800 Amp Panel feed

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AJToady

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I have been searching and perhaps the wrong phrases in my search. However, the question is this; I am looking to install an 800 Amp 3 phase 480V panel and will be running two conduits with parallel 500MCM THHN-2 copper wires. At each termination there will be Burndy crimped lugs rated 90C. The lugs will be installed with the approved tool. By my calcs the total ampacity of the feeders will be 860 Amps based on table 310.15(B)(16). They will be fed by an 800 amp breaker and the length will not be too long to create an excessive voltage drop.See any problems with any of this?? Thanks in advance.
 
I agree with Roger, your use of 90° C terminals will not change the 75° C rating of the equipment that they're connected to so you'll end up with a 760 amp feeder. You still can use an 800 amp OCPD if the calculated load is 760 amps or less based on the next standard size up rule.
 
infinity said:
I agree with Roger, your use of 90° C terminals will not change the 75° C rating of the equipment that they're connected to so you'll end up with a 760 amp feeder. You still can use an 800 amp OCPD if the calculated load is 760 amps or less based on the next standard size up rule.
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same opinion here.
 
In addition to the above statement, the terminations will be to the buss attach points in the gear, it has crimp lugs for the termination. Secondly I have addressed the panel attachment points with the manufacturer. The breaker and gear are rated. My question maybe should be, if approved by the equipment supplier to directly attach crimp lugs, are the feeders actually 860 amp capable as listed in the code?
 
It would be more up to the listing of the equipment, manufacturers and suppliers will say a lot of things to make a sale and if I were the inspector their claims wouldn't carry much weight without some substantiation.

Roger
 
So in conclusion, apparently even if the lugs, wire, and devices are approved it's a path hard to follow. I have heard conflicting stories on using parralel 500's for an 800amp feed also. What is the general consensus on that? 75C rates the wire at 380amps, so 760 amps of parallel current capacity when seperate conduits. Next size up breaker, 800amp. Not over 800 amp so should be OK. Voltage drop less than 3% with the length of run. Comments???
 
AJToady said:
So in conclusion, apparently even if the lugs, wire, and devices are approved it's a path hard to follow. I have heard conflicting stories on using parralel 500's for an 800amp feed also. What is the general consensus on that? 75C rates the wire at 380amps, so 760 amps of parallel current capacity when seperate conduits. Next size up breaker, 800amp. Not over 800 amp so should be OK. Voltage drop less than 3% with the length of run. Comments???
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The next size up rule only is allowed if the calculated load does not exceed the conductor adjusted ampacity.
 
texie said:
The next size up rule only is allowed if the calculated load does not exceed the conductor adjusted ampacity.
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Adjustments normally are applied for the insulation @ 90C ampacities.

The conductor itself normally can still be no smaller than the termination @ 75C ampacity based on calculated load, but can be protected at next size up for 800 amp devices or less, unless other protection is permitted by other sections mentioned in 240.3.
 
kwired said:
Adjustments normally are applied for the insulation @ 90C ampacities.

The conductor itself normally can still be no smaller than the termination @ 75C ampacity based on calculated load, but can be protected at next size up for 800 amp devices or less, unless other protection is permitted by other sections mentioned in 240.3.
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Why are you trying to explain derating to me? If you have, say, a 780 amp load you can't use 2X 500 MCM and use the next size up rule to an 800 amp breaker.
 
texie said:
If you have, say, a 780 amp load you can't use 2X 500 MCM and use the next size up rule to an 800 amp breaker.
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That's correct. Because you wouldn't have enough ampacity for the load. The reason you see 2 sets of 500 kcmil on so many 800A breakers, is that it is an example where the installer is taking credit for 240.4(B), and the load (or 1.25*continuous load) doesn't exceed 760A, which is the ampacity of 2x500's in parallel. In other words, if a 760A breaker existed as a non-standard rating, and you could theoretically use it instead of the 800A standard size you are actually using, then you can use 2 sets of 500 kcmil to connect to the 800A breaker instead.

Given a 780A load on an 800A breaker, you'd either need 3 sets of 300 kcmil Cu, or 2 sets of 600 kcmil cu.
 
OK, lets talk this through, it has taken the wrong direction in where the terminations would be. The point in the gear is a buss connection from the 2000Amp frame breaker. The trip unit will be 800 Amp. Lugs are already there in the gear, provided by the manufacturer. The point of attachment in the panel will be to the buss attach points and not a breaker. So the original question was, would 90c lugs and wire supply a combined ampacity of 860amps with 2 sets of 500mcm?
 
AJToady said:
OK, lets talk this through, it has taken the wrong direction in where the terminations would be. The point in the gear is a buss connection from the 2000Amp frame breaker. The trip unit will be 800 Amp. Lugs are already there in the gear, provided by the manufacturer. The point of attachment in the panel will be to the buss attach points and not a breaker. So the original question was, would 90c lugs and wire supply a combined ampacity of 860amps with 2 sets of 500mcm?
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The answer is still no. It does seem like 90 degrees should be fine in this case where the conductors terminate on a bus some distance from the breaker, but if you check with the manufacturer I'll bet it's still 75.
 
texie said:
Why are you trying to explain derating to me? If you have, say, a 780 amp load you can't use 2X 500 MCM and use the next size up rule to an 800 amp breaker.
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Calculated load and ampacity adjustments are not the same thing. Calculated load will need to be 760 or less or ampacity adjustments will never matter, you will have exceeded termination temp rating regardless of what adjustments allow for other portions of the circuit.
 
AJToady said:
So the original question was, would 90c lugs and wire supply a combined ampacity of 860amps with 2 sets of 500mcm?
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Same answer as previously mentioned, no. The equipment that the lug is connected to is only rated for 75 degrees C.
 
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