Transformer Power

[jross]

Here?s my problem: Have a three-phase Delta 450VAC 30A source through a 25A distribution breaker to three single-phase Delta 5KVA 450/120VAC step-down transformers. Are the following assumptions true? If not, where am I going wrong?

Source Power to Dist Panel: 450VAC x 30A x 1.732 = 23.382 kVA (30A per Phase)

Source Power to Transformers: 450VAC x 25A x 1.732 = 19.485 kVA (25A per Phase)

Output Power of banked 5KVA XFMRs: 15000/120VAC/1.732 = 72.17A per Phase

Based on above I am assuming I have 72.17 x 3 phases = 216.51A of single phase capacity to operate systems equipment.

Is this true?

Thanks,

J. Ross

 

[bob]

Re: Transformer Power

Output Power of banked 5KVA XFMRs: No 15000/120VAC/1.732 = 72.17A per Phase

If you connect your transformers in a 3 phase wye configurations the output will be 208 volts.
Therefore you would take 15000/(208 x 1.73) and
get 41 amps per phase. You do not multiply by 3.

[ July 18, 2004, 07:22 PM: Message edited by: bob ]

 

[jross]

Re: Transformer Power

Thanks for the reply. Based on your calculations (assuming secondary is wired Wye), I have 41 Amps per phase for a total of 123 Amps for system equipment.

Any difference if the secondary is wired DELTA?

J. Ross

 

[charlie b]

Re: Transformer Power

Originally posted by jross: Based on your calculations (assuming secondary is wired Wye), I have 41 Amps per phase for a total of 123 Amps for system equipment.

No, you have 41 amps per phase for a total of 41 amps for system equipment. Actually, 41.6 is closer to the actual amps. You are making the error of adding ?amps? to ?amps,? and thinking that the answer will be ?amps.? When dealing with 3-phase systems, an ?amp? is not an ?amp.? What you are adding is ?amps at a zero degree phase angle? to ?amps at a 120 degree phase angle? to ?amps at a 240 degree phase angle.? You can add apples to oranges to bananas, and you can express your result as ?fruit,? but you cannot express the result as either ?apples,? or as ?oranges,? or as ?bananas.?

Any difference if the secondary is wired DELTA?

Yes. Your earlier calculation (15000 / (120 * 1.732) = 72.17) applies here. Please note that 41.6 amps at 208 volts represents the same amount of power as 72.17 amps at 120 volts. The constant factor here is the power available from the bank of transformers.

 

[Ed MacLaren]

Re: Transformer Power

Jross, try this sometime. Put your clamp-on ammeter around each wire of a three phase circuit that has a balanced load.
Let's say, for example, the meter reads 10 amps on each wire. Now, clamp the meter around any two wires. The reading will still be 10 amps.

Referring to the sketch below, what do you think the reading would be if you clamp the meter around all three wires?

With regard to the transformers described in your question, you could operate three single phase loads, each rated at 41.6 amps, 120 volts, without overloading the transformers.

Ed

[ July 19, 2004, 11:53 AM: Message edited by: Ed MacLaren ]

 

[jross]

Re: Transformer Power

Thanks guys?, opened my eyes to quit a lot.

Ed - The answer is "10 Amps."

The referenced bank of 5 kVA transformers is wired 120VAC Delta to the A/B/C bus of a power distribution panel. I now understand this panel, and all it?s single phase 120 VAC loads (i.e., A-B, B-C, C-A), represent the ?load? seen by the three-phase output of the transformer bank and that I must not wire up individual 120VAC equipment loads that exceed a total of 72 Amps.

The Engineer on this shipboard project is having a nice vacation. I?m just the installer trying to answer a bunch of questions coming out of the walls at me. The tech data sheet for the system being installed states the total Amp requirement is 150 Amps. It?s now obvious to me the existing bank of 5 kVA transformers won?t meet the bill.

