unbalanced load calc for 3 phase delta system

[rattus]

Re: unbalanced load calc for 3 phase delta system

Originally posted by RUWired:
rattus,E-L=E-Px1.732
Line voltage =Phase voltage x 1.732
Rick

Rick, your formula applies to a wye configuration. For example, in a 120V/208V wye,

Vline = 120Vx1.732 = 208V

For a delta config,

Eline = Ephase = 240V in this case.

Vectors--or phasors depending on your age--are not for the faint of heart. That is the reason we have formulas which work quite well when when properly applied.

 

[bob]

Re: unbalanced load calc for 3 phase delta system

RU

In this case it is 240 3 phase and the multiplier is 415. If i have a 192 amp 240v 3 phase load and want to find kva, i multiply 192x415= 79680kva(80).Rick

As Rattus said this method is not correct for a delta config. The phase amperage is equal to the phase load / phase voltage or 80kva/.24v= 333 amps. If you want the line current its 333 x 1.73 which = 576 amps.

If i have a 3 phase load drawing 80 kva, i don't say i have a 240kva load, i say i have an 80kva load.

You don't have 80 kva of load. You have 240 kva of load. You can express it as 80 kva per phase.

[ February 09, 2006, 02:57 PM: Message edited by: bob ]

 

[kalanjeya]

Re: unbalanced load calc for 3 phase delta system

Originally posted by rnolan:
I have a 120/240 3 phase, 4 wire (center tap) delta system (B stinger). My 3 phase load is 80 kva per phase (A, B, and C). My single phase load is 34 kva phase A and 41 kva phase C. What are the amps for each phase and how is this calculated? What size (amp rating) panel is needed?

Simple RNOLAN, See here.........

Your threephase load is 80Kva per phase, o.K !

So ,

Phase A= 80Kva
Phase B= 80Kva
Phase C= 80Kva .... Am I right ?

Again Take Your Single phase load....

34 kva phase A and 41 kva phase C , O.K ?

So, add these Two there, you will get

Phase A= 80+34=114 Kva &

Phase B= remain same , i.e, =80 Kva

Phase C= 80+41 = 121 Kva

I think , I am right here.....

So , see the current here

Phase A= 114/ 1.732*240= 0.274 KA = 274A

Phase B= 80/ 1.732*240= 0.192 KA = 192A

Phase C= 121/ 1.732*240= 0.291 KA = 291A

How is it ?......

And for calculating Feeder size ,

you can take 125 % of the load current...

Thanks

Jeyakumar

 

[rattus]

Re: unbalanced load calc for 3 phase delta system

jeyakumar,

Sorry, but your answers are wrong.

First:

For a wye system which this is not:

Vline = Vphase x 1.732
Iline = Iphase

For a delta system which this is:

Vline = Vphase (no multiplier)
Iline = Iphase x 1.732 (with a balanced load)
(can be used for Ib in this case)

Second:

You must compute the currents in the 41KVA and 34KVA loads at 120V. You can't just lump them in with the 80KVA 240V load.

Third:

You must combine the phase currents with vector math to obtain Ia and Ic. You can do this graphically if you don't know trig, but that is slow and clumsy.

[ February 09, 2006, 11:26 AM: Message edited by: rattus ]

 

[bob]

Re: unbalanced load calc for 3 phase delta system

You must compute the currents in the 41KVA and 34KVA loads at 120V. You can't just lump them in with the 80KVA 240V load.

Rattus
The 120 volt load is 41 + 34 kw. You could consider 34kw + 34kw at 240 = 283 amps and
41 - 34 = 7kw at 120 amps. The total would then
be 333 + 283 = 616 amps and the other load would be 333 + 283 + 58 (7kw at 120) = 675 amps.

[ February 09, 2006, 04:52 PM: Message edited by: bob ]

 

[rattus]

Re: unbalanced load calc for 3 phase delta system

Bob,
Yes, I did that in my analysis, but I was trying to keep the subsequent explanation as simple and straightforward as possible.

 

[RUWired]

Re: unbalanced load calc for 3 phase delta system

Rattus,I think you may have gone the wrong way with the math.I stopped using my memory and went into the books and found that;
EL=EP
IL=IPx1.73
IP=IL/1.73
I agree with your 80000/240=333.(this would be the line current ) IL
Then; IP= 333/1.732 (192 amps)
These formulas are out of AEH 7th edition.
Rick

 

[rattus]

Re: unbalanced load calc for 3 phase delta system

Originally posted by RUWired:
Rattus,I think you may have gone the wrong way with the math.I stopped using my memory and went into the books and found that;
EL=EP
IL=IPx1.73
IP=IL/1.73
I agree with your 80000/240=333.(this would be the line current ) IL
Then; IP= 333/1.732 (192 amps)
These formulas are out of AEH 7th edition.
Rick

Rick,

In a balanced delta system, say 80KVA/phase,

Iload = Iphase = 80KVA/240V = 333A in this case.

Iline = 1.732xIphase = 1.732x333A = 577A

Papp = VlinexIlinex1.732 = 240Vx577Ax1.732 = 240KVA.

Ask Bob, ask Zif, ask anyone.

 

[Zifkwong]

Re: unbalanced load calc for 3 phase delta system

I second rattus

 

[RUWired]

Re: unbalanced load calc for 3 phase delta system

rattus,the numbers you post are correct if you have a 240kva load,and i do understand how you arrive at that figure.I personally don't figure 80kva loads to be 240kva loads,especially when it comes to figuring branch circuits.I never had a problem in undersizing wires in both delta/Y transformers.This idea is new to me and one i will consider in theory only.
Rick

 

[bob]

Re: unbalanced load calc for 3 phase delta system

RU

rattus,the numbers you post are correct if you have a 240kva load,and i do understand how you arrive at that figure.I personally don't figure 80kva loads to be 240kva loads,especially when it comes to figuring branch circuits.I never had a problem in undersizing wires in both delta/Y transformers.This idea is new to me and one i will consider in theory only.

