service conductor sizing

[chad lawson]

I am a cnc machine tool electronic technician with a strong industrial electric back ground. The company i work for is currenty expanding and would like to better understand the conductor size need for our new service and if our old service is adequate to continue running the old building. I have a 2500 amp 480/277 main breaker rated for continous duty, our new electrical service contractor has told us our current service which consists of 6 sets of 500kcmil conductors is not rated for continous duty rather about 1800 amps. i need to no if this is correct or not he mentioned our previous contractor used the 90c chart rather than the 60c. I will also be installing a seperate 2000amp 480/277 service for the new construction and would appreciate some help sizing this service. The problem i have is management wants to believe our new contractor but would like to be reasurred that our last contractor sized things wrong before we go through the expense of changing the conduits and conductors on our exsisting service.

 

[chad lawson]

i would like to add the existing service runs aprroxamtely twenty feet underground to a pad mount transformer provided by the utility provider.

 

[bsh]

I am assuming that the facility will require the full 2500 amps that the main breaker will allow. Since the service is run underground the cable ampacity needs to be derated because the earth surrounding the underground duct provides thermal insulation and will not allow the heat generated in the cable to be dissipated as easily as cable/conduit in free air. Using Annex B in the the NEC will provide you with information as to how much current each cable can carry. The IEEE Brown Book also has information about calculating this ampacity. The ampacity will be based on whether the cable is direct buried or in conduit, spacing of cables(conduit) in the duct, type of soil, etc. If you read the first part of Annex B it will provide you with additional information as to why derating is required in underground installations. I believe that the existing 6 sets of 500KCM cable will allow considerably less than 2000 amps of continous current.

 

[iwire]

6 sets of 500 is at best 2280 amps in most cases.

This should be corrected.

 

[augie47]

Bob,
Help me here. My UL "Molded Case Circuit Breaker Guide" under "Continuous Duty" breakers states that the may be marked for 75? or for 90? terminations. Disregarding the transformer terminations for a moment, if the breaker is marked for 90? termination, could you not use the 430 amp rating for 500 kcmil (thus giving you 2580) ?

 

[Pierre C Belarge]

6sets x 380A = 2280 amps. Rounding up is not permitted for this size as per 240.4(C).
If there is more than one service disconnecting means, the conductors may be permitted based on load calculation as per 230.90Ex#3.

Is there more than one service disconnecting means for your existing service? I see you mentioned one, but I am asking anyway, sometimes the obvious is overlooked.


If there is one disconnecting means, it would probably be a lot less expensive to add an additional service disconnect, say a 100A house panel.
This way the conductors and wiring will not need to be changed.


Who supplies the conductors for the existing service? If supplied by the POCO, the size is not determined by the NEC.

 

[Pierre C Belarge]

Bob,
Help me here. My UL "Molded Case Circuit Breaker Guide" under "Continuous Duty" breakers states that the may be marked for 75? or for 90? terminations. Disregarding the transformer terminations for a moment, if the breaker is marked for 90? termination, could you not use the 430 amp rating for 500 kcmil (thus giving you 2580) ?

No. There are a lot of lugs/bus, etc... that are listed and labeled for 90C. The circuit breaker will most likely be labeled either 60C or 75C.

Some larger breakers may be listed at 90C, I have not seen them though.

The "weakest link" process is attached to the termination temperature of wiring SYSTEM. The circuit breaker enclosure also would have to be listed/labeled for 90C, and I am not aware of any enclosures with that rating.

 

[iwire]

Bob,
Help me here. My UL "Molded Case Circuit Breaker Guide" under "Continuous Duty" breakers states that the may be marked for 75? or for 90? terminations. Disregarding the transformer terminations for a moment, if the breaker is marked for 90? termination, could you not use the 430 amp rating for 500 kcmil (thus giving you 2580) ?

No. There are a lot of lugs/bus, etc... that are listed and labeled for 90C. The circuit breaker will most likely be labeled either 60C or 75C.

Some larger breakers may be listed at 90C, I have not seen them though.

It is my understanding that there are no breakers under 600 volts that allow sizing the conductors at 90C.

There are large breakers that require the use of 90 C conductors but they have to be sized at no more than the 75 C rating.

 

[augie47]

Thanks. I knew that I had not seen one, but the wording in the UL book made me question.
As a side note of curiosity, are transformer terminations limited to 75? ?

 

[wawireguy]

I'd have a professional do a load calc with your new loads figured in. From what you're saying it doesn't really sound as if someone has done this. Get your load calc then it's straightforward. Either your conductors are large enough or they are not. Might be able to swap out the main breaker if it's over size for the conductors and large enough for the calculated load.

 

[Pierre C Belarge]

It is my understanding that there are no breakers under 600 volts that allow sizing the conductors at 90C.

There are large breakers that require the use of 90 C conductors but they have to be sized at no more than the 75 C rating.

I agree with this, except I cannot say that I am aware of all breakers and their ratings.

There are so many different types of breakers 600v and less, it staggers my mind.


All of the breakers I have seen installed are rated at 75C.

 

[benaround]

Chad,

Sounds like enough going on to get an EE involved, IMO, you don't want to get the wrong

equipment when it's in this range.

 

[chad lawson]

The problem i have is that we are a rapidly expanding company as our owner has been going out and buying other businesses and relocating them to our fascility in this down turn economy we have continued to grow our company. My square d main breaker has the surelogic display panel. I was running at 1700 amps continous last summer with the air conditioning load. I have since added thirty machines of various size and have a four hundred amp machine coming soon. We also started dirt work on a new addition to house even more equipment yet. So i would like to get my existing service capable of 2500 continous to ensure future growth is available in that building. I also have been running my fluke power analyzer on my mains for several weeks and realized we have major quality issues which i wondered if could be caused by undersizing at the main and virtually all distribution panels in our fascility. For example our last contractor installed a 250 amp 480/277 panel running 250 feet with 4/0 copper in suspended ridgid metalic conduit. I know this is not proper and may be causing alot of our voltage loss problems. I have seen a b and c phase as low as 205 to ground at the main lugs and fear this is causing a lot of problems on our cncs and robots.

 

[zbang]

Chad, from the sound of it, and especially as the company is doing new contruction, you/they really need to get a EE involved immediately. The cost ofthe EE could save big $$$ later. Also, since you're building and I assume there are building permits, the building departments really like to see the PE stamp on the drrawings of larger projects.

That said, since you've already done some measurements and have logs out of the Fluke, that should make the EE's work faster and easier.

 

[glene77is]

Pierre,

"" If there is one disconnecting means, it would probably be a lot less expensive to add an additional service disconnect, say a 100A house panel. This way the conductors and wiring will not need to be changed. ""

What layout do you have in mind?