service conductor sizing

[kp542]

I am looking at a drawing showing an 800 amp service supplied by (2) sets of 4 500 MCM THWN conductors...

The service will supply (4) subpanels: (2) 200 amp, (1) 400 amp, & (1) 100 amp...

Are the service entrance conductors sized correctly?

 

[EvertonFerreira]

I am looking at a drawing showing an 800 amp service supplied by (2) sets of 4 500 MCM THWN conductors...

The service will supply (4) subpanels: (2) 200 amp, (1) 400 amp, & (1) 100 amp...

Are the service entrance conductors sized correctly?

Whats is distance?


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[texie]

I am looking at a drawing showing an 800 amp service supplied by (2) sets of 4 500 MCM THWN conductors...

The service will supply (4) subpanels: (2) 200 amp, (1) 400 amp, & (1) 100 amp...

Are the service entrance conductors sized correctly?

Maybe. What is the calculated load?

 

[kp542]

Total calculated load of the 4 panels is 452 amps...

Distance is approx. 50 feet.

 

[augie47]

You say "subpanels". Is there a main service panel, if so, what amperage main and are we taking 3 phase or single phase and one conduit or two ?
(99% chaneg you are fine, but there a a couple of "traps".

 

[kp542]

The service conductors feed a CT cabinet which feeds 4 meters on the outside of the building...Each meter feeds a MCB panel on the interior... I should not have characterized them as "subpanels"... can you enlighten me on the 'traps" I should look for in the future?

 

[Dennis Alwon]

There should not be an issue at all, iMO.

 

[augie47]

Just a case of CMA before answering...
Since we were a little short on details,, there was a slim chance it was a 3 phase service fed by one conduit in which case we would have had at least 6 current carrying conductors requiring a 80% adjustment factor. 500 kcmil with a 90° rating of 430 amps would then be rated 688 amps and, if there was a "main" involved, would require a 700 amp main.

In your situation you are fine with 500's, in fact you could actually get by parallel 4/0s or larger with your calculated load.

 

[kp542]

Very sorry about the lack of details...It is 120 / 208 3 phase power...The SE conductors are in (2) conduits: 4 500 mcm conductors in each...

thanks for the help...

 

[rlundsrud]

It's been a while however...

I thought with 800 amp you need 2 sets of 600 mcm.

 

[infinity]

500 kcmil=380 amps*2=760 amps, next standard size 800 amps. Conductors are adequately sized.

 

[Dennis Alwon]

500 kcmil=380 amps*2=760 amps, next standard size 800 amps. Conductors are adequately sized.

This is written in art. 240.4(B)

 

[kp542]

Last question...

If there are (4) 500 MCM conductors in each set, why is each set 380 amps, which I thought was the allowable amperage of (1) conductor...

 

[infinity]

Last question...

If there are (4) 500 MCM conductors in each set, why is each set 380 amps, which I thought was the allowable amperage of (1) conductor...

Each set consists of 3 phase conductors and a neutral conductor. Set #1 has an ampacity of 380 amps, set #2 also has an ampacity of 380 amps. Since they're parallel conductors (connected on both ends) they're now a 760 amp conductor.

 

[kp542]

So the 760 amps flows to each of the (3) phases?
I mistakenly thought that the (3) ungrounded conductors in each set carried a total of 1140 amps ( 3 x 380)...

Just trying to get a better handle on sizing service entrance conductors...

 

[Cow]

I am looking at a drawing showing an 800 amp service supplied by (2) sets of 4 500 MCM THWN conductors...

The service will supply (4) subpanels: (2) 200 amp, (1) 400 amp, & (1) 100 amp...

Are the service entrance conductors sized correctly?

Total calculated load of the 4 panels is 452 amps...

Distance is approx. 50 feet.

With 452 amps being the calculated load and 4 service disconnects, one could use parallel 4/0 CU and still meet NEC requirements allowing us to size to the load since there isn't one single 800 amp main.

 

[kwired]

The service conductors feed a CT cabinet which feeds 4 meters on the outside of the building...Each meter feeds a MCB panel on the interior... I should not have characterized them as "subpanels"... can you enlighten me on the 'traps" I should look for in the future?

Does that mean there is 4 service disconnecting means on this?

If so this is one application where the common supply conductors only need to have an ampacity equal or greater then the calculated load. Even though you add the numbers of the main breakers and come up with 800 - if load calculations only come up to say 570 amps - you are legal code wise if you have at least 570 amps of supply conductor. Good design practice may be to allow for future load addition, but as is in my example 570 is allowable now.

Leads to individual overcurent devices must have ampacity in accordance with that device setting though.

 

[kingpb]

Has nothing to do with load. It has to do with that there is an 800A panel and the owner must want full capacity so you need 800A worth of cable. NEC is for minimum safety guidelines not a design manual.

 

[infinity]

With 452 amps being the calculated load and 4 service disconnects, one could use parallel 4/0 CU and still meet NEC requirements allowing us to size to the load since there isn't one single 800 amp main.

Since he said sub-panels I would assume that there is a main but if not you're correct.

 

[JFletcher]

It's been a while however...

I thought with 800 amp you need 2 sets of 600 mcm.

801 amp and larger, the wires must carry the full capacity of the main breaker, no rounding allowed. I have been frequently tripped up by this as well.

if the calculated load was above 760 amps, or D rating brought the wire capacity under load, then yeah you have to go to 600 versus 500