I'm looking at a device to monitor my power. It hooks around the main power feed wires that feed the breaker panel. It has a sensor for each of the two main wires (240 volt standard setup). I measured the amps separately for each feed wire with a clamp around meter and get what seems like the total draw on each wire. So for the "left" wire I get around 80 amps. For the "right wire" I get around 80 amps. 80 Amps is just about what I would expect to be getting based on what's running. If there is really 80 amps on each wire that would be a total of 160 amps which I cannot imagine I could possibly be using. It's a 200 amp service.. I'd have to have literally everything in the house on, stove, oven, water heater, both AC's, toasters, coffee maker, etc and I don't have all that stuff on. So why does the monitor need to measure on both of the wires? It looks like either one alone gives the amp amount. The only thing I can think of that might explain it is all teh big stuff taht was running was 240 volt stuff, the only 120 stuff was lights. Is it possible there was 82 amps on one side and 77 on the other and the large 240 draw masks the single side draw?
Amp draw measurment on main power feed residential
Jul 03, 2020
32 Replies
No that is really 80 amps. If the ungrounded legs are really that balanced, neutral current will be around zero so you are seeing about
19.2kva (roughly the same as watts that you get billed for). That is still pretty high tho. What does the PoCo meter say you are pulling?
Yeah, that'd be like two 4-ton central units at the same time...we've got two, 1@2.5,1@2 and they're <50 A combined.
Great! Though, it doesn't completely have to hook around to get the same reading. Does it say 'ammeter'? It probably does and those are called 'clamps' around the two cables (probably called '500 wire' 2 phase).
Which you just told us.
Called an 'ammeter', which you just described to us.
Of one phase (out of three phases)
Of another phase out of the three phases.
No. You can, but you don't 'have' to. Each device when on, draws only as much as it needs.
So that such a reading can be taken if need be.
Yes, but at three different volt amp phases in the AC power circuit.
Just two phases of 120v each.
Without starting the argument over about what is single phase vs 3 phase, there are only 2 ungrounded conductors (hot wires if you like) and a grounded conductor AKA neutral. if you have a higher current on one ungrounded conductor than the other, the difference should show up on the neutral although in reality some will leak out of the ground electrode system. The meter is designed to sum these out and charge you for your total usage.
Not another fidhing expedition - - - - \
The 200 amp sevice is 200 amps 240volts - so 200 amps per leg - or 400 amps at 120 volts as long as the loads are balanced. An engineer (of any sort) should be able to come up with that answer with a single google search.
Not getting drawn in any farther. (hopefully)
No, get educated
No - get educated
Crusher doesn't have 3 phase (or even 2 phase) power
No, one center tapped single phase
It's 80A at 240V or 160A at 120V, depending on what the loads are. They would only need to monitor one hot if all the loads were 240V or if there were also 120V loads and they were perfectly balanced, eg 30A on one leg, 30A on the other. If not, then the power is
240V * (min of the amps in the two hots) + 120V * (the difference between the two). That's without the complication of power factor.
Agree. This isn't a three phase service.
[snip]
There is no actual 400A there. The SAME 200A flows through each leg (with balanced loads).
Correct, but it will support 400A worth of 120V loads if they are balanced so half is on each hot leg. If it's totally unbalanced, then you can only support 200A on one leg. If it's somewhere in between, which is the usual case, then you can support somewhere between 200A and 400A of 120V loads.
It is a lot easier to understand if you just say 48kva max and you can split that between 120 and 240v loads. The ungrounded conductors are still limited to 200a and usually more than that, for a while, depending on the trip curve of the main breaker.
[snip]
Yes.
You can assume so, because you aren't actually there. Crusher is. Residential service at the two cable 500 wire cable interface could be AB, BC or AC two-phase 240v. It depends upon how the utility classifies it.
It is single phase so there is no A-anything. You only have a 240v transformer winding that is grounded on the center tap. "Two phase" is pretty much obsolete in the US and probably anywhere in the world. For those last few customers in Philadelphia and maybe somewhere in New York who won't get rid of their 100 year old 2 phase motors, the utility can do a transformer trick to make it from the 3 phase medium voltage distribution.
Crusher is generally / usually only about half there. Residential power anywhere in North America except in some multi unit residential buildings is center tapped single phase. Easyto tell by checking voltage. In a MURB with 3 phase power you will have something like 115/208 while anywhere else you will have something like 120/240 or 115 /230. This is NOT 2 phase - it is center tapped single phase 3 wire plus ground - shared neutral
It's very reasonable to assume that residential service in the US isn't three phase, because it's extremely rare. I've never encountered it. I've never seen a poster here that had it and if they did, they made that clear. But on top of that, Ashton clearly described a 240V/120V split-phase residential service in his question and that sure isn't 3 phases. I'm sure 3 phase residential exists in rare cases, for example some home that has a large shop as part of it that requires three phase. And the service into the house isn't two cables either, it's generally a single cable using three conductors. None of this has anything to do with his questions.
I've encountered it on a military base in the housing section.
Of course the base had 3 phase power for the other buildings but housing you probably wouldn't expect, and not all of housing had it.
But at one time AC units were replaced, and somebody before my time used three phase air conditioners (heating was gas). At least that's what I think happened.
The impact on the average occupant was that their 120 was fine, but where they expected 240 for a dryer or a range they got 208 instead.
That's why you have to actually be there maybe with the prints too rather than to just assume.
I wouldn't have mentioned it except it might be of interest because it is vanishingly rare in the US. We lived in Germany and I think it did exist there but I didn't work on that.
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