Single Phase 240 vac Current Measurement Explanation???

[Chip0628]

:blink:Trying to understand current measurement: At the inlet to a 240vac, 200a single-phase (3-wire) residential panel (measured at the overhead service drop), I measure 25 amps on one leg, 35 amps on the other leg and ten amps on the neutral. Can anybody provide a simple explanation of how much total current is being drawn?

I have read ten explanations on other forums and gotten ten different answers; add the legs together, add together & subtract the neutral, use just the highest measurement... Anybody know for sure??? Thx!

 

[Dennis Alwon]

Sounds like a bad transformer to me. I would have the power company deal with this if it is at the service drop. Test the voltage with the main disco off and if you get the same it is a power company issue

 

[Chip0628]

Not a Bad XFMR?

Not a Bad XFMR?

Sorry maybe I wasn't clear; the question was about interpreting my Amprobe readings at the panel. I assume the neutral current is normal for an unbalanced load condition and don't see how the transformer could be involved.

 

[GoldDigger]

Sounds like a bad transformer to me. I would have the power company deal with this if it is at the service drop. Test the voltage with the main disco off and if you get the same it is a power company issue

Are you assuming that this measurement was made with no load on the panel? That is not how I read it.
If the OP is just asking what "total current" would mean in this example, with an unbalanced load, my answer would be that total current is not well defined for a 120/240 or three phase situation.
Total kVA is almost cleanly defined, and would be (25 + 35) x 120 in this case.
Whenever different voltages or phases are available to drive a load, total current is ambiguous unless you also specify the voltage for that current. Hence the use of kVA instead.
The fact that you can get ten explanations with at least three different answers is a good sign that the term (not defined or used in the NEC) should not be used!

 

[n1ist]

Could be a 25A 240V load, and 10A of 110V all on one leg.
/mike

 

[Chip0628]

Thanks but still not clear - 240v Sngl Phase Current @ the Panel

Thanks but still not clear - 240v Sngl Phase Current @ the Panel

Are you assuming that this measurement was made with no load on the panel? That is not how I read it.
If the OP is just asking what "total current" would mean in this example, with an unbalanced load, my answer would be that total current is not well defined for a 120/240 or three phase situation.
Total kVA is almost cleanly defined, and would be (25 + 35) x 120 in this case.
Whenever different voltages or phases are available to drive a load, total current is ambiguous unless you also specify the voltage for that current. Hence the use of kVA instead.
The fact that you can get ten explanations with at least three different answers is a good sign that the term (not defined or used in the NEC) should not be used!

Thanks - I think I'm halfway there. Yes these are the measurements of the normal load conditions at the 200A Panel.

But if a panel is called a "200 Amp Panel" WHERE is that 200 Amps be measured? 100A on each leg? 200A on each leg? See?

 

[GoldDigger]

Thanks - I think I'm halfway there. Yes these are the measurements of the normal load conditions at the 200A Panel.

But if a panel is called a "200 Amp Panel" WHERE is that 200 Amps be measured? 100A on each leg? 200A on each leg? See?

Now you have asked a question with an answer. That 200A is the maximum current on any one hot leg, independent of what is going on on the other leg(s).
We would NOT call that 400A total on 120/240 or 600A total on three phase!

 

[Chip0628]

Great! Now we're getting somewhere

Great! Now we're getting somewhere

Now you have asked a question with an answer. That 200A is the maximum current on any one hot leg, independent of what is going on on the other leg(s).
We would NOT call that 400A total on 120/240 or 600A total on three phase!

OK - So assuming the loads are ALL either 240V or perfectly balanced across both legs, wouldn't BOTH hot legs read 200A? (Assuming 0A neutral current)

 

[GoldDigger]

OK - So assuming the loads are ALL either 240V or perfectly balanced across both legs, wouldn't BOTH hot legs read 200A? (Assuming 0A neutral current)

Yup.
Which makes a 200A panel appropriate, except for that pesky 125% that sometimes appears.

 

[Chip0628]

Yes but this is just for the Theory

Yes but this is just for the Theory

Yup.
Which makes a 200A panel appropriate, except for that pesky 125% that sometimes appears.

THANKS for the explanation. So, we have 200A across the 240V supply. Continuing, couldn't you also have two (2) 200A, 120V hypothetical loads across each phase-to-neutral, for 200A on each hot leg? Again, assuming 0A neutral current.

 

[gar]

141020-1949 EDT

Chip0628:

The panel current rating is based on the current capability of an individual bus bar and its termination points. This is essentially a thermal problem in one fashion or another. This also includes all the power dissipated in the breakers as well as in the bus bars, and the ambient temperature around the panel. Put the panel in a 200 F hot house and the rating will be greatly reduced (possibly zero).

