transformer magnetizing inrush current

[PE (always learning)]

Hello,

Hope everyone is doing well. I have a question pertaining to transformer magnetizing inrush current. I'm doing a project where a 112.5 KVA dry type transformer is being protected by a 175A thermal mag breaker on the primary side. I have been checking the breaker instantaneous trip curve with an inrush current of 8 times the FLA and the breaker manages to not have any nuisance tripping when I assume this magnitude of inrush, but when using 10 times the FLA the breaker would trip in the instantaneous region. The breaker has been adjusted as far as it can go in the instantaneous region as well. I have seen from numerous sources that for low voltage transformers an inrush current of 8 - 12 times the FLA is assumed for inrush current at 0.1 seconds, but that a lot engineers usually just assume 12 times the FLA to go with worst case. I have also seen that the inrush can be far greater than this too sometimes depending on different variables. Would assuming 8 times be playing with fire and should I just tell them to replace the breaker with something that can handle higher inrush current or am I thinking too much about this? When performing selective coordination studies is 12 times the FLA always assumed?

Best Regards,
Engineer in training

 

[Ingenieur]

assuming 480/3 vac
it doesn't trip at 8 x fla but trips at 10?

 

[kwired]

Source impedance and supply conductors can make every installation using the same breaker and transformer have different results.

 

[PE (always learning)]

Thanks for the responses. The system voltage is 480/277V, 3 phase and the transformer is stepping down to 120/208V, 3 phase. Yes, the breaker works at 8 times the FLA at 0.1 seconds, but doesn't work at 10 times the FLA at 0.1 Seconds. I've attached a picture of what I'm talking about since a picture is worth one thousand words.


Best Regards

 

[ron]

If I'm specifying a project in advance, I use 12x.

Sometimes you can get a way with 8x, but why take the chance? You can close the breaker a few times and depending on where on the sin wave you are closing it, you may get different in rush results.

 

[Ingenieur]

Thanks for the responses. The system voltage is 480/277V, 3 phase and the transformer is stepping down to 120/208V, 3 phase. Yes, the breaker works at 8 times the FLA at 0.1 seconds, but doesn't work at 10 times the FLA at 0.1 Seconds. I've attached a picture of what I'm talking about since a picture is worth one thousand words.


Best Regards View attachment 19122 View attachment 19123

by 'work' do you mean it 'trips' at 8x or 'holds'?

 

[topgone]

What breaker make are you using?

 

[electrofelon]

You can close the breaker a few times and depending on where on the sin wave you are closing it, you may get different in rush results.

Right which is why I never worry too much about this and wonder why many seem to obsess about it. If it doesn't take, just try it again, big deal. I understand there are certain applications where one would want this to be more highly engineered, but for most general applications I don't see any reason to go with anything other than a standard breaker at 125% fla.

 

[steve66]

IMO the new energy efficiency requirements for transformers are creating lower impedances, and higher inrush currents.

Even if we know exactly what number to use, it may not be the right number to use tomorrow.

And, no, I don't worry too much about it for most typical projects, but its obviously important to use a high enough inrush number for data centers and hospitals.

 

[brian john]

Is the transformer supported by a generator, with a fast transfer from Generator to Utility?

In practice (depending on the impedance of the transformer) I have typically measured 4-6 times the rating.

 

[publicgood]

...for most general applications I don't see any reason to go with anything other than a standard breaker at 125% fla.

I disagree. The code allows you more in the OP example. The feeder size doesn’t change. The cost for increasing the breaker size is insignificant to the total project cost. IMO, risking 125% just doesn’t make sense.

Note if you use max 250% you may need to increase the conductor size to ensure feeder protection

 

[publicgood]

Hello,

Hope everyone is doing well. I have a question pertaining to transformer magnetizing inrush current. I'm doing a project where a 112.5 KVA dry type transformer is being protected by a 175A thermal mag breaker on the primary side. I have been checking the breaker instantaneous trip curve with an inrush current of 8 times the FLA and the breaker manages to not have any nuisance tripping when I assume this magnitude of inrush, but when using 10 times the FLA the breaker would trip in the instantaneous region. The breaker has been adjusted as far as it can go in the instantaneous region as well. I have seen from numerous sources that for low voltage transformers an inrush current of 8 - 12 times the FLA is assumed for inrush current at 0.1 seconds, but that a lot engineers usually just assume 12 times the FLA to go with worst case. I have also seen that the inrush can be far greater than this too sometimes depending on different variables. Would assuming 8 times be playing with fire and should I just tell them to replace the breaker with something that can handle higher inrush current or am I thinking too much about this? When performing selective coordination studies is 12 times the FLA always assumed?

Best Regards,
Engineer in training

You sound like you are studying with shop drawing available, given you have a specific breaker in the TCC. You have the transformer data to look up and verify then?

 

[electrofelon]

I disagree. The code allows you more in the OP example. The feeder size doesn’t change. The cost for increasing the breaker size is insignificant to the total project cost. IMO, risking 125% just doesn’t make sense.

Note if you use max 250% you may need to increase the conductor size to ensure feeder protection

You are free do disagree. Note also I didnt say the OP couldnt go more than 125%, just that I didnt feel it was necessary. I find it is often not a trivial cost to go higher. As you note, you have to go with larger conductors. Thats larger raceway too. Maybe a larger frame size or switch size. Take what is probably the most common transformer install, a 75KVA 480 to 120/208. 100 amp equipment works out nice cost wise for the primary side. I havent had any problems. Sure, if you throw in transfer equipment or some installation that is being turned on and off frequently for some reason, maybe you would want to go higher. The vast majority of transformers I see have probably been on constantly for years.

 

[publicgood]

As you note, you have to go with larger conductors.

If you go all the way to 250%, yes. 175% typically not.

The vast majority of transformers I see have probably been on constantly for years.

And it is those instances where I have run across not being able to close back in, after years of being closed and finally having to shutdown for maintenance.

The one exception may be if you have an adjustable breaker to give you some headroom.

 

[kwired]

If you go all the way to 250%, yes. 175% typically not.



And it is those instances where I have run across not being able to close back in, after years of being closed and finally having to shutdown for maintenance.

The one exception may be if you have an adjustable breaker to give you some headroom.

Tripping when manually closing it isn't as big of a deal as tripping when it is automatically transferred or simply when power has been off for whatever reason and is restored. You are right there to reset and try again when manually closing it.