what % is a xformer primary Likely to trip at?

[travish]

450.3 (B) ocp in both primary and sec. 480 - 240/120 150KVA

Primary ocp sized at 250% Secondary ocp sized at 125% you have to feed the transformer with the same size wire that is on the secondary side, 450a wire. My application transformer will be right beside the panel feeding it, and right next to that the panel that the transformer is feeding will be mounted, so there is not alot of cost for the wire, but it seems unrealistic to feed with 1 size wire, double the amps thru the xformer then leave the xformer with the same size wire that came in at 1/2 the amps. I do understand inrush current (I think).

How likely is it that the primary ocp would trip if sized at 125% or some number lower than 250?

Is there a rule of thumb that you use?

 

[kwired]

but it seems unrealistic to feed with 1 size wire, double the amps thru the xformer then leave the xformer with the same size wire that came in at 1/2 the amps.

What you are forgetting here is that the secondary voltage is not the same as the primary voltage.

If you multiply primary volts x amps you get same VA as if you multiply secondary volts x amps, disregarding any inefficiencies which will be small anyway.

So your 150 KVA transformer at 150KVA load will draw 312.5 amps on primary and 625 amps on secondary - but power (VA) is same for both.

 

[travish]

sorry Kwired, I must not have stated the question clearly

pri 150000/480=312/1.73=180a x 250% = 450a ocd
sec 150000/240=625/1.73=360a x 125% = 450a ocd

so the trans pri can be protected by 250% and sec can be protected by125% since we cannot run wire less than the ocd it has to be 700mcm (or paralleled) on both sides

or am i missing something

 

[kwired]

sorry Kwired, I must not have stated the question clearly

pri 150000/480=312/1.73=180a x 250% = 450a ocd
sec 150000/240=625/1.73=360a x 125% = 450a ocd

so the trans pri can be protected by 250% and sec can be protected by125% since we cannot run wire less than the ocd it has to be 700mcm (or paralleled) on both sides

or am i missing something

What if I told you numbers you get from 450.3(B) are maximum ratings or settings and you can go lower if you wish?

 

[travish]

yea i thought that, see the last 2 questions in the 1st post

 

[kwired]

If you think about it - a transformer with secondary protection is protected from overload by the secondary overcurrent device. The primary device if at maximimum allowed level is only going to protect from short circuits and ground faults on the primary side, and will do so just fine sized at 250%.

If the transformer is allowed to have primary protection only then this protection needs sized closer to rating otherwise it provides no overload protection.

 

[travish]

I am talking about inrush current tripping the primary breaker on the transformer, sorry for the confusion.

is inrush current dependant upon what loads on the secondary are connected at the time of energization, or is it just dependant on the size of the coil in the transformer?

 

[jim dungar]

is inrush current dependant upon what loads on the secondary are connected at the time of energization, or is it just dependant on the size of the coil in the transformer?

The magnitude of the inrush is primarily a factor of the design of the transformer, it is almost never a function of the load.
In fact, there is quite a bit of anecdotal evidence that a transformer with a secondary load has a lower inrush current than an unloaded one.

 

[gar]

120628-2043 EDT

travish:

I do not know anything about 450.

But consider this logic. If the OCP was designed to protect the transformer, then it would be sized much closer to the transformer rating, maybe like the 125%. If it were sized like this to protect the transformer, then you would likely have an excessive number of trips when applying power to the transformer from transformer inrush current.

The primary OCP is probably allowed to be sized much larger to tolerate inrush. Now what purpose does this OCP serve. It can trip on very large overloads, but you want the wire leading to the OCP to be sized for the rating of the OCP.

Since the OCP % ratio was 2 to 1 and the transformer ratio was 2 to 1 this made primary and secondary wires sizes the same. As an example it might have been better if the transformer ratio was 1 to 1 or 4 to 1, then the wire sizes would not have been the same.

Back in the early 1960s I invented a circuit for a circuit breaker company that would tolerate very high inrush current yet it could be set to trip as close as you wanted to any value. For example 1% over your desired maximum load.

This obviously is not a simple thermal trip breaker.

For some inrush current waveforms see photos P1 thru p8 at my web site http://beta-a2.com/EE-photos.html

.

 

[travish]

thanks Jim,

that is the info that I was looking. Appoligize to all for being questions not being asked very clear,

looks like manufactures could publish the suspected or calculated inrush if it is design related. could save the end user some money on sizing the primary circuit

Travis

 

[kwired]

thanks Jim,

that is the info that I was looking. Appoligize to all for being questions not being asked very clear,

looks like manufactures could publish the suspected or calculated inrush if it is design related. could save the end user some money on sizing the primary circuit

Travis

The amount of inrush current is going to vary with conditions in the field, like size and length of conductors, not only for the transformer feed but all the way back to the source, the amount of current the source is capable of delivering, whether or not conductors are run in magnetic or non magnetic raceway, loading and voltage conditions of other equipment at time of energizing the transformer, and of course the deenergized impedance of the transformer - which is one thing the manufacturer will know.

This is true for any inductive load when first energized.

 

[steve66]

450.3 (B) ocp in both primary and sec. 480 - 240/120 150KVA

Primary ocp sized at 250% Secondary ocp sized at 125% you have to feed the transformer with the same size wire that is on the secondary side, 450a wire. My application transformer will be right beside the panel feeding it, and right next to that the panel that the transformer is feeding will be mounted, so there is not alot of cost for the wire, but it seems unrealistic to feed with 1 size wire, double the amps thru the xformer then leave the xformer with the same size wire that came in at 1/2 the amps. I do understand inrush current (I think).

How likely is it that the primary ocp would trip if sized at 125% or some number lower than 250?

Is there a rule of thumb that you use?

If the length of wiring between the primary OCP and the secondary OCP is less than 25', you can use the tap rule in 240.21(A)(3) and reduce the wire size on the primary.

 

[hurk27]

I'm a little confused??? the OP is asking about the size of the conductors being the same as the secondary conductors, I have never sized the primary conductors to the rating of the OCPD just like in motor startup, you size the conductors for the load served and the OCPD for the inrush up to 250% (assuming proper secondary protection)

If so, why would you have primaries as large as the secondaries ?:huh:

 

[travish]

I did not think there was an exeption to allow sizing the primary conductors for the load and the the breaker for the inrush

 

[david luchini]

I'm a little confused??? the OP is asking about the size of the conductors being the same as the secondary conductors, I have never sized the primary conductors to the rating of the OCPD just like in motor startup, you size the conductors for the load served and the OCPD for the inrush up to 250% (assuming proper secondary protection)

If so, why would you have primaries as large as the secondaries ?:huh:

The primary conductors must be protected per 240.4. You can size the primary OCPD up to 250%, but the conductors should be sized per the primary OCPD size.

 

[kwired]

The primary conductors must be protected per 240.4. You can size the primary OCPD up to 250%, but the conductors should be sized per the primary OCPD size.

Or the 25 foot feeder tap rule in 240.21(B)(3) allows higher overcurrent protection - with conditions.

 

[steve66]

I did not think there was an exeption to allow sizing the primary conductors for the load and the the breaker for the inrush

There is for limited lengths:

If the length of wiring between the primary OCP and the secondary OCP is less than 25', you can use the tap rule in 240.21(A)(3) and reduce the wire size on the primary.

Edit: Kwired has the reference right: It should be (B)(3), not (A)(3).