240v heater mis-stated question

[dm9289]

I posted a question in a manner that was apparantly missunderstood due to my poor wording so let me try it again.

Lets say you have a 120v baseboard heater the current flows from hot thru the element and then thru the neutral as its path. But a 240v baseboard heater (no neutral). A differing phase hooked to each side of the element. My problem to explain is how the current flows phase to phase with no neutral and why that does not cause overcurrent.

Perhaps someone has some good reference material or a good explanation, I unfortunately know it works but not why.

Thanks
Dave

 

[boboelectric]

Draw a 15 kva xfmr circuit with center tap . then you'll see.

 

[LarryFine]

Dave, a given load only cares what the voltage between its terminals is, and not which, if any, of the conductors is grounded.

See if this helps any: http://forums.mikeholt.com/showpost.php?p=708650&postcount=4

 

[cpal]

I. My problem to explain is how the current flows phase to phase with no neutral and why that does not cause overcurrent.Thanks
Dave

to whom are you trying to explain this to?

 

[gar]

091203-1553 EST

dm9289:

I hope Larry's reference that he presented, and note he wrote it, helps answer your question. It is very well written.

My problem to explain is how the current flows phase to phase with no neutral and why that does not cause over-current.

You indicated that a 240 V heater was placed across the 240 supply. We have to assume that since this heater was designed for your application that its wattage, and therefore current, is within a level that is compatible with the branch circuit you are connecting to.

Suppose the heater is rated at 2400 W and 240 V. Then its operating current is 10 A. This is well within a safe value for a 20 A breaker. Next consider a similar 2400 W heater but designed for 120 V. It requires 20 A for operation and this would exceed the capability of 20 A breaker in a 120 V circuit.

Next consider a 1200 W heater designed for 120 V. Its current load at 120 is 10 A. Erroneously connect this 1200 W 120 V heater to 240 V from a 30 A breaker. The current drawn will be 20 A and will not trip the 30 A breaker. The power dissipation in the heater will be 4800 W or 4 times its rating. It will burn up and moderately fast. If the breaker had been 20 A the heater would probably burn up before the breaker tripped.

The steady state current thru any stable impedance is Voltage/Zimpedance no matter what the circuit is connected to said impedance.

.

 

[LarryFine]

To add, voltage between any two points will drive a current, limited (mostly) by the intentional resistance built into the load in question. Neither conductor needs to be grounded for this to occur.

In an ideal world, all of the voltage would be 'dropped' across the load, and none in any of the source or supply conductors. In a properly designed system, most of the voltage does make it to the load.

Think back to my battery example, a 120/240v 1ph supply actually is two 120v supplies in series, whether it's a single, center-tapped winding, two separate windings, or even two separate transformers.

I hope Larry's reference that he presented, and note he wrote it, helps answer your question. It is very well written.

Danke!

 

[rattus]

From my experience:

From my experience:

I posted a question in a manner that was apparantly missunderstood due to my poor wording so let me try it again.

Lets say you have a 120v baseboard heater the current flows from hot thru the element and then thru the neutral as its path. But a 240v baseboard heater (no neutral). A differing phase hooked to each side of the element. My problem to explain is how the current flows phase to phase with no neutral and why that does not cause overcurrent.

Perhaps someone has some good reference material or a good explanation, I unfortunately know it works but not why.

Thanks
Dave

Ages ago, I made a service call on a 120V print dryer which no longer heated and the motor ran too fast. The company electrician had wired it into 240V which burned out the heating elements and made the universal motor run too fast.

Yes, it did draw too much current--for a few seconds. Didn't hurt the motor though which was in series with a rheostat. You know, one of those wirewound things with a knob.

 

[LarryFine]

Ages ago, I made a service call on a 120V print dryer which no longer heated and the motor ran too fast. The company electrician had wired it into 240V which burned out the heating elements and made the universal motor run too fast.

Too bad it hadn't occured to him to at least try placing the two parts of the load in series.

 

[rattus]

Too Bad:

Too Bad:

Too bad it hadn't occured to him to at least try placing the two parts of the load in series.

Too bad he didn't look at the nameplate, or notice the 120V plug on the power cord!

 

[benaround]

Lets say you have a 120v baseboard heater the current flows from hot thru the element and then thru the neutral as its path. But a 240v baseboard heater (no neutral). A differing phase hooked to each side of the element. My problem to explain is how the current flows phase to phase with no neutral and why that does not cause overcurrent.

Dave,

Just remember that the neutral is one side of each phase coil tied together.

 

[dm9289]

Thank you for all your advice it is starting to clear up in my head now, I always understood how to hook it up but not how it worked.

Dave