800a Mdp

[K&R]

Ok I have a 800A 120/208V 3P4W 10KAIC MDP to install. The prints are telling me I need 6- 3 1/2" conduits with 4- 500MCM in each conduit. Thats a total of 24- 500MCM to go into this panel. Each conduit is 200' long. That is a total of 4800 feet of 500MCM at $8.33 this is $39,984.00 + Tax. Hell im not even sure where to put all the wire. By my calculations I should only need 3- conduits with 4- 500MCM in each one. Hell I think even 3- conduits with 4- 350MCM in each would deliver the 800A required.

Ok where am I going wrong here? I either have prints by a crackhead electrical engineer or I am missing something. The best part is I tried to contact the engineer and he is no longer in business.

I keep coming up with a min. of 4- 2/0 per phase for my calculations. I also derated the wire by 80%.

 

[infinity]

I would agree that 6 set of 500's is overkill. Given the length of 200' and possibly the use of this feeder for something in the future may have led the engineer to design it this way. Or maybe he simply made a mistake. You'll need to contact the guy paying the bill since the engineer is no longer available to see if he wants to pay for something that he may not need. Or maybe you should just do the job as designed and increase your profit.

 

[K&R]

The problem is that the wire for the service alone will be in the $50,000 range and the owner is having a hard time understanding a total cost for just the electrical for an addition to his restraunt running $125,000 and honestly I don't blame him. He really is not adding alot but this service seems to be where the cost overruns are located. I have a meeting with the owner on Friday but was hoping to figure something out for him before then. I am going to call the AHJ in the morning and see if they would ok a 800A service with 4 conduits and 4- 250MCM in each for this size service. It would cut the total price of the service down to about 1/2.

 

[K&R]

I guess my real question is do my calculations seem correct. Would 4 conduits with 4- 2/0 meet the minium NEC requirments for the 800A service?

 

[roger]

I guess my real question is do my calculations seem correct. Would 4 conduits with 4- 2/0 meet the minium NEC requirments for the 800A service?

No, it would not, it would be 100 amps short.

Connected load and Voltage drop at this distance would be key players in the actual conductor size, so we can't just assume 4 sets of say 3/0 would be sufficient with out knowing more.

Roger

 

[Pierre C Belarge]

Why don't you install 2 raceways with 500's. I am not in the contracting business anymore, so I do not know the cost of materials. It would seem that you would have 2 less pulls, less material and maybe the copper cost would be a little less overall.
Your original post with the prints by the engineer are excessive.

The actual load will determine if the 500's are adequate. My guess is that there seems to be a lot of overkill in sizing of conductors/services these days as some do not really understand how to size them.

 

[K&R]

OK I have to ask what may be a dumb question but what would the auctual load have to do with it if it has a 800A main disconect? I thought it would just be sized to the 800A. BTW the VD is 6.27% using 300MCM and the max length is 250.18'. So it seems that 3 raceways with 4-300MCM should deliver the 800A needed. The auctual load for the building according to the prints is about 350A. Another thing I found funny on the prints it that a 125A 42ckts panel has been specified even though the 4 breakers in it are at 80% capicity. Why would someone spec a panel that has 42ckts at 125A when its at max capicity with only 4ckts in it. Seems strange to me because in my area it had to be custom ordered. They had the same specs in a 32ckt panel.

 

[ces]

more questions

more questions

I have a couple of questions and thoughts if I may
-as I see it 800A @208V for 250ft in copper is (2) 3-1/2"C 4-500kcmil and the vdrop for this segment at 800A is 2.1%. 760A of cable is fine per 240.4(B)
-6.27% vdrop appears high I assume it takes into account vdrop elsewhere in the system?
-in practice the available fault current at every 800A board I've seen exceeded 10kA, this might be worth double checking and does it include load contribution. As I see it there would have to be less than 15KA available at the beginning of this run (using parallel 500 CU) to get below 10kA
-however I would definitely use aluminum not copper and would look to 90'C cable and terminations
-depending on anticipated additional loads I would consider a 400A or 600A service. For 350A connected load 400A usually is fine. Given the apparent oversizing error it might be worth checking the load calc that determined the 350A

 

[ces]

I guess my real question is do my calculations seem correct. Would 4 conduits with 4- 2/0 meet the minium NEC requirments for the 800A service?

if 90'C copper 195A cable is used 2/O looks fine to me but vdrop approaches 4% for the segment when 100% loaded. I frequently do vdrop calcs based on load or 80% of the circuit capacity which would be around 2% or 3%

 

[roger]

CES, what gear would you spec with 90 deg terminations?