There is no need to consider it as theory. It a fact.

My 3 phase load is 80 kva per phase (A, B, and C).

This is the origional post. As you see it says
80 kva per phase. Each phase has 80 kva. 3 phases have 240 kva.

I personally don't figure 80kva loads to be 240kva loads,especially when it comes to figuring branch circuits.

What does the branch circuit have to do with it?

rattus,E-L=E-Px1.732
Line voltage =Phase voltage x 1.732
Rick

That is incorrect for a delta system. Eline and Ephase are the same in a delta. The Iline is = to
Iphase x 1.73. Your statement is for a wye system.
It think you may need to hit the books again and understand the difference between a delta system and a Wye system.

[ February 09, 2006, 11:33 PM: Message edited by: bob ]

 

[ramsy]

Re: unbalanced load calc for 3 phase delta system

bob wrote: ..What does the branch circuit have to do with it?

I thought electrical-trade jargon for branch-circuit "line" voltage was defined as (H-N) Phase-to-Ground, as opposed to branch-circuit "Phase" voltage, short for Phase-to-Phase.

Maybe thats why some texts refer to transformer phases as coils or windings, so engineer vs trade jargon are not exact opposites.

It's easier for me to remember open Delta(V) and WYE coils as series circuits, where voltage sums 1? or vectors 3? * 1.732, and closed Delta coils as parallel circuits, where current sums 1? or vectors * Sqrt(phases).

I also thought spsnyder saved me from this confusion when he said, The formula S=1.73*I(line)*V(line-line) holds true for delta or wye configurations.. Can this provide a universal field check against Delta & Wye load calcs? vs rattus's Papp = Vline*Iline*1.732 which is only good for Delta's.?

 

[rattus]

Re: unbalanced load calc for 3 phase delta system

Roger,

I believe "phase voltage" always means the voltage across the xfrmr coils and "phase current" means the current through the xfrmr coils--delta or wye.

"Line voltage" always means line-line voltage and "line current" always means current through the lines.

For delta:

Vline-line = Vphase
Iline = Iphase x 1.732 (balanced load)

For wye:

Vline-line = Vphase x 1.732 (e.g., 120V/208V)
Iline = Iphase

The formula for total apparent power,

Pa = Vline-line x Iline x 1.732, (balanced)

applies to delta or wye. With an amprobe and voltmeter one can determine the apparent power without knowing the configuration.

If the engineers and electricians are speaking different languages, we are in big trouble.

I would think that the NEC may define these terms for us to put the question to rest.

Edited to remove confusion about Vline.

[ February 11, 2006, 09:19 AM: Message edited by: rattus ]

 

[bob]

Re: unbalanced load calc for 3 phase delta system

The formula S=1.73*I(line)V(line-line) holds true for delta or wye configurations. In a Wye configuration I(line) = I(phase). In a delta configuration V(line) = V(phase).

Ramsey
This is a true statement for caculating power(KVA).

 

[engy]

Re: unbalanced load calc for 3 phase delta system

"Pa = Vline x Iline x 1.732, (balanced)"

This is a very good point, a lot of simple calculations are based on BALANCED load.

Once you start getting into unbalanced loads like a center-tapped delta with a mixture of 1&3 phase loads, the math changes a lot, 1+1 no longer equals 2

 

[RUWired]

Re: unbalanced load calc for 3 phase delta system

rattus, i finally see where our differences are.When someone says to me they have a "connected" load of 80 kva a-b-c,i think they are saying its 80 between a and b, 80 between b and c, 80 between c and a.Your saying each line to the coil is 80kva making a-b 240kva and so on. now making it a "connected" load of 240 kva.(and then doing the math from there).
Rick

 

[rattus]

Re: unbalanced load calc for 3 phase delta system

engy,

You have just emphasized the fact that formulas are valuable tools when properly applied. I would also add that all these formulas are derived from proven theory. Furthermore, one should understand the basis of the formula even if one does not understand the theory behind it.

Rattus

 

[rattus]

Re: unbalanced load calc for 3 phase delta system

Terminology:

I just dragged out my ancient AC circuits book. It says in so many words that "line voltage" means "line to line voltage", delta or wye.

 

[kalanjeya]

Re: unbalanced load calc for 3 phase delta system

Originally posted by rnolan:
What are the amps for each phase and how is this calculated? What size (amp rating) panel is needed?

.

So , What is the conclusion here ?

Ia = ?

Ib = ?

Ic = ?

Please .........

Jeyakumar

 

[ramsy]

Re: unbalanced load calc for 3 phase delta system

rattus wrote:.."Line voltage" always means line-line voltage ..If the engineers and electricians are speaking different languages, we are in big trouble.

I read the 2000 NEC term for "Line" & "line connected" 200.10, 280.21, including Annex D examples, as one phase of the xfrmr. NEC defines "line-voltage (120 volts nominal" in 550.15(H). "Line Isolation Monitors" also measure "line to ground" current in 517.2, as opposed to "phase-to-phase" or "line-to-line."

Pa = Vline x Iline x 1.732 for NEC convention could be written;

VA = E(?-to-?) * I(?-to-?) * Sqrt(?), (balanced)

Works for 1-12? open/closed delta or wye.

For Delta(V) & WYE series coils:
I? = I?-to-?
E sums 1? or vectors * Sqrt(?)

For open Delta parallel coils:
E? = E?-to-?
I sums 1? or vectors * Sqrt(?)