1 mva transformer

[chris kennedy]

269 amps load, how would i do it, v drop

Need to know how many sets of what size conductors you would use to determine (R).

269A secondary load on a 1MVA transformer? Transformers like to operate close to their potential.

 

[RUWired]

269 amps load, how would i do it, v drop

There are the basic voltage drop formulas as well as........
(C) Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general formula:
where:
TC= conductor temperature in degrees Celsius (?C)
TA= ambient temperature in degrees Celsius (?C)
TD= dielectric loss temperature rise
RDC= dc resistance of conductor at temperature TC
YC= component ac resistance resulting from skin effect and proximity effect
RCA= effective thermal resistance between conductor and surrounding ambient
FPN: See Annex B for examples of formula applications.



But with that many secondary conductors, conduit type, overhead under ground and configure will make a difference

 

[badgerfan08]

Need to know how many sets of what size conductors you would use to determine (R).

269A secondary load on a 1MVA transformer? Transformers like to operate close to their potential.

wouldnt i have to size the wire to the secondary breakers size, 3000a. so i could use 8 500 kc/mils per set

 

[bob]

269 amps load, how would i do it, v drop

If you only have 269 amps, why do you want a 3000 amp breaker. This is sounding like you need some help with this project. This forum is not a design forum. WE can not design this for you. You need some one with experience in this area to assist you.

 

[badgerfan08]

If you only have 269 amps, why do you want a 3000 amp breaker. This is sounding like you need some help with this project. This forum is not a design forum. WE can not design this for you. You need some one with experience in this area to assist you.

it is a hypothetical question. I am just learning this stuff, this is the info the teacher gave us. Its fictional Bob, I'm just looking for some guidance.

 

[mayanees]

EE Response

EE Response

I saw a comment "I'm not an engineer, but what I'd do is..."
So here's a response from an engineer....
... which is no more qualified than a response from an electrician, because we're all following the same code! specifically NEC Table 450.3(B)

In addition to adherence to the code, there's the other major factor... the load.

If the load has significant inrush potential, size the primary OverCurrent Protective Device (OCPD) to 250% with cable to match. Secondary at 125%. That's also the safest cover-your-a$$ call. Any loads that trip the primary OCPD are too large for the transformer. It's also the most expensive approach for the user because the wire is so much larger.
If the load is light, resistive in startup nature, or there are many many small loads, then you could reduce the size of the primary OCPD. Even to the point where you could use 125% primary only protection per the table.
But the risk there is the magnetizing inrush of the transformer. The primary OCPD must be able to withstand that inrush, which can be 8-12X the fla of the transformer, for an 8-20 cycle duration...

The last engineering firm I worked for standardized on 175% for the primary. It hasn't failed them in the last 5 years.

Bit it's hard for me to drop down from the 250%, especially now because I'm in the critical power market, where my mission is to maximize uptime.

The best way to look at this is on a Time-Current-Curve where you can plot the characteristics of all the devices in order to assure the transformer magnetizing inrush is tolerable with the chosen OCPDs.

John M

 

[relochris]

I appreciate all of the suggestions and links you all have offered. I take no offense at the harsh words of some. I agree that I need to start being more of a business man or things are going to start slipping away. More great food for thought from this forum. Thank you