Voltage / Ampere

[LarryFine]

Here's the original post:

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law" but when I compare the 220/110 volts in a given load it goes the other way. For 220V given 5000 watts the current I got is 22.72A, for 110V the same load I got 45.45A. does the statemnet correct or its just a typograpical error? or there is an Theoretical / Technical answer for this? Power is the product of I squared times the resistance so the given load is not the resistance @ all. this is only the reason I see... can you explain it further to me.

Now, Ed refers to a 220v load and a 110v load as "the same load", which we know is an incorrect term. A load that can be used on two different voltages is not a constant resistance; it is a mutliple load that can be combined in more than one way.

It is possible to have a load that can be reconnected for different voltages, in which case each part of the load can be looked at as a constant resistance, but it is up to the installer to make sure it is set up properly for the applied voltage.

When talking about theory this way, we hyave to first decide which parameters are constants and which are variables. For a given load, we take the resistance to be a constant. The voltage can be a variable, which causes the current, and the resulting power, to vary as well.

If we ask about a load that can be set up for more than one voltage, it must be treated as a different load for each setup. Once we have established that, we can then discuss the results of varying the voltage across that constant resistance.

In normal electrical work, there is no such thing as a constant power or current, since power and current are products of voltage and resistance. If you want to ask about Ohm's Law calculations, first determine which two parameters are given, then figure the rest.

 

[websparky]

Duh....

Duh....

Al,

When you said:

In the real world, if I double the volts on a fixed R, the current doubles. Ohm's Law, plain and simple.

I was thinking WATTS. Sorry, my mistake!

As an example:
Let's say we have a fixed resistance of 2400 ohms and a voltage of 120. We would take 120 and divide it by 2400 to find amps, right? If so, we get .05 amps. If we simply double the volts to 240 and divide it by 2400 we get .10 amps, right?

For watts, if we had 2400 watts and 120 volts we would have 20 amps. If we had 2400 watts and 240 volts we would have 10 amps.

Thanks for the help!

 

[al hildenbrand]

Dave,

My pleasure.

Eduardo,

See what I mean about red herring? Thinking about power is hard not to do.

Ohm's Law is not about power. It's only the relationship of E, I and R.

Is that helping?

 

[LarryFine]

For watts, if we had 2400 watts and 120 volts we would have 20 amps. If we had 2400 watts and 240 volts we would have 10 amps.

This is correct mathematically, but confusing electrically. To end up with those wattages, the resistances must be very different. A manufacturer can only construct equipment with a set resistance, and give an expected wattage based on an expected voltage.

In these examples, the 120v 2400w load would have a 6-ohm resistance, while the 240v 2400w load would have a 24-ohm resistance. Notice that the doubled voltage requires a quadrupled resistance to keep the power constant.

 

[kingpb]

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law" but when I compare the 220/110 volts in a given load it goes the other way. For 220V given 5000 watts the current I got is 22.72A, for 110V the same load I got 45.45A. does the statemnet correct or its just a typograpical error? or there is an Theoretical / Technical answer for this? Power is the product of I squared times the resistance so the given load is not the resistance @ all. this is only the reason I see... can you explain it further to me.

Thank you!!!

Eduardo

Keep it in mathematical terms and don't get caught up in examples:

In mathematics, two quantities are called proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio. V = IR, or V/I = R, where R is a constant. Then if V goes up, I has to go up.

Two variables are inversely proportional (or varying inversely) if one of the variables is directly proportional with the multiplicative inverse of the other, or equivalently if their product is a constant.

Where if you have V=IR and the product of IR is constant, then if you allow V to go up, and I goes down, then R is changing proportinally with V, Where V and I are inversely proportional.

The bottom line is you need to understand the theory and know what is changing, and what is constant. Depending on when it is happening, it can also have an effect. A good example is to observe what happens with voltages and current during motor starting vs. motor steady state.

 

[Rick Mack]

This is correct mathematically, but confusing electrically. To end up with those wattages, the resistances must be very different. A manufacturer can only construct equipment with a set resistance, and give an expected wattage based on an expected voltage.

In these examples, the 120v 2400w load would have a 6-ohm resistance, while the 240v 2400w load would have a 24-ohm resistance. Notice that the doubled voltage requires a quadrupled resistance to keep the power constant.

This is correct and to expand on it a little bit, think of a motor with two windings.The motor can be wired for 120v or 240v.

