Transformer upgrade

[Jason T]

Hi Everyone,

Long time reader of this forum but first time posting.

I have a question regarding a transformer installation. Currently there's a 1500KVA transformer installed feeding a 2000A main disconnect. A proposal was written to upgrade to a 3000KVA transformer in order to have room for future expansion. I am reviewing the proposal and everything seems fine with the cable sizing and load side fused protection.

My question is the AIC rating for the 2000A main disconnect. The main disconnect is rated for 100KA or short circuit current rating, which isn't the same as the AIC rating. According to the calculation the max short circuit fault current should be around 62.2KA for the 3000KVA transformer (based on 480V secondary with a 5.8% impedance). If the short circuit current rating is sufficient to carry the fault current, does it matter what the AIC rating is if we size the primary protection of the transformer accordingly? Or do we have to make sure that the AIC for the main disconnect is rated at least 65KA?

Thanks in advance!

 

[charlie b]

A disconnect does not have an AIC rating. The "I" word is all about the device's ability to Interrupt current, and a simple on/off switch (such as a disconnect) cannot do that. So I am confused by your description. Since the disconnect has an SCCR (100 KA) that is higher than the SCCA (62.5 KA), you are good to go.

 

[kwired]

Hi Everyone,

Long time reader of this forum but first time posting.

I have a question regarding a transformer installation. Currently there's a 1500KVA transformer installed feeding a 2000A main disconnect. A proposal was written to upgrade to a 3000KVA transformer in order to have room for future expansion. I am reviewing the proposal and everything seems fine with the cable sizing and load side fused protection.

My question is the AIC rating for the 2000A main disconnect. The main disconnect is rated for 100KA or short circuit current rating, which isn't the same as the AIC rating. According to the calculation the max short circuit fault current should be around 62.2KA for the 3000KVA transformer (based on 480V secondary with a 5.8% impedance). If the short circuit current rating is sufficient to carry the fault current, does it matter what the AIC rating is if we size the primary protection of the transformer accordingly? Or do we have to make sure that the AIC for the main disconnect is rated at least 65KA?

Thanks in advance!

SCCR is withstand rating of things like bus bars - in particular the mounting methods need to withstand the forces imposed on them when the bus is carrying such a heavy current.

AIC is interrupt rating and applies to anything that makes/breaks the circuit. A disconnecting means will have a AIC rating, and that AIC is probably more limiting overall to the complete disconnect then the SCCR of the remaining fixed components is.

Primary protection doesn't necessarily limit the short time fault current level that can be reached on the secondary, and I would think this gets worse the higher the primary to secondary ratio is, and that is assuming you set the primary protection close to primary rated current, allowing for surge current when energizing or for riding through startup of large motors or similar only worsens that condition as well.

 

[charlie b]

I beg to disagree with these two statements:

AIC is interrupt rating and applies to anything that makes/breaks the circuit. A disconnecting means will have a AIC rating.

I submit that AIC applies to anything that automatically breaks a circuit in response to a high value of current. Thus, an unfused disconnect would not have an AIC rating.

 

[Jraef]

Aside from the main, your available fault current will increase to everything down stream as well. A thorough analysis and protection coordination study should be performed on a change such as that.

 

[Jason T]

A disconnect does not have an AIC rating. The "I" word is all about the device's ability to Interrupt current, and a simple on/off switch (such as a disconnect) cannot do that. So I am confused by your description. Since the disconnect has an SCCR (100 KA) that is higher than the SCCA (62.5 KA), you are good to go.

thank you for the reply! I believe it's a fused disconnect so I guess the fuse rating is the AIC rating? Or am I just completely mixing up terminologies here? :weeping:

 

[Jason T]

Aside from the main, your available fault current will increase to everything down stream as well. A thorough analysis and protection coordination study should be performed on a change such as that.

Thank you for the suggestion, it looks like I will have to dig deeper into this scenario.

