Calculating draw 220A circuit Question
rjw
| Posted in Energy, Heating & Insulation on
I just want to confirm that for an 220/240v electric water heater which draws 4500 watts, max (upper or lower, they don’t operate at the same time), to calculate the draw on either leg I’d divide 4500 by 220 and get 20.5 amps per “leg.”
“Man’s capacity for justice makes democracy possible, but man’s inclination to injustice makes democracy necessary.”
Reinhold Neihburh: ‘The Children of Light and the Children of Darkness’
Replies
good question. Total draw will be the 4,500 watts total amps is 20.5 amps. Is this per leg or just simply the total. Check out the couple of examples at this site.
http://www.ehow.com/how_5328646_calculate-amperage-draw.html
Don't know if this answers your question, but why are you concerned about e.g. 'per leg' amps rather than just amps?
>>but why are you concerned about e.g. 'per leg' amps rather than just amps?I was just having a brain fart: 4500 watt water heater on 2 30 amp breakers
"Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Neihburh: 'The Children of Light and the Children of Darkness'http://rjw-progressive.blogspot.com/
yes,
Yes. All the current flows through both legs, so 20½ amps flow in one leg and out the other.
, so 20½ amps flow in one leg and out the other.
Huh?
Resistive load, two hot legs.
Better reword that statement.
so 20½ amps flow in one leg and out the other.
Huh?
Resistive load, two hot legs.
Better reword that statement.
I don't get it! What's the problem?SamTA Pragmatic Classical Liberal, aka Libertarian.
I'm always right! Except when I'm not.
It should be in one ear and out the other.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
240v
Two hot legs ( goes innies).
One nuetral (goes outie).
277v
One hot leg (goes innie)
One nuetral (goes outie)
If 20 amps is flowing through both hots, the neutral current is zero.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
yer right
240v
Two hot legs ( goes innies).
One nuetral (goes outie).
There's no neutral on his water heater tho.......
yer right too. the white wire is generaly attached to the ground screw.
point I was trying to make is that with two hot wires supplying a WH one of then can't be returning current to the source. try that and you'll buck phases and let the smoke out of the breaker, bus and whatever else it can get out of.
>>...try that and you'll buck phases and let the smoke out of the breaker, bus and whatever else it can get out of.<<
What am I missing.......
Every 240v HWH I have ever seen (or connected) was connected one of 3 ways.
"Modern" Romex (meaning with ground):
x-2 Romex (usually 10-2 w/ground) to 2 pole breaker - connections at breaker box = black to 1 pole of breaker, white to other pole of breaker (good jobs will be recoded black or red, but not always); bare ground to ground buss (in a sub-panel) or neutral / ground buss in a SEP.
At HWH : black to L1, white to L2 terminal; bare to frame or ground screw.
"Old" Romex (meaning no ground conductor within the cable):
x-3 Romex (again generally, #10)- connections at breaker panel = red to one pole of breaker, black to the other plole of breaker; white to neutral buss.
Connections at HWH = red to L1, black to L2 (or vice versa); white to frame.
OR
x-2 Romex (usually 10-2) to 2 pole breaker - connections at breaker box = black to 1 pole of breaker, white to other pole of breaker.
Black and white connected to L1, L2 at HWH; sometimes a ground wire run from the frame to a cold water pipe (but not always, and usually poorly connected if present at all).
No neutral conductor present.
JimNever underestimate the value of a sharp pencil or good light.
The original response was made to the remark that current came in on one wire and out on the other.
Now how does that make sense if both L1 and L2 are hot?
Because it's true. Electricity can't go in unless it's also coming out (at least not at normal voltages). When electrons are going in one wire they're coming out the other. 1/120th of a second later the direction reverses.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Not a licensed electrician, or an electrical engineer. But do have plenty of "mechanical" or "practical" experience with residential electricity, and my work has an excellent track record of functioning reliably and has passed several hundred inspections, for myself and customers.
