Wiring size for primary & secondary transformer

[AmazingTrans]

Hi there,

I have overcurrent question about circuit breaker. I have been using the rule of thumb for continuous load where if it is a fuse I size the size of the fuse to 125%, and if it is a ckt. brkr, I size to 150%.
Of course i could not find anythign about the 150% for ckt brkr. So i would like to revise this rule I knew.


1) Typically, do i size to 125% or 150% or more for a circuit breaker?

2) What is it is for a adjustable circuit breaker?

3) What is it is for a fixed circuit breaker?

4) I don't understand this phrase. Does it mean if i have a standard rating of a fuse and fixed circuit breaker, the 125% does not apply ? what is a standard rating of a fuse mean as well?
"Where 125% of the primary current doesn't correspond to a standard rating of a fuse or nonadjustable circuit breaker as listed in 240.6(A), the next higher rating in Table 450.3(B) is acceptable (Note 1)."

5) If i have a transformer of 250kVA, where the primary is 480VAC, and secondary 400Vac with a fixed and or adjustable circuit breaker of 400Amps, what do i size my primary wire going into the transformer and going out of the transformer? Based of the NEC Ampacity chart of 90°C (http://www.usawire-cable.com/pdfs/nec%20ampacities.pdf)
From my calcs:
Primary : 300.71 Amps (250,000 / 480 / 1.732)
Primary x 125% : 375.89 Amps

Secondary : 360.85 Amps (250,000 / 400 / 1.732)
Secondary x 125% : 541.28 Amps

Hope to hear from everyone!

 

[Paul1955]

Standard ampere ratings are listed in 240.6. The next higher standard size fuse or CB can be used if your calculations don't equal a standard size.

I think your calcs for the secondary should be 451A instead of 541?

 

[Paul1955]

Mike Holt had a great article in the December EC&M magazine that goes in to detail on transformer calculations:

http://ecmweb.com/code-basics/transformer-calculations

 

[augie47]

Table 450.3 is a guide for transformer protection. It does not address conductors and their protection.
If you elect to protect the transformer by protecting the transformer at 125% you can chose the next higher breaker (standard or adjustable) or fuse.
One you select that breaker or fuse you choose a conductor that is sized to carry the current
allowed by that breaker.
110,14 would normally require you select a conductor based on it's 75° rating.
(The 90° rating is most often used only for de=-rating purposes)

 

[topgone]

TRANSFORMER PROTECTION USING CIRCUIT BREAKERS

TRANSFORMER PROTECTION USING CIRCUIT BREAKERS

To do away with tripping of main overcurrent protection due to energization inrush, you can opt to choose a primary breaker sized not more than 250% of transformer FLA and use the secondary breaker to protect against overload currents (125% of transformer FLA).

 

[don_resqcapt19]

To do away with tripping of main overcurrent protection due to energization inrush, you can opt to choose a primary breaker sized not more than 250% of transformer FLA and use the secondary breaker to protect against overload currents (125% of transformer FLA).

But that also requires larger primary feeder conductors. They have to have an ampacity equal to or greater than the rating of the primary OCPD.

Also remember that the rules in Article 450 are only for the protection of the transformer itself and you have to look to Article 240 for the protection of the conductors. Specifically to 240.21(C) for the protection of the secondary conductors. Most of the time, you can use a single OCPD to protect both the transformer secondary and the secondary conductors.

 

[topgone]

But that also requires larger primary feeder conductors. They have to have an ampacity equal to or greater than the rating of the primary OCPD.

Also remember that the rules in Article 450 are only for the protection of the transformer itself and you have to look to Article 240 for the protection of the conductors. Specifically to 240.21(C) for the protection of the secondary conductors. Most of the time, you can use a single OCPD to protect both the transformer secondary and the secondary conductors.

I beg to differ. Your primary supply conductors need not be upsized just because you have chosen an OCPD at 250%. The current that goes thru the secondary, goes thru the primary though smaller in proportion. We all know that the primary current is just secondary amps divided by the transformation ratio and the sizing of conductors are fixed at 125% of each expected FLA.

When the secondary current becomes overloaded, the secondary OCPD trips accordingly. The primary current also disappears at the same time. The tripping secondary breaker also protects your primary windings and the supply conductors. You are only using the primary breaker as your protection against short circuit of the equipment, not overload protection.

 

[augie47]

I beg to differ. Your primary supply conductors need not be upsized just because you have chosen an OCPD at 250%. The current that goes thru the secondary, goes thru the primary though smaller in proportion. We all know that the primary current is just secondary amps divided by the transformation ratio and the sizing of conductors are fixed at 125% of each expected FLA.

