Transformer OCP??

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jahilliard

Senior Member
Today I ran into a scenario where an office bldg had flooded and a restoration company had come in to clean up and had several pieces of equipment plugged in and running at once. They called me to check out everything in the electrical room because they smelled something burning. When I arrived I measured the load on the panel with the receptacles within that floor with all the equipment. Amperage was reading 179 Amps continuos. There is an enclosed CB rated at 175 Amps feeding that panel, it had already tripped a couple times I was told. The ECB is fed from a step down transformer rated at 125 Amps and was too hot to touch and obviously was creating the burning smell. This exact application is on all 4 floors. Unles I am overlooking something it seems to me the OCP is too high (175A) to protect a 125A rated transformer, even at 125%. Am I wrong or what?? Thanks.
 
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jahilliard

Senior Member
Ok, I may be talking to myself tonight but by looking at tables 450.3(A) and (B) it sems to be saying...if the transformer is rated at 125 A x 125% = 156A and the next highest standard rating for OCP according to sec 240.6 is 175A then all is in order???
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
Ok, I may be talking to myself tonight but by looking at tables 450.3(A) and (B) it sems to be saying...if the transformer is rated at 125 A x 125% = 156A and the next highest standard rating for OCP according to sec 240.6 is 175A then all is in order???

yes, but I'm having trouble coming up with "standard" transformer sizes based on your numbers. Can you give some more specifics...ie: kva and single/3 phase
 

jahilliard

Senior Member
The transformer has a rating of 125A on the nameplate, if that's what you're asking. I was referencing "standard" as far as OCP. HV side is 277/480 and secondary is 120/208 3 ph.
 

Cold Fusion

Senior Member
Location
way north
... Unles I am overlooking something it seems to me the OCP is too high (175A) to protect a 125A rated transformer, even at 125%. Am I wrong or what?? Thanks.
OCP is fine. Load is too high. Bad design, not a bad OCP.

The OCP does not protect the transformer, the design does. The OCP is there to put out the fire when the transformer fails. (edit to add - not exactly true for a secondary main - but the part about the design protecting the transformer is true)

The fix? Do a real load calc, put in right sized transformer, transformer feeder, and panelboards to serve the load.

All four floors the same, makes it sound like it was original installation. The owner would be justified is having the engineer slapped around the block. Unless, of course the EC "Valued engineered the job to death". In which case the owner should slap herself around the block for not listening to the engineer.

cf
 
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kingpb

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Location
SE USA as far as you can go
Occupation
Engineer, Registered
OCP is fine. Load is too high. Bad design, not a bad OCP.

The OCP does not protect the transformer, the design does. The OCP is there to put out the fire when the transformer fails. (edit to add - not exactly true for a secondary main - but the part about the design protecting the transformer is true)

cf

The OCP on the HV side does protect the transformer. The selection should be high enough to not trip on Inrush, but low enough to trip before the current hits the thermal damage curve. This is much overlooked by the over simplified approach the code takes when selecting transforer protection.
 

Cold Fusion

Senior Member
Location
way north
The OCP on the HV side does protect the transformer. The selection should be high enough to not trip on Inrush, but low enough to trip before the current hits the thermal damage curve. ...
Maybe you are right and I just have never paid enough attention. However, every coordination I've seen, plots an ANSI damage point for the xfm - but not a damage curve. The primary OCP is set to be inside of the damage point. I have not seen a damage curve on a coordination sheet.

The coordination sheets I am thinking of are in the 20MVA to 1MVA range. For xfm smaller than this, there is not even a damage point on the curves.

However, you are likely a designer of these systems and I am concerned with building, commissioning, keeping these systems running. So, you could well be more knowledgable that I.

For now, I'll stick with my philosophy that the primary OCP does not protect the transformer. It just puts the fire out.

... This is much overlooked by the over simplified approach the code takes when selecting transforer protection.
I'd agree with that. I don't ever recall saying the NEC was even up to being a poor design document.

cf
 

Volta

Senior Member
Location
Columbus, Ohio
OCP is fine. Load is too high. Bad design, not a bad OCP.
Probably. But also the "several pieces of equipment plugged in and running at once" might have been multiple pumps and a dozen dehumidifiers. That equipment was not included in the design of the building. Likely all 120 volt appliances, and there may have been a severe unbalance. I doubt each phase leg was running continuously at exactly 179 amps.
 

jahilliard

Senior Member
The last post is exactly right...there was a crazy unbalance. 179A to 89A on other legs and I didn't understand why exactly. The most important question I have now is...should I recommend any changes to be made or just walk away? The owners and property managers are concerned if this is a hazard of any kind and should be corrected?? Any opinions?
 

Cold Fusion

Senior Member
Location
way north
Probably. But also the "several pieces of equipment plugged in and running at once" might have been multiple pumps and a dozen dehumidifiers. That equipment was not included in the design of the building. Likely all 120 volt appliances, and there may have been a severe unbalance. I doubt each phase leg was running continuously at exactly 179 amps.
So, moron operators instead of moron designers. Good call.

cf
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
IMO something that may be is being overlooked is that the transformer becomes the point of supply to the downstream panel. The secondary protection in this case is not intended to protect the conductors from the LV side of the transformer to the panel. They need to be protected according to 310.15 which cannot be loaded to more than 125%. So if your using the 175A breaker, wouldn't it mean that you need to have at least #1/0 AWG copper feeding the panel. The panel better also be rated for at least 175A as well. Otherwise I believe you do have some code issues.

Keep in mind the table 450.3(A) and (B) are MAXIMUM permissible, and should not be used as a selection guide. The design needs to be done while keeping coordinated overcurrent protection in mind.

A transformer will deliver more than it's rated power KVA however the life of the transformer is significantly impacted. During the overpower situation voltage regulation becomes poor, and the losses are significantly higher.

If the panel is rated properly you need to at least balance the load so you have no more than 125A per phase.
 

Volta

Senior Member
Location
Columbus, Ohio
...there was a crazy unbalance. 179A to 89A on other legs and I didn't understand why exactly. The most important question I have now is...should I recommend any changes to be made or just walk away? The owners and property managers are concerned if this is a hazard of any kind and should be corrected?? Any opinions?

I would at least see what the current draw is now that the flood restoration equipment is gone. It may be that the legs are much closer to being balanced now. As Cold pointed out, the persons plugging in the equipment aren't looking for an education on which receptacles to use, they just use the closest ones. It may have been a bad design fluke that left all of the easily accessible receptacles on one already full leg.

But either way, it sounds likely that the transformer is operating at full capacity already and a half day to do some data gathering and load calcs may be warranted.
 
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