Here is my rudimentary, "practical" understanding / interpretation of how current flows in a 240V circuit, like this water heater:
>>that current came in on one wire and out on the other.
Now how does that make sense if both L1 and L2 are hot?<<
Remember, the following is just my "practical" or "mechanical" understanding; namely a "pure" 240vac device does not require a neutral but, a combination circuit such as a stove with a timer, lights, etc. does require a neutral to return the power from the 120v devices only.....
A wire carrying Alternating Current (AC) is not "hot" 100% of the time, in reality it turns on and off at a rate of 60 times per second (frequency). ON-OFF-ON-OFF-ON-OFF, etc., making it "hot" only 1/2 of the time.
In a 120V line, this means that the "hot" is only energized 1/2 of the time, and the neutral is only "returning" current 1/2 of the time.
This alternating pattern allows a "shared neutral" circuit with 2- 120V breakers on opposite phases to share a single neutral wire (and not overload that shared wire, it can/will overload the neutral if both breakers are on the same phase). Breaker #1 returns through the "shared" neutral wire, then breaker #2 uses the same wire to return. So the source breaker that returns through the single neutral wire forms a pattern of returns that look like 1-2-1-2-1-2-1, etc.
If there is a load on only one of the breakers connected in a shared neutral circuit then the "shared" neutral wire carries a pattern which looks like 1-0-1-0-1-0.... or, 2-0-2-0-2-0 etc. If a load is applied to the other breaker in the shared neutral circuit, then the pattern of "returns" goes back to 1-2-1-2-1-2 etc.
Long ago, it was explained to me, that in a "pure" 240V circuit (like a HWH with no 120v loads on either leg), the above alternating pattern allowed the power from one pole of the double pole breaker (L1) to return through the conductor connected to the other pole of the breaker (L2). Thus eliminating the need for a neutral(s) in the circuit.
This "practical" explanation may be completely incorrect, but from a "mechanical" installation standpoint - it works everytime, and the resulting connections never raise the eyebrow of the inspector dejour........
Edumacation / correction welcomed!
Jim
Never underestimate the value of a sharp pencil or good light.
More or less correct, but it should be noted that the same thing would occur on a DC circuit, if, for example, one "hot" was +12V and the other "hot" was -12V. No requirement for AC, simply the requirement that voltage between the two wires is twice what the voltage would be between "hot" and neutral/ground.We could go on to explain how there's no real requirement for a neutral or ground per se in an electric power system, just a difference in voltage between two wires. But that would probably just confuse things in this case.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Your explanation is pretty shaky and I'm not sure you should be passing it on. For starters, voltage in both legs varies as a sine wave, not on and off. In single phase 240, the sine wave of one leg is the inverse of the other, so they add to deliver the higher AC voltage.
Edited 10/16/2009 12:44 pm ET by PeteBradley
>>Your explanation is pretty shaky and I'm not sure you should be passing it on.<<
I 'splained it was rudementary, "practical", "mechanical", and possibly totally incorrect. Added the disclaimer that I was not a licensed electrician or EE........
Where were you when the question was asked?
Do you really think the question was asked by someone who will understand sine waves?
I'm not sure I do, thanks for the edumication which will require more research on my part.
Sorry,
Jim
Never underestimate the value of a sharp pencil or good light.
Edited 10/16/2009 1:29 pm ET by JTC1
Edited 10/16/2009 1:40 pm ET by JTC1
Why post something that you think is "possibly totally incorrect"? How about letting people who actually know the answer respond instead?
Edited 10/16/2009 3:13 pm ET by PeteBradley
I would say you are just about a day late to the party.......
Suggest you re-read the entire thread, start paying especially close attention at post xx.24 from Dave; that would be this one:
>the white wire is generaly attached to the ground screw.
point I was trying to make is that with two hot wires supplying a WH one of then can't be returning current to the source. try that and you'll buck phases and let the smoke out of the breaker, bus and whatever else it can get out of.<
Think about how well / safely the wiring installation he describes will function.
