240.21(B)(3) Taps supplying a transformer

[bryce_wynia]

I have a rare case here of a 1600A 208V Main gear where I need to feed a 500kVA step-up to feed 400kw of load of 480V equipment. In this case could i use the Main breaker as my OCPD for the primary side of the transformer (utilizing this tap rule) or would the transformer require its own OCPD ? (The distribution is a 4kV system that will not allow a 480v service voltage, hence the need for this backwards install.

 

[don_resqcapt19]

The maximum permitted primary protection for a transformer is 250% of the rated primary current. 450.3(B)

 

[bryce_wynia]

The maximum permitted primary protection for a transformer is 250% of the rated primary current. 450.3(B)

Wouldn't the 1600A main fall under those stipulations?

 

[don_resqcapt19]

Actually I did not run the numbers.

So looking at the numbers the full load current on the 208 volt side is 1388 amps. I would have a bit of a concern on the 1600 amp main tripping when you energize the transformer as the general rule of thumb is that the primary protection needs to be around 125% of the primary current. This inrush would be even worse of your are reverse feeding a 480/208Y/120 volt transformer and not purchasing one intended to be used to step 208 up to 480.

Not sure what you get out of the tap rule, as it appears to me that the conductors required for the load would be much larger than what the tap rule would permit. What size conductors are you proposing?

 

[bryce_wynia]

Actually I did not run the numbers.

So looking at the numbers the full load current on the 208 volt side is 1388 amps. I would have a bit of a concern on the 1600 amp main tripping when you energize the transformer as the general rule of thumb is that the primary protection needs to be around 125% of the primary current. This inrush would be even worse of your are reverse feeding a 480/208Y/120 volt transformer and not purchasing one intended to be used to step 208 up to 480.

Not sure what you get out of the tap rule, as it appears to me that the conductors required for the load would be much larger than what the tap rule would permit. What size conductors are you proposing?

 

[bryce_wynia]

I have not figured the conductors yet, will have to be 4-5 parallel runs i am sure. I wanted to verify it was even possible first. I was a bit worried about the inrush as well, a transformer intended to be a step up was the plan.

 

[jim dungar]

the general rule of thumb is that the primary protection needs to be around 125% of the primary current.

This may not be as much of a problem with an adjustable trip breaker, but you have to plot the transformer versus the breaker to make sure. One trick would be to use a 2000A frame breaker with the long time adjusted down to 1600A and the short time adjusted to max.

 

[don_resqcapt19]

I have not figured the conductors yet, will have to be 4-5 parallel runs i am sure. I wanted to verify it was even possible first. I was a bit worried about the inrush as well, a transformer intended to be a step up was the plan.

Might not be physically possible to use that tap rule...would be tough to keep the total length of one primary conductor plus one secondary conductor to 25' or less. Often with that size equipment, you use up 5 or 6 feet just at the terminations.

 

[electrofelon]

One other option might be to tap on the line side of the disconnect and add a 2nd service disconnect per 230.40 ex#2 to serve the transformer (assuming the "main breaker" is a service disconnect). Of course That may add substantial cost for that new disconnect. One advantage is you could now oversize your transformer breaker because it can exceed the size of the common service conductors.

What kind of building is this? I'll be the first to say that it doesn't always matter if a breaker trips occasionally when trying to start a transformer. Seems like many think it's the end of the world and must be avoided at all costs.

 

[jim dungar]

What kind of building is this? I'll be the first to say that it doesn't always matter if a breaker trips occasionally when trying to start a transformer. Seems like many think it's the end of the world and must be avoided at all costs.

I had one COPS site that tripped a breaker every time the ATS transferred during weekly tests.
I would not like to have a main breaker trip when the power is restored after a utility 'blip' during a storm.

 

[electrofelon]

I had one COPS site that tripped a breaker every time the ATS transferred during weekly tests.
I would not like to have a main breaker trip when the power is restored after a utility 'blip' during a storm.

There are certainly applications where you would want the OCPD to hold 100% of the time. In other cases it can not be that big a deal if it trips on inrush and needs a 2nd attempt.

What is COPS?

 

[jim dungar]

There are certainly applications where you would want the OCPD to hold 100% of the time. In other cases it can not be that big a deal if it trips on inrush and needs a 2nd attempt.

How many people are going to like having to go and reclose their 1600A main because the utility had a power blip during a storm? A one second outage becomes a call to a contractor who needs to investigate before simply reclosing.

What is COPS?

That was an NEC term for critical operation centers, see Art 708 (2017).

 

[electrofelon]

How many people are going to like having to go and reclose their 1600A main because the utility had a power blip during a storm? A one second outage becomes a call to a contractor who needs to investigate before simply reclosing.

Maybe. Perhaps that is serving a dedicated piece of equipment that doesn't automatically restart and there are qualified people at the facility. It really depends on the situation.

 

[jim dungar]

Maybe. Perhaps that is serving a dedicated piece of equipment that doesn't automatically restart and there are qualified people at the facility. It really depends on the situation.

OSHA, NFPA70E, and many companies require all trips to be investigated before an OCPD is reclosed. How do you know it was transformer inrush and not an insulation failure that caused the trip?
This is why many designers default to the maximum allowed 250% for transformers and others are reluctant to go even as low as 125%.

The choice is yours. It is a low risk level for a branch circuit to a transformer feeding a coffee pot and a much higher risk when it affects a large building service.

 

[electrofelon]

OSHA, NFPA70E, and many companies require all trips to be investigated before an OCPD is reclosed. How do you know it was transformer inrush and not an insulation failure that caused the trip?
This is why many designers default to the maximum allowed 250% for transformers and others are reluctant to go even as low as 125%.

The choice is yours. It is a low risk level for a branch circuit to a transformer feeding a coffee pot and a much higher risk when it affects a large building service.

Yes I would almost certainly apply maximum inrush trip design if this was serving an entire building. Besides, something like that is most likely designed and built from scratch so it's much easier and cheaper to coordinate. It's typically adding to an existing system where you run into not being able to readily install the frame you need and/or making a tap to a separate enclosure can be a pain. Where I have encountered this it was dedicated equipment that would reset and not restart automatically. It just wasn't worth spending thousands of dollars to guarantee it would start 100% of the time. I told the client they may need to reset the breaker after a power outage.

Not sure of the OPs situation, but he could easily be looking at $10,000 to provide a setup that starts 100% of the time. He'll have to decide if it's worth it.

 

[don_resqcapt19]

There are certainly applications where you would want the OCPD to hold 100% of the time. In other cases it can not be that big a deal if it trips on inrush and needs a 2nd attempt.

What is COPS?

With the safety rules in many industrial plants, resetting a breaker of that size can be a big deal. Many times only some of their maintenance staff has the training that the plant electrical safety plan requires to reset a breaker of that size.

 

[electrofelon]

With the safety rules in many industrial plants, resetting a breaker of that size can be a big deal. Many times only some of their maintenance staff has the training that the plant electrical safety plan requires to reset a breaker of that size.

Well if that's the case for the OP and/or they don't want to possibly reset it after an outage once every couple years, then he should spend the 10 grand and design it for a 100% chance of holding .