Help solve a debate between my twin brother and I (both electrical engineers) read message below for details

[PE (always learning)]

Just to expand the conversation, can we discuss fuses vs circuit breakers for an application like this? I always thought a fuse would be better for applications with high inrush, but I don't recall ever actually looking at the corresponding trip curves.

I could show them in the morning if you want to see 90 amp typical fuses vs Siemens inrush

 

[electrofelon]

So another thing: I have a 25kVA single phase transformer. It's one of those octagonal shaped acme dry types. Wired as 208 primary, 240 secondary. It was connected to a 100A QO breaker, and the panel that was in was fed from a 150A I-line breaker. When that thing was turned on, it would trip either the 100A or the 150A "a lot". This is going back a few years, but I would say one out of three attempts one of those breakers would trip. Why is this setup so trippy? This doesn't seem to match the results that we have been discussing. Maybe we have to be careful about generalizing a breaker/transformer FLA proportion with a nuisance tripping likelihood?

 

[ptonsparky]

I'm not surprised. That's a 120 A.

 

[ptonsparky]

I'm not surprised. That's a 120 A.

You wouldn’t expect a motor with FLA of 120 amp to start on a 100 or 150 amp CB.

 

[electrofelon]

I'm not surprised. That's a 120 A.

True , Its a bit higher than I thought, I spaced and was using 240 which would be a bit lower 104A. But still, what is different than the situation in the OP? Transformer inrush can vary significantly perhaps? I don't have the inrush profile for this unit.

 

[Jpflex]

My twin brother and I are both electrical engineers and sometimes get into some fierce arguments when it comes to design.
Today we had an argument on primary breaker sizing for a 75 kVA transformer. The transformer was feeding a secondary 200 amp main circuit breaker panel at 120/208V, 3 phase, 4 wire and the primary was 480V, 3 phase and he used a 125 amp breaker (which is fine, but read on)

He told me that it is industry standard to size the overcurrent protection on the primary at 125 amps in this scenario to which I disagreed with. I told him that a 100 amp or even a 90 amp breaker would be fine in this scenario based on table 450.3(B). 450.3(B) clearly states that the 250% is a maximum overcurrent protection for transformers 1000 volts and less, but does not state minimum protection.

I know that having a higher primary breaker size is always nice in a scenario like this to avoid tripping from transformer inrush, but I have looked at the actual transformer inrush charts for many transformers and the real values are always well below the breaker trip curve values in the instantaneous region.

Again, I go back to his argument about using a 125 amp breaker as industry standard to which I say no. I say using a 100 amp breaker would be just fine and would probably save you money as usually going from 100 amps to a 125 amp breaker leads to using a sub fed breaker which costs more money.

Anyway, let me know your thoughts so we can know who wins.

I also got 90 amperes for primary OCPD
30,000 VA / 480 E / 1.732 = 36 i amperes

Max OCPD = 36 i x 250% = 90.21 i

However, 90.21 gets rounded down to 90 since the fraction is less than 0.5

And 90 is a standard size breaker so i would say 90i OCPD

I do not have my code book so I don’t know if there is an exemption to allow the next higher size breaker over 90.21 for transformer feeder primary or if a higher breaker is allowed if inrush current is a problem?

 

[ptonsparky]

True , Its a bit higher than I thought, I spaced and was using 240 which would be a bit lower 104A. But still, what is different than the situation in the OP? Transformer inrush can vary significantly perhaps? I don't have the inrush profile for this unit.

He is at least matching the FLA, while you aren't even close @ 100 amp.
You may need to know the trip curve of the CB more than current of the xfmr.

 

[Jpflex]

Let me see if I understand the argument. He said, "The sky is blue." You replied, "No, you are wrong. The grass is green." Do I have that right?

1. I agree that 125 amps is industry standard. I've never seen any other size used for this application.

2. I also agree that a 100 amp would "likely" be successful. I say "likely" because we, as engineers, generally have very limited control over the exact makes and models of components that the contractor purchases based on our drawings and specifications.