Using the 90 deg column for conductors would mean every link in the chain would have to be rated at least 90 deg.

Roger

 

[K&R]

if 90'C copper 195A cable is used 2/O looks fine to me but vdrop approaches 4% for the segment when 100% loaded. I frequently do vdrop calcs based on load or 80% of the circuit capacity which would be around 2% or 3%

I see what happened. I wrote this after being awake for a long stretch. I ment to say 4- 3 1/2" conduits with 4- 4/0 THHN per phase.

 

[ces]

Roger, I would have thought we have a single length of cable with lugs at the 800A board and at the point of service
Gary

 

[roger]

Gary, even with that being the case, the terminations (lugs) are not the only items that would need to be listed for use at 90 deg, all of the "equipment" would have to be including the breaker, enclosures, etc...,

Roger

 

[ces]

Roger, that is news to me and something I would like to know more about.
For the main 800A breaker I believe that if it's lugs are 90'C we stop at that point. In the case of enclosures I did not think they had temperature ratings?
Gary

 

[roger]

Gary, see 110.14, take note that the conductors temperature rating is based on the equipments listing and identification of its temperature rating.

Here is 110.14(C)(b)(2)

(2) Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75?C (167?F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors

Roger

 

[ces]

Roger, thanks - how I read that is it means only the circuit breaker terminations is listed and identified. I'm just not aware of anywhere else that temperature ratings occur, and think that this meets 110.14(C)(b)(2)

Perhaps this is a good question for a manufacturer?

Gary

 

[mdshunk]

I know that the insulating bushings have different temperature ratings. The brown fiber type are higher rated than the grey and blue plastic one's. I've yet to run into an instance where I thought it mattered. They all seem sufficiently high enough.

 

[don_resqcapt19]

Gary,

Roger, thanks - how I read that is it means only the circuit breaker terminations is listed and identified. I'm just not aware of anywhere else that temperature ratings occur, and think that this meets 110.14(C)(b)(2)

This section requires that the equipment (cuircuit breaker) be listed for use with 90?C conductors, not just the terminations on the equipment. I am not aware of any breakers for 600 volts or less that are listed for use with 90?C ampacities.
Don

 

[ces]

90'C terminations

90'C terminations

Okay I just asked Square D Tim Milbert about this - Roger and Don you are right on the money - seems the problem is that UL does not test equipment at 90'C they only test at 75'C. This includes all equipment including non-fused switches. However Tim said a way around this is to install our own buswork for example, and use 90'C rated lugs on the bus, then the cable can be used at its 90'C rating.

In a second followup conversation with Square D (this time Stanley K) I asked -if UL don't test to 90'C then how could lugs be UL listed? Stanley clarified that Square D only have 75'C lugs. That we would likely need to go to independant testing to get the higher rating.

Where this leaves me is forget about 90'C ratings of cable unless there is a dire need such as existing conduits that are too small -and I'm willing to to do some fabrication and/or use independant lab tested equipment. Sure sounds like a headache.

While the cost to the industry for all the extra copper and aluminum must be considerable I assume the cost for UL to do all the testing at an additional temperature would be also be considerable. Having said that if they would just test a handfull of non-fused and fused switches safety switches... now that would be useful.

Gary

 

[don_resqcapt19]

Gary,

While the cost to the industry for all the extra copper and aluminum must be considerable

What about the cost to the owner who has to pay for the electricity that heats the conductors? Over the life of the installation the cost to make this heat will far exceed the cost of the wire. You are not doing your clients any favors by trying to use 90?C at its rated ampacity.
Don