Based on the power source being provided (i.e., 450VAC, 30A, 23382 VA), is it possible to replace the 25A breaker in the 450VAC panel with a 30A breaker and replace the bank of transformers with higher rated units (or a single 3-phase unit) to get the full 23382 VA to the system loads? (quickie calc = approximately 112 Amps made available). This gets us closer to the requirement, but we?ll still probably need to obtain another feed from Engineering spaces.

Thanks for your patience and putting up with my ignorance.

jross

 

[charlie b]

Re: Transformer Power

Jross: You still have a few fundamental misconceptions.

Originally posted by jross: Ed - The answer is "10 Amps."

No, the answer is ?Zero Amps.? If you have a delta system, and clamp an ammeter around all three phase conductors, the reading must necessarily always be zero. If it is not, then you have a ground fault. Similarly, if you have a wye system, and clamp an ammeter around all three phase conductors and the neutral, the reading must necessarily always be zero. Here again, if it is not, then you have a ground fault. This is a requirement driven by Kirchhoff?s Current Law, which can be stated as follows, ?The sum of all currents entering and leaving a single node is zero amps.?

I now understand this panel, and all it?s single phase 120 VAC loads (i.e., A-B, B-C, C-A)

I fear that you do not yet understand this panel. If the loads are single phase, 120 volts, then they would be wired A-N, or B-N, or C-N. If instead you are wiring a load A-B, or B-C, or C-A, then you are giving the loads a single phase 208 volt power supply.

The tech data sheet for the system being installed states the total Amp requirement is 150 Amps. It?s now obvious to me the existing bank of 5 kVA transformers won?t meet the bill.

Is that requirement for 150 amps at 120 volts? If so, you need 150 x 120 x 1.732, or 31kVA. At present, you have only half that.

Based on the power source being provided (i.e., 450VAC, 30A, 23382 VA), is it possible to replace the 25A breaker in the 450VAC panel with a 30A breaker and replace the bank of transformers with higher rated units (or a single 3-phase unit) to get the full 23382 VA to the system loads?

First, you need 31kVA for the system loads, not just 23.4kVA. You can get that from a 30kVA transformer (or bank of three 10kVA transformers wired as a delta). But the rated primary current (at 450 volts) would be 38.5 amps. Sure, you could use a 30 amp primary breaker, but it would give you frequent false trips. NEC allows the primary of a transformer to be protected by up to 250% of the primary rated voltage. That means you could use a 100 amp primary breaker. But that is not the only issue. You would need to verify that the primary feeder conductors are capable of handling the current that you propose to put through them.

Please do not take this wrong, but I think you need to let the engineer get back from vacation, and not try to design this project yourself. There are too many variables that you need to understand, and I don?t think you are ready yet.

 

[jross]

Re: Transformer Power

Will do Charlie B...,

By the way..., US Navy shipboard 120 VAC Delta distribution panels don't have N. Volts are phase to ground (ship hull)= 60 VAC, Phase to Phase = 120 VAC (if everything balanced). Wiring to equipment is indeed A-B, B-C, C-A. I'm a bit slow, but every Navy ship I've been on is wired this way.

This ends this string.

 

[charlie b]

Re: Transformer Power

Originally posted by jross: US Navy shipboard 120 VAC Delta distribution panels don't have Neutrals . . . every Navy ship I've been on is wired this way.

I am well aware of that fact, being a retired Commander in the US Navy Reserve, and having spend three tours in Electrical Departments of nuclear power ships. But I was not aware you were working on a navy ship.

Volts are phase to ground (ship hull)= 60 VAC, Phase to Phase = 120 VAC (if everything balanced).

Untrue, and true, in that order. You are right in saying that phase-to-phase is 120 volts, and that therefore (contrary to what I had said earlier) a single phase 120 volt load would be wired A-B or A-C or B-C. However, the voltage from any single phase to the hull can by anything. Since there is no connection to ground (other than during the occasional checks of the ground test system), the voltage between any point in the distribution system and the hull floats at will.

This ends this string.

Fine by me. Good luck.