What will happen when the breaker is thrown to "on"? How long will it remain "on" with that white wire connected to the ground screw? Assuming the breaker does not trip, what happens if there is a puddle on the concrete floor that the installer happens to be standing in and touches the frame of the water heater? Lacking 2 hots at L1 and L2, will the heater perform well? At all?
Now note xx.25 where I described the mechanics of wiring a 240v residential water heater, 3 different scenarios which I have seen. No theory presented, mechanics of proper installation are flawless.
Pay close attention again when you get to xx.33 where Dave is obviously still confused.
Note that by this time other posters are backing me up in my conversation with Dave.
You have already responded to my xx.35 post where I presented my rudimentary, "practical" understanding of 240 current flow. Theory shaky, mechanics still flawless.
>Why post something that you think is "possibly totally incorrect"?<
Because my post xx.35 dealt strictly with electrical theory, not installation mechanics, and nobody else was coming up with an answer - you hadn't posted yet!
Keep in mind this is a construction forum, mechanics are quite possibly more important than the "perfect" theory behind a statement. Disclaimers like I posted are fairly normal here - "not a licensed electrician or EE", "practical", etc.
To the guy in his basement attempting to connect his HWH, accurate, safe, code-compliant, mechanical guidance are probably more important than perfect theory presentations. I try to help when I can, and frequently include disclaimers.
>How about letting people who actually know the answer respond instead?<
You did, and presented good, though somewhat sketchy information, but you were a day late to the party.
So, please explain, how would the fact that current varies by sine wave rather than on-off (as I incorrectly described) affect the safe installation of the water heater? Could Dave finally figure out that a separate neutral conductor was not required from the information which you supplied?
Get my point?
Jim
Never underestimate the value of a sharp pencil or good light.
There are lots of folks on BT who can explain how to safely install a water heater without getting into guesswork about how A/C works. My comment about a sine wave was directed to you not the original poster.I've taken this as far as I'm going to, so feel free to have the last word if you must. Maybe you can even put in a bunch of disclaimers about how it could be totally bogus but you expect people to read it anyway.
"For starters, voltage in both legs varies as a sine wave, not on and off. In single phase 240, the sine wave of one leg is the inverse of the other, so they add to deliver the higher AC voltage."That is not entirely correct either.The problem is that the terms voltage, hot, neutral, supply, return, and phase are all RELATIVELY.Voltage is only meaningfull as a measurement between TWO POINTS. Often the 2nd (reference point)is implied and everyone knows what that reference point is and understands what is being said. However, when that "reference" gets confused then we end up with a thread like this.Now PHASE gets even more confusing.Phase is the time or angular difference between ONE signal measured to it's reference and a SECOND signal measured to the same reference.Now the water heater is a 240 volt load that connects to 2 power wires. There are ONLY 2 wires. So one of them is a reference. But thee is only ONE SIGNAL and thus phase has no meaning.The water heater does not know if it is hooked to a split 240 single phase system, 240 system (with out without one side ground) as in England, a 3-phase 240 delta with one side center taped or even a grounded corner 240 delta with the load between the two high legs.In all of those cases the WH sees 240 between the 2 terminals and with only 2 connections phase is meaningless..
William the Geezer, the sequel to Billy the Kid - Shoe
> In all of those cases the WH sees 240 between the 2 terminals and with only 2 connections phase is meaningless.However, if you have two hots and neutral then phase is VERY meaningful.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Here's another explanation that might be helpful in clarifying this for some readers.
In a 240 volt system, power is provided to the main panel through three conductors -- two hots and one neutral. The neutral is connected to ground in the panel. We'll call ground zero volts. Because the neutral is connected to ground it is also at zero volts in the panel.
One of the hots is 120 volts "above" zero and the other is 120 volts "below" zero. So the voltage between either hot and neutral/ground is 120 volts. Between the two hots is 240 volts -- they are in effect 240 volts apart.
