Jarrett McFatter
Member
We have a 800 amp sub panel being feed with a 800 amp breaker. The engineer is saying us 2x500 mcm cu per phase, is this correct or should 2x600 mcm cu be used?
Why do you believe the 600's should be used?
what type of wire? TW - THWN - THWN-2
what are the terminal ratings? 60c, 75c, or 90c
500 mcm = 320 - 380 - 430
Few more inputs..
What length ? and is it underground or on a cable tray or is it a busbar?
cheers
Lugs are 75c
Cable is THHN
500 mcm cu is rated at 380 amp x 2 = 760 amps.
600 mcm is 420 amps x 2 = 840 amps.
Doesn't the cable for a sub feeder 800 amps and less need the be 100% rated for the over current protection?
:-? Charlie...hold my hand for bit?Why is nobody asking the only question that really matters: "What is the calculated load"? :-?
So, what is the load? If it is 760 amps or lower, then you can use a pair of 500's. If it is 761 amps or more, you cannot.
2005 NEC said:240.4(B) Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of
the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multi outlet branch circuit supplying receptacles for cord and- plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.
Celtic, you are on step 3. I am talking about step 2. If the load is, for example, 750 amps (as calculated in step 1), then I can use a pair of 500's. That is because the ampacity of a pair of 500's is 760 amps, and that is more than the load.
Now that I have picked the conductors, let's pick the OCPD. 760 amps is not a standard setting, and it is lower than 800, so I can use an OCPD setting of 800, which is the next higher standard value.
(B) Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of
the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
If we have an 800A OCPD, it stands to reason this is not going to be a BC ~ but rather a feeder ~ that will be supplying "multi outlet branch circuit supplying receptacles for cord and- plug-connected portable loads.".(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord and- plug-connected portable loads.
Celtic, this is trickier than I had expected, especially since I am struggling to find code articles to prove my point.Why are we calculating step 1?
Celtic, this is trickier than I had expected, especially since I am struggling to find code articles to prove my point.
I think we would all agree that if you had a load of 91 amps, you could not use a #4 copper conductor to serve that load. Am I right? The 310.16 ampacity is 85 amps, and the load is 91 amps. Bad choice of conductors, perhaps, but where exactly in the code does it say I can?t do that?
(3) The next higher standard rating selected does not exceed 800 amperes.
Let me start there, and I?ll tell you later where this question leads me.
Nothing in 240 is going to say anything about what conductor I can, or cannot use, for a given load. 240 is all about the breaker or fuse. I am talking about selecting a conductor to serve a load.A #4 is 85A [T310.16]...and would not be allowed for 91A...240.4(B)(3) disallows it:
OK, now to finish the story. I see where we are having trouble communicating. The question is not whether an 800 amp breaker can be used to protect a pair of 500 MCM copper conductors. It can, for the reasons you gave. I agree with you on everything you have said. Happy now?No problem...we can pick it up later on.