3. I have nothing to offer on the question of relative costs.

4. If you called for a 90 amp breaker, you would be getting the blame for what will almost certainly be occasional (if not downright numerous) unnecessary trips.

5. A PE's job includes protecting the client's interests. That does include costs, but it also includes convenience (e.g., future operational and maintenance considerations).

I don’t understand how a 125 ampere breaker can be used if the code limits the max primary current to 250%?

36 x 2.50 = 90i

 

[ptonsparky]

I also got 90 amperes for primary OCPD
30,000 VA / 480 E / 1.732 = 36 i amperes

Max OCPD = 36 i x 250% = 90.21 i

However, 90.21 gets rounded down to 90 since the fraction is less than 0.5

And 90 is a standard size breaker so i would say 90i OCPD

I do not have my code book so I don’t know if there is an exemption to allow the next higher size breaker over 90.21 for transformer feeder primary or if a higher breaker is allowed if inrush current is a problem?

Yes, higher breaker is allowed, but you must protect the conductors, which my mean they need to be larger.

 

[ptonsparky]

I also got 90 amperes for primary OCPD
30,000 VA / 480 E / 1.732 = 36 i amperes

Max OCPD = 36 i x 250% = 90.21 i

However, 90.21 gets rounded down to 90 since the fraction is less than 0.5

And 90 is a standard size breaker so i would say 90i OCPD

I do not have my code book so I don’t know if there is an exemption to allow the next higher size breaker over 90.21 for transformer feeder primary or if a higher breaker is allowed if inrush current is a problem?

Where did 30,000 come from?

Did I miss something? 75Kva is the OPs transformer.

 

[jim dungar]

I don’t understand how a 125 ampere breaker can be used if the code limits the max primary current to 250%?

36 x 2.50 = 90i

Are you looking at the primary current of a 3-phase 75kVA transformer with a 480V input?

 

[PE (always learning)]

Guys, w

I also got 90 amperes for primary OCPD
30,000 VA / 480 E / 1.732 = 36 i amperes

Max OCPD = 36 i x 250% = 90.21 i

However, 90.21 gets rounded down to 90 since the fraction is less than 0.5

And 90 is a standard size breaker so i would say 90i OCPD

I do not have my code book so I don’t know if there is an exemption to allow the next higher size breaker over 90.21 for transformer feeder primary or if a higher breaker is allowed if inrush current is a problem?

We are talking about a 75 kVA transformer not a 30 kVA transformer...

 

[PE (always learning)]

As requested, attached you will find common 90 amp fuses tested with a 75 kVA, 480V 3 phase primary, 208V 3 phase 4 wire secondary, Siemens Series J Typical Performance Data for an Aluminum 3 phase dry type K1 without electrostatic shield transformer at 150 degree temp rise. Again, this is using the absolute peak inrush. Reference attachments for information. As you can see, all the typical fuses clear the absolute peak inrush, although they are a little closer than the breaker curves were.

Best Regards

 

[PE (always learning)]

Are you looking at the primary current of a 3-phase 75kVA transformer with a 480V input?

I think jpflex was confused, for some reason, he thought we were talking about a 30 kVA transformer, but yes we are talking about a 75 kVA transformer

 

[Jpflex]

Guys, w


We are talking about a 75 kVA transformer not a 30 kVA transformer...

Ok I see 75,000 VA / 480 E / 1.732 = 90.21 primary amperes

Round down 90 i x 250% = 225 i and this is a standard size breaker for primary?

 

[PE (always learning)]

Ok I see 75,000 VA / 480 E / 1.732 = 90.21 primary amperes

Round down 90 i x 250% = 225 i and this is a standard size breaker for primary?

You don't have to size the primary at 250%, that is a maximum suggestion so that the primary overcurrent protection doesn't trip from the transformer inrush upon energizing. That is why I am showing the time current curve charts, to let people see that the reality is you can size some of your breakers a lot lower on the primary side if you know the manufacturer's inrush information.