 

[iwire]

Seems like leaving the existing system in place and adding a second 1500KVA system would be a easier way to go. Less disruptive and no need to upgrade the existing.

 

[Jason T]

Seems like leaving the existing system in place and adding a second 1500KVA system would be a easier way to go. Less disruptive and no need to upgrade the existing.

that would be a viable alternative.

 

[Mike01]

SCCR

SCCR

To provide an example to what Charlie b already pointed out when you look at non-fused switch [safety switch] from most manufactures you will find a similar table that indicates the non-fused safety switch standard rating is a SCCR not AIC rating. Where the NEC defines the Short-Circuit Current Rating as “The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria”. Most manufacturers list non-fused safety switches at 10kA SCCR, with any manufactures circuit breakers. They will also list higher ratings that can be achieved when the upstream OCPD is a listed upstream circuit breaker or fuse as indicated in the specific switch manufactures literature. A couple of interesting questions come out of this.

1. If a non-fused safety switch is utilized where the rating is increased above 10kA and coordinated with the upstream OCPD [fuse or breaker] does the switch require a “Series Rated Label” since the device is being used above its listed rating based on the OCPD upstream?
3. For increased ratings when using a breaker upstream, it appears you are limited to the “specific manufacturer”, I was looking but could not find if you can mix and match manufactures to get a higher rating as long as the devices is classified as a “3-cycle” device?

 

[Jason T]

1. If a non-fused safety switch is utilized where the rating is increased above 10kA and coordinated with the upstream OCPD [fuse or breaker] does the switch require a “Series Rated Label” since the device is being used above its listed rating based on the OCPD upstream?
2. For increased ratings when using a breaker upstream, it appears you are limited to the “specific manufacturer”, I was looking but could not find if you can mix and match manufactures to get a higher rating as long as the devices is classified as a “3-cycle” device?

I would think if the upstream OCPD is a current limiting fuse that limited a short circuit fault current to below the SCCR of the OCPD down stream it would be ok. Let's say the short circuit fault current is at 65KA but the current limiting fuse caused the peak let through current to be 30KA then all the down stream equipment can be rated at 30KA. Please let me know if that thought process is nuts.

 

[charlie b]

I would think if the upstream OCPD is a current limiting fuse that limited a short circuit fault current to below the SCCR of the OCPD down stream it would be ok. Let's say the short circuit fault current is at 65KA but the current limiting fuse caused the peak let through current to be 30KA then all the down stream equipment can be rated at 30KA. Please let me know if that thought process is nuts.

It's not nuts. But it's not right either, for reasons that are very much non-self-evident. I am only learning about this now, for reasons I described in a separate recent thread about elevator SCCR values.

To start with, don't take the "L" word ("Limiting") at face value. For a CLF to function in its current-limiting capacity, certain conditions must be met. Having a separate OCPD downstream of the CLF can negate the conditions required for the CLF to function. That is because the downstream OCPD possesses what is known as "dynamic impedance." When the downstream breaker starts to open, or the downstream fuse starts to melt, an arc is drawn across the breaker contacts or the fuse ferrules. That arc has impedance, and it limits the fault current that can flow to the point that the CLF is no longer in its operating range. That is not a very good explanation, but as I say I am just learning this concept.

 

[Jason T]

It's not nuts. But it's not right either, for reasons that are very much non-self-evident. I am only learning about this now, for reasons I described in a separate recent thread about elevator SCCR values.

To start with, don't take the "L" word ("Limiting") at face value. For a CLF to function in its current-limiting capacity, certain conditions must be met. Having a separate OCPD downstream of the CLF can negate the conditions required for the CLF to function. That is because the downstream OCPD possesses what is known as "dynamic impedance." When the downstream breaker starts to open, or the downstream fuse starts to melt, an arc is drawn across the breaker contacts or the fuse ferrules. That arc has impedance, and it limits the fault current that can flow to the point that the CLF is no longer in its operating range. That is not a very good explanation, but as I say I am just learning this concept.

very interesting point. I will have to research more on the effects. thanks again!

