# 3 Phase Delta Power Question

#### egg

Joined Sep 16, 2005
2
Hello,
This site was reffered to me from a friend in my trade (which is glass blowing). I have a question, but I preface it with the notion that I am not an electrician. I'm trying to figure out power for my 3 phase furnace in order to choose the right heating element set.

I think I've gotten my head around most everything but one part:

To figure out watts, I understand the formula is:
P=V*I*1.73

But what I'm confused about is the Amp portion.. is it?:

P=V*<VOLTS DEVIDED BY OHMS>*1.73

or

P=V*<VOLTS DEVIDED BY OHMS TIMES 1.73>*1.73

Though the <VOLTS DEVIDED BY OHMS TIMES 1.73> should be the total current draw, but
I am thinking to figure power, it is the first formula.
If it was the second, the 1.73 would be a factor twice.

Donovan

#### thingmaker3

Joined May 16, 2005
5,084
Delta, or wye?

Delta:
Line Current = Phase Current x √3
Line Voltage = Phase Voltage

Wye:
Line Current = Phase Current
Line Voltage = Phase Voltage x √3

#### egg

Joined Sep 16, 2005
2
Originally posted by thingmaker3@Sep 16 2005, 11:19 AM
Delta, or wye?

Delta:
Line Current = Phase Current x √3
Line Voltage = Phase Voltage

Wye:
Line Current = Phase Current
Line Voltage = Phase Voltage x √3
[post=10380]Quoted post[/post]​
Delta...

The above doesn't give me Watts (power) does it? √3 is basically 1.73 which I have as a factor in my formula. I'm not sure what "Phase Current" is... but I think it is simply volts divided by ohms (in one phase of the Delta).

My question was not how to figure current, but whether to use {what I think you are calling} "Phase Voltage" in the Power (WATTS) formula or to use "Line Current" in the Power (WATTS) formula.

I hope that is clearer... I don't know all right terms.
Thanks,
Donovan

#### Erin G.

Joined Mar 3, 2005
167
Donovan,

Are you wanting to know how many amps you elements will draw when on?

#### thingmaker3

Joined May 16, 2005
5,084
Originally posted by egg@Sep 16 2005, 11:54 AM
My question was not how to figure current, but whether to use {what I think you are calling} "Phase Voltage" in the Power (WATTS) formula or to use "Line Current" in the Power (WATTS) formula.
[post=10381]Quoted post[/post]​
Sorry, Donovan. I can be a little dense at times.

Output power will be

(voltage across a phase) x (current through a phase) x √3

Phase voltage is the voltage measured across one phase.

I've used Google to barely scratch the surface of glass kilns, but it looks like the elements are rated in Volts and Amperes.

With a delta system, you'll want three sets of elements that can handle your phase voltage (same as line voltage in delta). They will each need to handle phase current (not line current) as well. If you know power available from your outlet, divide that by voltage and then divide by 1.73. This will give you the amperage rating each set of elements must be rated for. If the elements in a set are in parallel, smaller amperage elements may be used. (IE - six elements total, you can use half the ampere rating... nine elements total, you can use 1/3 the ampere rating.)

One last consideration... it is always a good idea to have components that can handle 125% of the expected current. Power surges do happen.

#### recca02

Joined Apr 2, 2007
1,214
really late on this one,
but for future reference,
it is really confusing sometimes but u have to think of it in this way,
there are three phases.one load connected in each phaseor a three phase load.
so total power draw is
power= 3 x V(phase) x I(phase).
in terms of line values
power=√3xV(line)xI(line)
the two power are same.(since either vline is √3 times its phase value or the current is)
power in each phase is multiplication of phase values.
phase current = phase voltage/resistance in one phase.
phase voltage = voltage between line and neutral.

hope this clears things up.