So an appliance that requires 240 volts is connected between the two hots. An appliance that requires 120 volts is connected between either hot and the neutral. To supply 240 volts a neutral is not needed and some 240 volt appliances don't even have a connection for a neutral. Other 240 volt appliances use dual voltage, for example, a stove may use 240 volts for the heating elements but 120 volts for a light and clock. Wiring for such appliances includes a neutral. Internally the heating element would be connected to the two hots, and the light and clock would be connected between one of the hots and the neutral.
In a 120 volt circuit the current travels through the hot, the appliance, and the neutral; the hot and neutral carrying equal current. In a 240 volt circuit the current travels through one hot, the appliance, and the other hot; both hots carrying equal current. In a 120/240 volt appliance the current travels through all three conductors. One hot carries the full current, the other hot carries less, with the difference being carried by the neutral. The current of the higher amperage hot always equals the sum of the currents in the neutral and the other hot.
Thanks, nicely done.
This mechanic understood every word!
JimNever underestimate the value of a sharp pencil or good light.
> point I was trying to make is that with two hot wires supplying a WH one of then can't be returning current to the source. try that and you'll buck phases and let the smoke out of the breaker, bus and whatever else it can get out of.Kirchhoff's spinning in his grave.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Why would it need to at 240v?
Why would WHAT need to be 240V?
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
I've never seen a water heater with a neutral.
Doesn't need one, since the current through the neutral would be zero.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
I've never seen a WH with a reverse, for that matter. :)BruceT
A water heater with reverse is called a water cooler.
No rewording needed. It is correct.
in one leg and out the other
Only for 8.3 ms or so, then it is out one leg and in the other <G>
Correct formula, wrong number plugged in....
4500w divided by 240v = 18.75 amps
"220" is a common misnomer for 240v. As is "110" is for 120v.
Only your volt meter knows for sure.
Jim
Edited 10/6/2009 9:22 pm ET by JTC1
In my area, voltages run on he low side for 120/240, and I figured I'd use that lower figure to build in a little leeway in he calc.
"Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Neihburh: 'The Children of Light and the Children of Darkness'http://rjw-progressive.blogspot.com/
But, unlike a motor, the current through a resistive heating element doesn't go up as the voltage goes down. The current at 220V will be LESS than the current at 240V.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
>>But, unlike a motor, the current through a resistive heating element doesn't go up as the voltage goes down. The current at 220V will be LESS than the current at 240V.
Thanks - I was actually just trying to be conservative (high) as to how many amps the unit draws -
4500W/220V=20.45A
4500/240V=18.75.
"Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Neihburh: 'The Children of Light and the Children of Darkness'http://rjw-progressive.blogspot.com/
Right, but the power of the heating element is figured at 240V, so there's no need to "pad" that number. You do need to be wary of drawing too much for a given breaker -- 85% I believe is the rule for "mixed load" circuits, but I think it might be different for a fixed load like a water heater. And you need to be wary of using too large of a breaker, as the heater is designed with the expectation that it will be protected by a breaker of the appropriate size, and may be unsafe with a larger one. (The wiring, of course, can be safely oversized up until the point where it no longer fits in the connections, but the breaker still should be no larger than what the device "expects".)
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
It is 80% and that is for any "continuous" load and that is defined as being 3 hrs or more.At one time I thought someone said that water heaters where called out specifically as not being continuous. But either that person was wrong or I remembered wrong or they change the code since then. But I found it a couple of years ago and it code does specifically call out WH as needed a 20% derating on breaker and wiring.
.
William the Geezer, the sequel to Billy the Kid - Shoe
"Low voltage"....Reasonable, but you will still end up with #10 wire and 30A breaker.
JimNever underestimate the value of a sharp pencil or good light.
220, 221 what ever it takes.
What was that from??
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Mr Mom
That is the amps through one element and they are interlocked by the top thermostat so only one is on at a time.
Dr Kirchoff says the amps through one leg will be the same on the other. It is a rated 4500w load @ 240v so it is 18.75a. If you only had 230 or 220v the amps would be less. (the resistance didn't change)
amps would be less...
whoa there, remember this was posted by BOB! He is a constant power load<G>