 

[electrofelon]

An update on my end:. I just energized the single phase 25 KVA 240-240 transformer I discussed earlier in the thread. It is fed by a 125 amp breaker. The breaker does NOT like it and trips probably every 4th attempt. So I am at primary breaker being 120% of FLA.

Now I will be the first to say it doesn't always matter if a transformer doesn't reliably start - just try it again. In this case I don't consider the $1000 it would cost to put this on a 200 to be worth it, so I'm just going to advise the client there is a possibility they need to turn the breaker back in after an outage. But the point is, it looks like my experience is quite a bit different than the OP's so it appears you can't generalize on a OCPD/FLA ratio for reliable starting.

 

[topgone]

You don't have to size the primary at 250%, that is a maximum suggestion so that the primary overcurrent protection doesn't trip from the transformer inrush upon energizing. That is why I am showing the time current curve charts, to let people see that the reality is you can size some of your breakers a lot lower on the primary side if you know the manufacturer's inrush information.

Digging back, your time curve plot is so crowded and does not provide a good separation between curves. I am not wondering if those protective devices are racing against each other. A trip time separation of about 250 milliseconds works and you'll get better coordination.

 

[PE (always learning)]

An update on my end:. I just energized the single phase 25 KVA 240-240 transformer I discussed earlier in the thread. It is fed by a 125 amp breaker. The breaker does NOT like it and trips probably every 4th attempt. So I am at primary breaker being 120% of FLA.

Now I will be the first to say it doesn't always matter if a transformer doesn't reliably start - just try it again. In this case I don't consider the $1000 it would cost to put this on a 200 to be worth it, so I'm just going to advise the client there is a possibility they need to turn the breaker back in after an outage. But the point is, it looks like my experience is quite a bit different than the OP's so it appears you can't generalize on a OCPD/FLA ratio for reliable starting.

Digging back, your time curve plot is so crowded and does not provide a good separation between curves. I am not wondering if those protective devices are racing against each other. A trip time separation of about 250 milliseconds works and you'll get better coordination.

??? I thought those time plots were pretty clear. I can easily separate them one by one if it’s not clear enough

 

[PE (always learning)]

An update on my end:. I just energized the single phase 25 KVA 240-240 transformer I discussed earlier in the thread. It is fed by a 125 amp breaker. The breaker does NOT like it and trips probably every 4th attempt. So I am at primary breaker being 120% of FLA.

Now I will be the first to say it doesn't always matter if a transformer doesn't reliably start - just try it again. In this case I don't consider the $1000 it would cost to put this on a 200 to be worth it, so I'm just going to advise the client there is a possibility they need to turn the breaker back in after an outage. But the point is, it looks like my experience is quite a bit different than the OP's so it appears you can't generalize on a OCPD/FLA ratio for reliable starting.

My in

Digging back, your time curve plot is so crowded and does not provide a good separation between curves. I am not wondering if those protective devices are racing against each other. A trip time separation of about 250 milliseconds works and you'll get better coordination.

??? I thought those time plots were pretty clear. I can easily separate them one by one if it’s not clear enou

An update on my end:. I just energized the single phase 25 KVA 240-240 transformer I discussed earlier in the thread. It is fed by a 125 amp breaker. The breaker does NOT like it and trips probably every 4th attempt. So I am at primary breaker being 120% of FLA.

Now I will be the first to say it doesn't always matter if a transformer doesn't reliably start - just try it again. In this case I don't consider the $1000 it would cost to put this on a 200 to be worth it, so I'm just going to advise the client there is a possibility they need to turn the breaker back in after an outage. But the point is, it looks like my experience is quite a bit different than the OP's so it appears you can't generalize on a OCPD/FLA ratio for reliable starting.

all my data came straight from Siemens tested equipment. I don’t know what transformer your using or if you have any particular data available for the transformer you are having issues with.