Transformer protection

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Ponchik

Ponchik

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Electronologist
Please help me to clarify my understanding

Supply voltage is 240 Delta
Load is 208V rated at 45KVA

Need a 45KVA transformer

I size the secondary conductors and the secondary fuses as follows:
125A x 1.25 = 156 A
Secondary conductors 2/0
Secondary fused disconnect with 150A fuses. Maximum fuse FLA 125 x 175% = 218A

Primary side:
Supply voltage 240
45000/240/1.732 = 108A

Primary conductors
108 x 1.25 = 135 A = 1/0
Primary side OCPD 150A circuit breaker


Am I missing anything?

Is my understanding correct?


TIA.
 
You should size your secondary side conductors and OCPD to your load, although many people work from the transformer out like you did.

The primary side protective device is allowed to exceed, up to 250%, the primary full load current if the secondary side protection is limited to 125%.
It is extremely common that a primary side device at 125% will trip during the transformer turn on due to inrush current.
 
jim dungar said:
You should size your secondary side conductors and OCPD to your load, although many people work from the transformer out like you did.

The primary side protective device is allowed to exceed, up to 250%, the primary full load current if the secondary side protection is limited to 125%.
It is extremely common that a primary side device at 125% will trip during the transformer turn on due to inrush current.
Click to expand...
I am 99% sure the following statement is correct: The primary side protection of up to 250% does not mean the primary conductors get upsized accordingly.
 
Ponchik said:
I am 99% sure the following statement is correct: The primary side protection of up to 250% does not mean the primary conductors get upsized accordingly.
Click to expand...
The way I look at it is the feeder conductors get sized per Article 215.2 or whatever. Then 450 modifies Article 215 and allows the larger OCPD for the inrush.
 
electrofelon said:
Yes they do get upsized. There isn't a "free lunch" to set the primary side protection at 250% but not have the conductors rated for that current.
Click to expand...
If the load is fixed at 45KVA, why does the conductor size increase. The OCPD "can" increase due to inrush, but why the conductors?
 
Ponchik said:
If the load is fixed at 45KVA, why does the conductor size increase. The OCPD "can" increase due to inrush, but why the conductors?
Click to expand...
The primary conductors get sized to 125% of the rated primary current. The OCPD for the primary takes care of the short-circuit protection, hence you must choose a CB or fuse that will trip on short-circuits in the transformer but will not trip on momentary overloads. You are allowed by code to go as high as 250% of the primary rated amps (to take care of inrush) but see if the instantaneous trip of the breaker of choice will be enough if a short circuit happens.
 
Ponchik said:
If the load is fixed at 45KVA, why does the conductor size increase. The OCPD "can" increase due to inrush, but why the conductors?
Click to expand...
Remember, article 450 is about transformers only, no conductors. The transformer primary conductors are just "normal" conductors and must be protected at their rating. There's no special allowance that allows the ocpd for these conductors to be higher than normal
 
topgone said:
The primary conductors get sized to 125% of the rated primary current.
Click to expand...
Is this because your considering the transformer a continuous load?
I think of transformer primaries much like motor circuits see 240.4(G) -> 450.3
My understanding is the primary feeders are sized to the calculated load per article 215.
 
electrofelon said:
Remember, article 450 is about transformers only, no conductors. The transformer primary conductors are just "normal" conductors and must be protected at their rating. There's no special allowance that allows the ocpd for these conductors to be higher than normal
Click to expand...
If my conductors are sized according to the rating of the transformer and 450.3(B) allows higher OCPD, isn't that exactly opposite to :
There's no special allowance that allows the ocpd for these conductors to be higher than normal
 
tortuga said:
I think of transformer primaries much like motor circuits see 240.4(G) -> 450.3
Click to expand...
You can think of them that way......Right now I like to think of them as a warm slice of apple pie with a scoop of vanilla ice cream, but unfortunately secondary tie conductors are the only thing in article 450 referenced in 240.4(G) - no ice cream, no transformer primary conductors :confused:
 
Ponchik said:
Please help me to clarify my understanding

Supply voltage is 240 Delta
Load is 208V rated at 45KVA

Need a 45KVA transformer

I size the secondary conductors and the secondary fuses as follows:
125A x 1.25 = 156 A
Secondary conductors 2/0
Secondary fused disconnect with 150A fuses. Maximum fuse FLA 125 x 175% = 218A

Primary side:
Supply voltage 240
45000/240/1.732 = 108A

Primary conductors
108 x 1.25 = 135 A = 1/0
Primary side OCPD 150A circuit breaker


Am I missing anything?

Is my understanding correct?


TIA.
Click to expand...
I thought transformer primary current for single phase was calculated at transformer 45KVA / primary voltage Then conductors selected at amperes x multiplyer according to code

Secondary conductors being calculated at transformer KVA / secondary voltage x multiplier

But it seems you are sizing conductors not based on total transformer ampere output but based on total ampacity of connected load(s)?

Which is correct? Thanks

And secondary
 
Jpflex said:
But it seems you are sizing conductors not based on total transformer ampere output but based on total ampacity of connected load(s)?

Which is correct?
Click to expand...
For <600 volt transformers, there is no requirement to size the conductors relative to the transformer. For medium voltage transformers, the feeder supplying the primary must be rated equal to or greater than the transformer full load. Most of the time, the conductors will be sized to their respective OCPD's, and those will be sized to the transformer protection ratings given in 450.3.

Say you have a transformer twice as big as you need but it is deemed worth using, and dealing with the increased losses, rather than buying the properly sized unit. You are welcome to supply that with conductors and ocpds relative to your load (again for low voltage transformers only) , but you will likely get into issues with the primary ocpd tripping when starting the transformer.
 
electrofelon said:
For <600 volt transformers, there is no requirement to size the conductors relative to the transformer. For medium voltage transformers, the feeder supplying the primary must be rated equal to or greater than the transformer full load. Most of the time, the conductors will be sized to their respective OCPD's, and those will be sized to the transformer protection ratings given in 450.3.

Say you have a transformer twice as big as you need but it is deemed worth using, and dealing with the increased losses, rather than buying the properly sized unit. You are welcome to supply that with conductors and ocpds relative to your load (again for low voltage transformers only) , but you will likely get into issues with the primary ocpd tripping when starting the transformer.
Click to expand...

You are speaking of source primary or secondary voltage? not KVA transformer rating being under 600 volts correct?
 
This thread is a good example of how the NEC has made sizing transformer conductors and OCPD's so confusing. Can someone figure out a way to rewrite this section to make it user friendly?

I nominate Don. :giggle:
 
infinity said:
This thread is a good example of how the NEC has made sizing transformer conductors and OCPD's so confusing. Can someone figure out a way to rewrite this section to make it user friendly?

I nominate Don. :giggle:
Click to expand...
450 is not so bad once you understand it has nothing to do with the secondary side conductors, which are covered by 240 and often 408 when a panel board is involved.

450 is all about the maximum primary side protection. The default is 125%, but it can be higher only if an appropriately sized secondary side device exists.
 
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