800 Amp Service

[Cammack]

I have an 800 amp service with a 800 amp 100% rated breaker (75 deg C). Historical peak load is 500 amps. The service latteral is 2 sets of 500 kcmil rated for 760 total amps (380a x 2). I am wondering if this is a code violation. It appears to comply to 230.90 (A) exception #2 which refers to 240.4 (b) which states that you can go up to the next standard size breaker. Next size breaker from 760 is 800. Not sure if the intent is met here though since it may be implied that the breakers are standard breakers and not 100% rated. Anyone run up against this before?

 

[mayanees]

interesting ?

interesting ?

That's a good question because it smacks in the face of sensibility. You could theoretically run 800 amps all day, and the 760-amp cable would melt without tripping the breaker. And to make matters worse, it's parallel 500s, so one could/would be pulling more amperage than the other.

My opinion is that it is an NEC approved installation. And the specifics of the electrical system you described allow it to be a safe installation. (500 amp peak)

But if I'm doing the design, and it's for a connected load that could push the 760 amp capacity limit, I would run a true 800 amp capacity cable.

JM

 

[charlie b]

This is a common question, and a matter of some debate. My response is that you did not give us enough information. The statement that the ?historic peak is 500 amps? is a valuable tip, but it does not quite do the trick by itself.

What matters first and foremost is that the ampacity of the conductors (i.e., 760 amps) be at least as high as the calculated load. If this were a new construction project, you would be calculating the load in accordance with article 220. You can still do that, but it is often less practical for an existing facility. You can also calculate the existing load on the basis of a history of load measurements. Taking data every 15 minutes for a month is one way. Looking at a year?s worth of utility load data is another. But in either case, you need the measurement to be in units of power (i.e., kw), not energy (i.e., kw-hr.).

By the way, welcome to the forum.

 

[weressl]

That's a good question because it smacks in the face of sensibility. You could theoretically run 800 amps all day, and the 760-amp cable would melt without tripping the breaker. And to make matters worse, it's parallel 500s, so one could/would be pulling more amperage than the other.

My opinion is that it is an NEC approved installation. And the specifics of the electrical system you described allow it to be a safe installation. (500 amp peak)

But if I'm doing the design, and it's for a connected load that could push the 760 amp capacity limit, I would run a true 800 amp capacity cable.

JM

We can agree that you are in compliance with the NEC.

Presuming that you have made the appropriate ambient temperature corrections when you given us the data you may wish to take the following into consideration before making the decision.

Even if you're running 800A day-and-night there is a temperature difference between day and night. Meaning that the ambient temperature will be lower at night allowing the cable to operate closer to the normal operating temperature. There is safety margin in all design data. Meaning that you will not likely to "melt" the conductor insulation, heck it is rated for 90C. Your problem would be the 75C rated connectors holding the wires that may periodically operate above 75C due to the heat conducted from the higher wire temperatures, yet the connectors themselves do not exceed their current rating.

 

[hdpeng]

In accordance w/ NEC, up to & including 800A, 2-sets 500kCMIL (760A) meets code because your not supposed to load the breaker over 80% (640A), anyway. Over 800A, though, conductors must be 100% rated for breaker.

 

[hardworkingstiff]

In accordance w/ NEC, up to & including 800A, 2-sets 500kCMIL (760A) meets code because your not supposed to load the breaker over 80% (640A), anyway. Over 800A, though, conductors must be 100% rated for breaker.

David,

The way I understand it is you must do a load calculation and if that load calculation is =< 760-amps then you can use the 2-sets of 500kcmil (copper) to feed an 800-amp panel. If the load calculation came to 780-amps, then the 800-amp panel would still be OK, but the 2-sets of 500 would not.

 

[iwire]

(760A) meets code because your not supposed to load the breaker over 80% (640A), anyway.

David, you can load a standard 800 amp breaker to 800 amps non-continuously The NEC would allow that breaker to be loaded to 800 amps up to 3 hours at a time. 210.19(A)(1), 225.2(A)(1) and Article 100 'Continuous Load'

240.4(B) relies only on the calculated load to ensure that the conductor is not overloaded.

Exception 3 to 230.90(A) is very similar in it's reliance on the calculated load.

 

[hdpeng]

David, you can load a standard 800 amp breaker to 800 amps non-continuously The NEC would allow that breaker to be loaded to 800 amps up to 3 hours at a time. 210.19(A)(1), 225.2(A)(1) and Article 100 'Continuous Load'

240.4(B) relies only on the calculated load to ensure that the conductor is not overloaded.

Exception 3 to 230.90(A) is very similar in it's reliance on the calculated load.

As Ed McMahon used to say to Johnny... 'You are correct, sir." I was assuming 100% cont. load, & should have clarified that.