If you wire the windings in parallel and connect to 120v, then with 12 ohm windings, there would be 120v across each winding and 10 amps in each winding for a total of 20 amps supplied from the source. Also note the equivalent resistance of the parallel windings is 6 ohms. Using I squared R: (20)(20)(6) = 2400 watts.

On the other hand, if you connect the windings in series and connect 240v, then with 12 ohm windings, there would still be 120v across each winding and 10 amps in each winding for a total of 10 amps supplied from the source since these windings are connected in series. The equivalent resistance of the two windings in series is 24 ohms. Again using I squared R: (10)(10)(24)= 2400 watts.

 

[Eduardo Maun]

Thank you...

Thank you...

Thank you for all your valuable help, I have read all the explanation and it widened my knowledge. Hope all of you won't get tired helping people like me.
I really appreciate it...

regards,

Eduardo

 

[PAW2006]

Hello Everybody,
I'm going to make some assumptions here! (Yes I know what they say about assumptions).
The context of the book is estimating. i.e. making a profit.
We're talking about current here, so the tie between current and estimating is wire/equipment size. Which equates to $.
So let's just look at a single family dwelling. Forget DC. Forget 3Phase.
Two types of loads in a residence:
1) Resistive, Loads where R is fixed and E is specified. Therefore : We use I=E/R (Note, I is proportional to E, ) No change in I, No change in Wire size, (Load is next to panel) No change in $

2) Inductive/Capacitive, Motor loads among others. Loads where P is fixed (horsepower) Therefore: we use I = P/E. (Note I is inversely proportional to E ) Most motors are dual voltage. So we can wire them at the higher voltage and consequently reduce the current draw. A reduction in current equates to smaller conductors Which in turn leads to less materiel cost.
I think the author had to assume that the reader had the prerequisite knowledge, If not then they would either accept it or make the effort to find out just as Eduardo has.

 

[LarryFine]

Welcome to the forum. I hope "electrical stuff" means you're in the business.

The context of the book is estimating. i.e. making a profit.

Book? What book?

Most motors are dual voltage. So we can wire them at the higher voltage and consequently reduce the current draw. A reduction in current equates to smaller conductors Which in turn leads to less materiel cost.

That's no secret to most of us here. We all know that insulation is cheaper than conductor.

 

[PAW2006]

Welcome to the forum. I hope "electrical stuff" means you're in the business

Thanks Larry. Electrical stuff means: numerous different areas of the business, all involved moving electrons.

Book? What book?

I have read this " the higher the voltage the higher the current in a Electrical Estimate book

That's no secret to most of us here. We all know that insulation is cheaper than conductor.

Speaking of conductors and insulation: I recently striped about 6 years worth of rabbits. 581# $2.00 a Pound. I was able to put gas in my truck for a whole week! . Hmm, makes me think I should have someone else measure the wire in the first place.
PAW

 

[infinity]

Rabbits???

 

[PAW2006]

Rabbits???
The end cutoffs of wire (Typically 500mcm,) when you splice a swithgear or service or whatever. Smaller than 1/0 isn't worth the trouble. I came up through the ranks in Houston. Have worked different areas of the country since then, (1982) and find it common to hear different names for things.
What do you call them?

 

[LarryFine]

I only asked because of forum rules.

Oh, that book.

Rabbits? AddedThat's what I get for posting slowly)

 

[LarryFine]

What do you call them?

Short pieces of wire.



Q) "What do you call a boomerang that doesn't return?"

A) "A stick."

 

[infinity]

Rabbits???
The end cutoffs of wire (Typically 500mcm,) when you splice a swithgear or service or whatever. Smaller than 1/0 isn't worth the trouble. I came up through the ranks in Houston. Have worked different areas of the country since then, (1982) and find it common to hear different names for things.
What do you call them?

We call it Mongo.

 

[al hildenbrand]

With little imagination here, I know them simply as "scrap".

 

[Eduardo Maun]

Thank you...

Thank you...

Thank you for all your valuable help... I really appreciate it...

regards,

Eduardo

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law" but when I compare the 220/110 volts in a given load it goes the other way. For 220V given 5000 watts the current I got is 22.72A, for 110V the same load I got 45.45A. does the statemnet correct or its just a typograpical error? or there is an Theoretical / Technical answer for this? Power is the product of I squared times the resistance so the given load is not the resistance @ all. this is only the reason I see... can you explain it further to me.

Thank you!!!

Eduardo