three phase power to single phase water heater

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We have an installation to connect a three phase water heater. the unit has two 4500 watt elements inside. We are to connect three legs of the incoming three phase with the center leg connected as a "common" between the two elements. What is the calculated amperage load for each leg if operated at 208 volts?
If this were single phase, then the center leg would be the sum of the other two legs. But with three phase power, shouldn't the "three phase factor" (1.73) be involved in the calculations for the "common" conductor?

John
 

roger

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John, typically water heater elements work one at a time so you would calculate it as single phase 4500/208 = 21.6

Roger
 

winnie

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The 'three phase factor' is simply the square root of 3. It shows up in many, certainly not all, equations involving 3 phase circuits.

You don't say what the _rated_ voltage is for these heating elements, and you are not clear just how these heating elements are connected to the supply.

If you bring three phase legs to the unit, and connect each heating element between two phase legs, and call the current flowing through each heating element 'X', then the current through the separated legs will also be 'X', and the current through the common leg will be 1.732*X.

If you bring two phase legs and a neutral to the unit, and connect each heating element between a phase leg and neutral, and call the current flowing through each heating element 'Y', then the current through the separated legs will also be 'Y', and the current through the common neutral will be 'Y'. (no 1.732 in this situation)

-Jon
 

infinity

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winnie said:
The 'three phase factor' is simply the square root of 3. It shows up in many, certainly not all, equations involving 3 phase circuits.


Jon, correct me if I'm wrong but isn't the value of 1.73 attached to voltage portion of all calculations involving 3 phase?
 

rattus

Senior Member
I will answer for Winnie:

I will answer for Winnie:

infinity said:
Jon, correct me if I'm wrong but isn't the value of 1.73 attached to voltage portion of all calculations involving 3 phase?

No. not all. This is not the case with a delta configuration since there is no line to neutral voltage as there is with a wye.
 

LarryFine

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rattus said:
No. not all. This is not the case with a delta configuration since there is no line to neutral voltage as there is with a wye.
However, the same figure does apply to current calcs in Delta systems.
 

roger

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rattus said:
No. not all. This is not the case with a delta configuration since there is no line to neutral voltage as there is with a wye.

But 360 still works pretty good as a divisor or multiplier for the OP's voltage. :wink: :D

Roger
 

roger

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rattus said:
Roger, how about an example?

Rattus, think about it, 208 multiplied or divided by 1.732 (extend as far as you want to) is in perfect harmony with a circle.

Roger
 
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rattus

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roger said:
Rattus, think about it, 208 multiplied or divided by 1.732 (extend as far as you want to) is in perfect harmony with a circle.

Roger

Roger, does this fact have any useful application?
 

roger

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rattus said:
Roger, does this fact have any useful application?

Absolutely, as we discussed before, it's a short cut in performing calculations.

If you don't approve you should point out your objections to all the Electrician training courses that teach this.

If I am standing in a ravine with a compound bow getting ready to let my arrow go and you are standing next to me wanting to calculate wind speed, angle, altitude, etc..., I'll say to you, have a party, but by the time you give me my coordinates going through the long hand the arrow will be on it's way.

Roger
 
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rattus

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roger said:
Absolutely, as we discussed before, it's a short cut in performing calculations.

If you don't approve you should point out your objections to all the Electrician training courses that teach this.

If I am standing in a ravine with a compound bow getting ready to let my arrow go and you are standing next to me wanting to calculate wind speed, angle, altitude, etc..., I'll say to you, have a party, but by the time you give me my coordinates going through the long hand the arrow will be on it's way.

Roger

It is not a matter of approval; I don't understand how this shortcut is used. I certainly don't see the correlation between 360 degrees and 360 volts.

Let us see a calculation where this trick is used, then maybe it will sink in.
 

rattus

Senior Member
Ok:

Ok:

RayS said:
neat shortcut, I didn't know it.

208*1.73=360,
so
watts/360=amps

I see it now, but the line-line voltage must be 208V. If you are working with a balanced 208V system, and want to do rough calculations in your head, that is OK. But, what if the voltage is 240V? 480V? It is not that much trouble to use the standard formula expecially when calculators are so cheap.

The amount of time saved is miniscule in my opinion.
 

hardworkingstiff

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An interesting observation

An interesting observation

I seems that wiring this heater with a 3-phase circuit is more expensive than wiring as single-phase.

Assuming the 4500 watt elements are actually rated at 208-volts, you would need a 45-amp circuit wired as 3-phase and a 50-amp circuit wired as single-phase. The 2-pole breaker cost less, and you would have one less conductor. Both circuits would use a #8 conductor (copper), of course you need 50% more copper for the 3-phase circuit (assuming conduit ground).
 

sceepe

Senior Member
No. not all. This is not the case with a delta configuration since there is no line to neutral voltage as there is with a wye.

wait a minute. I must have missed something. Didn't the original post say he had a three phase water heater with only 2 elements. You can't make a delta configuration with two elements (or a Y config for that mater). If it were a three phase water heater it should have three elements.

I think he might have a single phase water heater designed for a two pole supply and they are trying to hit it with 3 phase. If the water heater is designed for single phase supply, the elements are rated at 4.5kw at line to neutral voltage (120V or 277). If you hit it with 208v, (line to line voltage) bad things may happen. Either you will only get slightly warm water or you will burn up the elements.
 

iwire

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sceepe said:
You can't make a delta configuration with two elements (or a Y config for that mater). If it were a three phase water heater it should have three elements..

I pictured two elements connected to phase B and one element apiece to phases A and C.
 

RUWired

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Pa.
roger said:
Absolutely, as we discussed before, it's a short cut in performing calculations. Roger

Roger,Although this multiplier short cut is used when all three phases are equally loaded, it can't be used in this application to find the amperage on the B phase.Two different formulas find B amps...9000/208x1.732/2=37.47 or 4500/208=21.63x1.732=37.47.
Rick
 
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