14, 2024, 08:13:33 pmThe power consumption of a pump goes approximately as the cube of its speed. So it could be economical if you could slow it down, but it would have to run longer to move the same amount of water through the filters. Or a smaller pump, as you say, but it might still have to run longer.

author: jmelson
author_rank: Super Contributor
timestamp: November 15, 2024, 01:43:54 am
content: Quote from: xrunner on October 25, 2024, 10:51:30 pmNote that RMS amps X RMS Volts does not take power factor into consideration. Induction motors draw nearly full load amps at any partial load condition. As the load increases, the current phase angle becomes more in line with the voltage, therefore power factor increases. At light load, the current is nearly 90 degrees lagging, so power factor is very low. A clamp-on ammeter misses this important part of the information. Your (residential) electric meter charges you for real power drawn.Jon

author: macboy
author_rank: Super Contributor
timestamp: November 15, 2024, 02:57:34 am
content: Quote from: IanB on November 14, 2024, 08:19:29 pmThis is very true. I run my pump at a low flow rate 24/7. It consumes less than 100 W but cycles the entire volume of the pool through the filter a few times per day. I also had 2 inch piping installed when I put in the pool, instead of 1.5 inch. This makes a difference by reducing back pressure on the pump.You also mentioned the furnace blower. Old AC furnace blower motors are much less efficient than modern variable speed DC ones. If the existing furnace is old enough, you could possibly justify replacement based on both fuel and electricity savings.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: November 16, 2024, 02:26:50 pm
content: Quote from: IanB on November 14, 2024, 08:19:29 pmI understand; Asynchronous motor speed is determined by the AC frequency isn't it.... but then in relation to the following:Quote from: jmelson on November 15, 2024, 01:43:54 amRemind me the relationship between power factor, current draw, and real power?So the tricky part here then becomes choosing the right pool pump motor, or possibly going with one of the newer expensive and unreliable motors, doesn't it

author: jmelson
author_rank: Super Contributor
timestamp: November 16, 2024, 04:47:05 pm
content: Quote from: Fried Chicken on November 16, 2024, 02:26:50 pmWell, power factor is expressed in a percent notation. Assuming a linear load, there is a phase shift between current and voltage. If the load is inductive, then the current lags the voltage, if capacitive, the current leads the voltage. There are tables that tell the power factor from phase angle, the calculation involves trigonometry and calculus. The Kill-a-Watt type device does it by measuring instantaneous voltage and current and multiplying them several hundred times a second, thus also making a correct reading for NON-linear loads.You cannot measure real power with a voltmeter and ammeter. A dial and pointer type wattmeter has a moving coil and a fixed coil. The magnetic attraction between these does the real-time multipliication.JonJon

author: themadhippy
author_rank: Super Contributor
timestamp: November 16, 2024, 04:59:07 pm
content: Quotenever heard anyone referring to the power factor as x%, 0.7 lagging or 0.9 leading yep,but a percentage?

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2024, 06:33:38 pm
content: A technical note from ABB:https://library.e.abb.com/public/aad7906d69854d018d2790e84a8aa9b5/Technical_Note_174_+IntroductionToPowerFactor.pdf?x-sign=JFVS3Ur%2FWEyJewcen4Ml%2F9dVXzsc2T4DbXyqtHPc3cfkctUU0WEF49gzDcrEb4WrSince the value of PF is a trigonometric cosine that varies between -1 and +1, a percentage is a reasonable way to express it.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 16, 2024, 10:15:02 pm
content: Quote from: themadhippy on November 16, 2024, 04:59:07 pmWhy not? Power factor is a value between zero and one, so why couldn't that simply be multiplied by 100 and expressed as %?

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2024, 10:23:35 pm
content: Technically, PF is between -1 and +1, depending on lagging or leading phase angle.