A very large number of electrical ratings are essentially based on some form of thermal problem.

.

 

[kwired]

THANKS for the explanation. So, we have 200A across the 240V supply. Continuing, couldn't you also have two (2) 200A, 120V hypothetical loads across each phase-to-neutral, for 200A on each hot leg? Again, assuming 0A neutral current.

Correct. I will say you are trying to calculate backwards to some degree. You should figure out how much load you have and try to balance the 120 volt loads as much as possible and then figure out what kind of amps you will have for maximum load, instead of figuring out what kind of load you can put on a 200 amp panel. (match the load to a suitable panel instead of matching a panel to the load).


In the OP you asked for a simple explanation of how much total current is being drawn. The answer is right there in your OP, 35 and 25, neutral is carrying the imbalance of 10. The 35 amp leg can draw 165 more amps before you need to be concerned about tripping a 200 amp breaker, the 25 amp leg can draw 175 more, or a 240 volt load of 165 can be added and 10 amps is still available for additional 120 volt load on the lighter loaded leg. (or an unlimited number of combinations that add up to these amounts)

 

[GoldDigger]

Correct. I will say you are trying to calculate backwards to some degree. You should figure out how much load you have and try to balance the 120 volt loads as much as possible and then figure out what kind of amps you will have for maximum load, instead of figuring out what kind of load you can put on a 200 amp panel. (match the load to a suitable panel instead of matching a panel to the load).


In the OP you asked for a simple explanation of how much total current is being drawn. The answer is right there in your OP, 35 and 25, neutral is carrying the imbalance of 10. The 35 amp leg can draw 165 more amps before you need to be concerned about tripping a 200 amp breaker, the 25 amp leg can draw 175 more, or a 240 volt load of 165 can be added and 10 amps is still available for additional 120 volt load on the lighter loaded leg. (or an unlimited number of combinations that add up to these amounts)

What is really comes down to, IMHO, is that Chip was looking for a single number that could somehow summarize those three numbers in a meaningful way, and, as you note, that does not really exist.

And the practical effect at the panel and the service of

200A of 240V load,
100A of 240V load and two balanced 100A 120V loads, and
two balanced 200A 120V loads​

will be exactly the same.
They will all total to 48,000VA.
The only problem with that single measurement is that it does not warn us if the load is unbalanced and overloading one bus or the other.

 

[iwire]

:blink:Trying to understand current measurement: At the inlet to a 240vac, 200a single-phase (3-wire) residential panel (measured at the overhead service drop), I measure 25 amps on one leg, 35 amps on the other leg and ten amps on the neutral. Can anybody provide a simple explanation of how much total current is being drawn?

I have read ten explanations on other forums and gotten ten different answers; add the legs together, add together & subtract the neutral, use just the highest measurement... Anybody know for sure??? Thx!

Forget current, you can't just add up the amps from a combination of 240 and 120 volt loads and get a total.

The way to determine the load on a service is by VA.

 

[kwired]

OP did ask what the "current" is. It is three different values in three different portions of the system, you can't combine them. You can get a combined VA, but that is not current, and as has been said doesn't tell you the balance of current within the system.

 

[iwire]

OP did ask what the "current" is

The OP asked what the 'total current' being drawn was which is not how we do things.

Can anybody provide a simple explanation of how much total current is being drawn?

I submit that you can't determine total current only average current based on VA.

And as you point out that ignores how the load is distributed across the three conductors.

 

[ronaldrc]

Why not look at it as two 120 volt circuits.

Circuit 1=120 x 35=4200 VA
Circuit 2=120 x 25=3000 VA

4200+3000=7200/240=30 amp.

 

[kwired]

Why not look at it as two 120 volt circuits.

Circuit 1=120 x 35=4200 VA
Circuit 2=120 x 25=3000 VA

4200+3000=7200/240=30 amp.

That is one of many possibilities to come up with values the OP had. We don't know if everything drawing current is 120 volt loads or if there is some 240 volt loads mixed in there.

 

[gar]

141021-2122 EDT

I pointed out in post #11 that a panel rating is really based on thermal factors.

Total current, however you want to define it, is not the criteria that determines the panel rating.

The current in each bus bar is the determining factor.

I can overload a 200 A panel by connecting a 200 A pure inductor from hot A to neutral, and a 200 A pure capacitive load from hot B to neutral. Neither bus bar A or B is overloaded. You figure out why.

This is a hard to create situation, and not something you would normally encounter. But by understanding this loading you will get a better understanding of how a panel rating would be developed.

.

 

[ronaldrc]

That is one of many possibilities to come up with values the OP had. We don't know if everything drawing current is 120 volt loads or if there is some 240 volt loads mixed in there.

Would it really matter?