When the secondary current becomes overloaded, the secondary OCPD trips accordingly. The primary current also disappears at the same time. The tripping secondary breaker also protects your primary windings and the supply conductors. You are only using the primary breaker as your protection against short circuit of the equipment, not overload protection.

I beg to say you are incorrect. There is nothing in 240.4 that allows you to vary from "conductors shall be protected against overcurrent in accordance with their ampacities" for, transformer primary conductors. The Code does take into account the ratio effect you
describe for secondary conductors in 240.4(F) but even then only for two-wire-two wire or delta-dealta transfomrer secondary conductors.

 

[AmazingTrans]

You are right, i calc that at 150% instead.

Standard ampere ratings are listed in 240.6. The next higher standard size fuse or CB can be used if your calculations don't equal a standard size.
I think your calcs for the secondary should be 451A instead of 541?

 

[AmazingTrans]

Can someone explain more on this, "you can opt to choose a primary breaker sized not more than 250% of transformer FLA"
I've seen this is many articles, and it says not more than 250%.
Do i evaluate this breaker depending on the inrush current? and as long as i maintain below 250% i am fine? (So, it can be 150% or 175% etc etc up to 250% right?)

To do away with tripping of main overcurrent protection due to energization inrush, you can opt to choose a primary breaker sized not more than 250% of transformer FLA and use the secondary breaker to protect against overload currents (125% of transformer FLA).

 

[AmazingTrans]

After reading everyone reply, i see that i can use 125% or more of up to 250%(for in-rush application) with circuit breaker.
Then, i guess what's the different with using fuses then? I size the fuse of 125% as well. Wouldn't i be using fuse over ckt. brkr. since it is cheaper?

 

[Dennis Alwon]

After reading everyone reply, i see that i can use 125% or more of up to 250%(for in-rush application) with circuit breaker.
Then, i guess what's the different with using fuses then? I size the fuse of 125% as well. Wouldn't i be using fuse over ckt. brkr. since it is cheaper?

You could use fuses -- the choice is yours but the percentage is the same for either I believe

 

[AmazingTrans]

Also, just say if i have a 200kva transformer , and at 125% it is 300.7Amps, can i just use a 300Amps breaker or do i have to up it even it is more that 0.1 Amps at 125%.

 

[jumper]

Also, just say if i have a 200kva transformer , and at 125% it is 300.7Amps, can i just use a 300Amps breaker or do i have to up it even it is more that 0.1 Amps at 125%.

You can use a 300A breaker. Upsizing not required.

 

[bhellmer]

The NEC tap rule in 240.21 (B) (3) allow transformer primary conductor sizing down to 1/3 of primary circuit breaker trip, but only when primary plus secondary conductors are not greater than 25'.

 

[petersonra]

After reading everyone reply, i see that i can use 125% or more of up to 250%(for in-rush application) with circuit breaker.
Then, i guess what's the different with using fuses then? I size the fuse of 125% as well. Wouldn't i be using fuse over ckt. brkr. since it is cheaper?

The 125% thing is not a minimum. You can make it 2% if you want. Might not be practical, but would meet code requirements.

There is a break even point where CBs are usually more cost effective than fuses. For me it is usually 100 or 200 amp frame sizes. I rarely if ever use fuses above 200A. A MCCB is almost always more cost effective.

Depends on various things such as whether I can also use the CB as the disconnecting means if needed, and sometimes the available SCC. It also depends on what our customer will accept. Some want brands of MCCB that are insanely priced (like AB or Eaton) while others will accept a more moderately priced MCCB. Larger frame MCCB tend to take up less room in a cabinet than the equivalent fused switches, or even just fuse blocks for larger sizes, so there can be cost savings there as well.

YMMV depending on your own situation and what your supplier charges you for various things. There is an astounding range of prices that electrical suppliers charge for the same part to different customers. One customer might be paying 11% of list price, while the next guy coming in the door pays list. It is a nutty game they play with discounts.

 

[bhellmer]

For low-voltage transformers, primary and secondary over-current protection trip settings are limited by maximums in NEC Table 450.3(B).

Primary over-current protection trip settings should also be great enough to clear transformer in-rush current. Plotting in-rush current on fuse or circuit breaker trip curves from manufacturer. Multiply transformer rated primary current by 12 and plot at 100ms. Multiply by 25 and plot at 10ms. Select a trip setting whose curve is above and to the right of both in-rush points.