Homework Help! Transformer Calculations and motor circuit

[SirCringe]

It is a design choice for you to go with the maximum value of 1600A. Barring and project, or test question, specs you don't have to be any larger than connected load.

Your secondary conductors need to match your secondary protection. You chose 1600A so you will also need 1600A of conductors.

Have you ever tried to install 1500 kcmil conductors? My experience has been to not design above 500kcmil CU or 750kcmil AL.

Ok I think its making sense now, since the secondary side we have a load of three 3 phase motors. The highest FLA of the three motors is 46.2A so I multiply that by 1.25 and add 30.8 A and 22 A (Other 2 motors) coming out with a total of 110.55 A. So a simple 2 AWG conductor can handle that amperage. So my secondary conductor from the transformer can be sized at 2 AWG? But what about the secondary overcurrent protection device? Would I just size it at my secondary fla load of 110.55 A * 1.25% ? 138 A rounding it up to a 150 A over current protection device?

 

[jim dungar]

But what about the secondary overcurrent protection device? Would I just size it at my secondary fla load of 110.55 A * 1.25% ? 138 A rounding it up to a 150 A over current protection device?

You could maybe, I don't know your instructor.

Is there a reason they spec'd a 500 kVA transformer. Do you need to allow for future growth in your equipment? If the usage grows in the future will they just provide additional secondary side equipment.
As JimD pointed out from the inf given we don't know if the instructor wants you to consider the full output current available from the transformer or just the require current for the motor load given.
Keep in mind Art 450 is for transformer protection only. Once you select the amperage of the OCP device you choose to use, you need to have conductors sized with those devices in mind.

 

[don_resqcapt19]

So here's what I got,
My over current protection device is 1600 A due to multiplying my FLA of 1204 by 125%.

Then I chose 2 parallel conductors of 1500 kcmil THWN Copper since one of them has a rating 625 A so by doubling it I get a amp rating of up to 1250A.

The round up rule never applies to transformer secondary conductors, so if you have a 1500 OCPD, you will need 1600 amps of conductors.
In my opinion conductor sizes larger than 600 kcmil are never practical both from an installation perspective and amount of copper.
For example, four sets of 600 kcmil will give you 1680 amps of conductor using a total of 2400 kcmil in place of your propose 3000 kcmil. Five sets of 400 would work with a total of 2000 kcmil, and six sets of 300 would use 1800 kcmil of copper.

 

[SirCringe]

So for the primary and secondary overcurrent protection device I've been getting two answers. From my understanding from nec 450.3(b) since I'm using less than 1000 volts and need overcurrent protection device for primary and secondary it states to use 250% for the primary and 125% for the secondary. Buy it does say max. So where in the nec does it say I can use 125% for the primary because that's what I'm being told I can use.

 

[augie47]

Same Table {450.3(b)}... If you elect to use primary protection only the maximum is 125% (next size up)
(For a single transformer 250% is seldom used as your OCP & primary conductor are more expensive)
Remember Art 450 is for transformer protection only. You would still need an OCP device for your panel (408.36)

(Make sure your question stated 500kva and nnot 50 kva )

 

[SirCringe]

Same Table {450.3(b)}... If you elect to use primary protection only the maximum is 125% (next size up)
(For a single transformer 250% is seldom used as your OCP & primary conductor are more expensive)
Remember Art 450 is for transformer protection only. You would still need an OCP device for your panel (408.36)

(Make sure your question stated 500kva and nnot 50 kva )

It definitely is 500kVA. If I'm being asked to use primary and secondary protection so 125% will be fine for primary and secondary?

 

[augie47]

It definitely is 500kVA. If I'm being asked to use primary and secondary protection so 125% will be fine for primary and secondary?

It would meet Code. The whole scenario is so far removed from a "real life" installation I have no idea what your class wants.

 

[topgone]

It definitely is 500kVA. If I'm being asked to use primary and secondary protection so 125% will be fine for primary and secondary?

Please remember that there is a significant inrush when you close the primary breaker/ switch of the transformer, thereby making a 125% primary protection setting trip most of the time. For a 500 kVA transformer, the inrush could be 2 to 6.5 X the full load amps. Unless your breaker has adjustable short-time overcurrent settings or is equipped with an electronic trip unit, a higher current setting to consider the inrush current can be had while the long-time setting is set at 125%.

 

[Tulsa Electrician]

You are a newly hired electrician at chemical processing facility. You have been asked for a preliminary assessment of a proposed installation. Given a 3-phase 500KVA 480/240 Delta-Delta Transformer, determine both the primary and secondary side protection. The transformer will feed a service panel 15 ft away, what size THWN conductors are required?


The service panel for the transformer problem feeds 3-3phase motor loads.
208V, 10 hp, S.F. 1.0, 250ft away from panel
208V, 15 hp, S.F. 1.10 300 ft away from panel
230V, 7 1/2 hp S.F. 1.10 150 ft away from panel
What are required starter sizes?, What are required overload heater sizes?, What are required feeder sizes (conductors)-limit VD to 3% and note VD is per phase. Size required feeder protection. Provide conduits sizes for all feeders (EMT).

Thanks in advance!

I would give them an assessment they ask for.

Provide them with your one line.
Provide your motor load information and sizing.


Assessment Ansawer: Based on provided information. The 500 kva and associated wireing will be excess for the loads given.

Recommendation: Review and revise.

Question: Reason for 240v delta.

Comment: 208 volt motors, requesting 240 delta system.
If additional KVA is for future loads a 120/208 wye may be a better choice. Then resize motor information for 208v.