 

[Tony S]

Seems like leaving the existing system in place and adding a second 1500KVA system would be a easier way to go. Less disruptive and no need to upgrade the existing.

That is the way I’d go, cheaper and less disruption.

The only down time would be connecting a bus-coupler between the panels. No need to upgrade existing protection or panel busbars. In the event of a transformer failure you still have power available.

In forty years I’ve only had two transformer failures, fortunately in each case there was a 2^nd unit to maintain supplies.

 

[Jason T]

That is the way I’d go, cheaper and less disruption.

The only down time would be connecting a bus-coupler between the panels. No need to upgrade existing protection or panel busbars. In the event of a transformer failure you still have power available.

In forty years I’ve only had two transformer failures, fortunately in each case there was a 2^nd unit to maintain supplies.

That is definitely another option worth exploring. Just comparing prices between doing that and running new cables and finding an affordable way to make the upgrade work, which doesn't seem likely at this point :blink:

 

[Phil Corso]

Jason T...

Do you have the space to install a 2nd 1,500 kVA xfmr?

Regards, Phil Corso

 

[Jason T]

Jason T...

Do you have the space to install a 2nd 1,500 kVA xfmr?

Regards, Phil Corso

Well, not readily available space, but we could create one. That's why I want to explore the option of upgrading to a 3000KVA first and see if I can make that work. Still trying to research how much effect dynamic impedance has on selective coordination.

 

[Toppcatt22]

TAKE THE 1.73 OUT OF YOUR CALCULATION

TAKE THE 1.73 OUT OF YOUR CALCULATION

I feel it is most important to point out that you have used 1.73 * 480 as the voltage for your SC calculation, which is incorrect. You use 1.73 to go from the Line-to-Neutral (single phase) to the Line-to-Line (three phase) voltage value. Because you scaled up what is already the 3-phase value, you have understated your available short circuit value by 45kA. That is the first issue that needs to be addressed.

Hope that helps.

Hi Everyone,

Long time reader of this forum but first time posting.

I have a question regarding a transformer installation. Currently there's a 1500KVA transformer installed feeding a 2000A main disconnect. A proposal was written to upgrade to a 3000KVA transformer in order to have room for future expansion. I am reviewing the proposal and everything seems fine with the cable sizing and load side fused protection.

My question is the AIC rating for the 2000A main disconnect. The main disconnect is rated for 100KA or short circuit current rating, which isn't the same as the AIC rating. According to the calculation the max short circuit fault current should be around 62.2KA for the 3000KVA transformer (based on 480V secondary with a 5.8% impedance). If the short circuit current rating is sufficient to carry the fault current, does it matter what the AIC rating is if we size the primary protection of the transformer accordingly? Or do we have to make sure that the AIC for the main disconnect is rated at least 65KA?

Thanks in advance!

 

[Jason T]

I feel it is most important to point out that you have used 1.73 * 480 as the voltage for your SC calculation, which is incorrect. You use 1.73 to go from the Line-to-Neutral (single phase) to the Line-to-Line (three phase) voltage value. Because you scaled up what is already the 3-phase value, you have understated your available short circuit value by 45kA. That is the first issue that needs to be addressed.

Hope that helps.

Really? I've always thought you had to divide by 1.732 to get the full load amperage for a transformer? can some verify?

 

[charlie b]

I feel it is most important to point out that you have used 1.73 * 480 as the voltage for your SC calculation, which is incorrect.

No, he correctly used 1.732 as part of the equation that gives us full load current based on KVA and voltage. Full load current equals KVA divided by voltage (line-to-line, or 480) and divided again by the square root of 3 (approximately 1.732).

Really? I've always thought you had to divide by 1.732 to get the full load amperage for a transformer? can some verify?

Consider it verified.