POCO xfmr temp ratings

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sceepe

sceepe

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I have a large commercial project where I want to use 90 rated conductors for service entrance conductors. These will run from a POCO pad mount to a secondary termination cabinet (STC). The STC has lugs rated for 90 deg c (according to the manufacturer). Does anybody know if the POCO secondary lugs are 90 deg rated?
 
Wait a minute, POCO is not required to follow NEC. So do the terminations on the pad mount secondaries have to comply with NEC. Guess it depends on where the service point is. So, where is the service point?????

Please don't tell to me to call the POCO, I already have.
 
I specified transformers for a utility for years and yes the lugs were rated for 90 degrees. In my experience, utilities generally don't use temperature ratings less than 90 C for their ampacities.

Jim T
 
You did not specify the voltage of the secondary. If it is above 480V then the medium voltage electrical switchgear has nothing less then 90 deg C capability, or higher depending on the method of connection. If you are going to be connecting to electrical equipment rated 480V or less, then you will be limited to the 75 Deg C at the switchgear (after your STC).

From an engineering standpoint, there is nothing wrong with using 90 deg C ratings, if available. It is my understanding some manufacturers are working toward 90 deg C terminals, but it may be some time before it becomes universal.
 
kingpb said:
From an engineering standpoint, there is nothing wrong with using 90 deg C ratings, if available. It is my understanding some manufacturers are working toward 90 deg C terminals, but it may be some time before it becomes universal.
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I doubt it. Don has some good opinions/facts on this matter. 90?C is awful warm to be operating equipment at.
 
kingpb,
From an engineering standpoint, there is nothing wrong with using 90 deg C ratings, if available. It is my understanding some manufacturers are working toward 90 deg C terminals, but it may be some time before it becomes universal.
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For one no customer is going to accept electrical equipment that is operating at 194?F. Also if the facility is covered by OHSA, they require protection from contact from anything that has a temperature over 140?F. If you are running the conductors at 194?F, the conduits will require insulation to protect the people. But the most important issue is the cost of making all of that heat. The customer is paying for electricity that is doing no useful work. The cost of the electrical energy needed to make this heat will be a very large number over the life of the electrical installation.
Don

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don_resqcapt19 said:
kingpb,

For one no customer is going to accept electrical equipment that is operating at 194?F. Also if the facility is covered by OHSA, they require protection from contact from anything that has a temperature over 140?F. If you are running the conductors at 194?F, the conduits will require insulation to protect the people. But the most important issue is the cost of making all of that heat. The customer is paying for electricity that is doing no useful work. The cost of the electrical energy needed to make this heat will be a very large number over the life of the electrical installation.
Don

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Don

I am not sure how you decided 75C is OK and 90C is not. If the facility is covered by OSHA, our 75C conductors fall into the category you listed above as requiring protection also. I suggest that the difference in opinion might be rooted in background as much as reason.

Utility people don't understand why 1/0 aluminum isn't good for 200 amps in the NEC world. We don't understand how it could be. As a utility engineer, I rated 1/0 aluminum triplex in open air at 205 amps. Like it or not, that wire rated that way is connected to thousands of houses each day.

I don't suggest loading conductors, at the design stage, to values which would create 90C temperatures, however I have never seen a load calculation that would provide an actual load anywhere near the calculated one. I do suggest that there is no problem designing with 90C ratings where equipment is properly rated, anymore than someone else's maximum of 75C.

Jim T
 
Don,

First, as Jim pointed out, 75 deg C is above your 140 deg F threshold. So, your argument on that account may be a little tough to defend. Second, the 90 deg C temperature is the cable temperature, not the conduit or raceway temperature. Based on my understanding of heat transfer, there will be a drop in temperature by the time a person were to be exposed to the outside of the conduit; that temperature, may or may not, be above your 140 deg F, maximum. I think the safety argument may be a little thin.

Next, I will however, have to concur with your point that the losses could add up to be a factor in how you design. I did a calc, looking at a 200A circuit which would require #4/0 AWG at 90 Deg C, and 250 KCMIL @ 75 deg C. Using the differential cost to use smaller cable, and conduit and use the 90 Deg C would be offset by roughly 1 year of additional utility expense, in other words break even. In year 2 and later, it is costing more money to go to 90 Deg C. I did not evaluate the incremental cooling cost for installations in conditioned spaces.

This of coarse applies to 600V and below. Seeing how the minimum design for medium voltage is 90 deg C.
 
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