500 kcmil feeding a 400 amp service

[LAYMAN JOE]

I have a 400 amp temporary service which is tapped to (2) service disconnecting means.
one is a 400 amp fused disconnect, fused at 300 amps.
the other is a 125 amp main breaker panelboard.
The total calculated load is 340 amps.

The service conductors are 500kcmil.

There is great debate over the use of 500 kcmil to feed a 400 amp service.

Is this a violation or what?

Your help is greatly appreciated.

Joe

 

[iwire]

As you have more than one service disconnect the only thing that matters as far as the service conductor size is the calculated load.

 

[Dennis Alwon]

Joe, assuming you are using thhn and copper then the 500mcm cable is good for 380 amps. Art. 240.4(B) states we may use the next higher rating of breaker-- 400 amps.

This is okay as long as the calculated load is not greater than 380 amps.

 

[LAYMAN JOE]

Thats how I read it...

I think we are in the minority. We shall see. hopefully many will comment.

 

[charlie b]

This is okay as long as the calculated load is not greater than 380 amps.

I agree. And if that puts me in the minority, then I would be interested in hearing how the majority sees it.

 

[iwire]

I am still curious to know if we have one service disconnect or two?

The result is the same either way but different code rules apply.

 

[rr]

As others have stated, it's fine.

Why do your coworkers disagree?

 

[LAYMAN JOE]

As others have stated, it's fine.

Why do your coworkers disagree?

I found this thread that represents the other side.

 

[charlie b]

I am still curious to know if we have one service disconnect or two?

The way I originally read the original description, I thought I understood that there were two service disconnects. I interpreted the situation as a utility transformer, with two sets of transformer secondary conductors, and with two overcurrent devices placed some distance away (using the tranformer version of the tap rules).

But with the emphasis being placed on the total calculated load being 340 amps (i.e., less than the ampacity of 500 MCM copper), I am not certain what wire (or wires) is (are) 500 MCM. At what point does the 500 split off and serve the two separate overcurrent devices? :-?

 

[LAYMAN JOE]

If you guys have time to post the code articles that support this claim, that would be awesome and much appreciated.

The claim:
With a 400 amp service, Conductors can be sized at 500 Kcmil as long as the calculated load is 380amps or less.

I will need full ammunition come inspection time.:wink:
I have a feeling it will fail. The AHJ told me my ground rods can NOT be greater than 6' apart. He told me that after telling him I put them ten feet apart from one another. :-?

 

[LAYMAN JOE]

The way I originally read the original description, I thought I understood that there were two service disconnects. I interpreted the situation as a utility transformer, with two sets of transformer secondary conductors, and with two overcurrent devices placed some distance away (using the tranformer version of the tap rules).

But with the emphasis being placed on the total calculated load being 340 amps (i.e., less than the ampacity of 500 MCM copper), I am not certain what wire (or wires) is (are) 500 MCM. At what point does the 500 split off and serve the two separate overcurrent devices? :-?

Sorry for the confusion...

Overhead service feeds through a meter, through a 4' trough into a 400a frame fused disconnect. In the trough, the cables are tapped and feed into a 125 amp main breaker panelboard.

This is a temporary service. The 400 amp disconnect feeds a double wide trailer, which is a temporary financial bank. the 125 amp panel feeds a work trailer.

500 mcm feeds the meter and the line side of 400 amp fused disconnect. The 500 mcm stops there.

 

[rr]

If you guys have time to post the code articles that support this claim, that would be awesome and much appreciated.

Others have posted Article 240.4(B).

The AHJ told me my ground rods can NOT be greater than 6' apart. He told me that after telling him I put them ten feet apart from one another.

I know he's the AHJ, but I disagree with him regarding this as well. Look at Articles 250.53(B) and 250.56 for distance requirements. The 6' rule is a minimum, not maximum.

 

[charlie b]

The AHJ told me my ground rods can NOT be greater than 6' apart. He told me that after telling him I put them ten feet apart from one another. :-?

250.53(B) says they must be at least 6 feet apart. Nothing gives a maximum distance.

 

[charlie b]

If you guys have time to post the code articles that support this claim, that would be awesome and much appreciated.

Others have posted Article 240.4(B).

I fear that that is not enough of an answer. 240.4 talks about protecting the conductor from overcurrent. The controversy has to do with having a calculated load under 380 amps, and protecting the cable over 380 amps. What is needed here is the code article that states that a conductor need have an ampacity no higher than the calculated load.

In other words, why is the feeder to a 400 amp panel allowed to have an ampacity lower than 400 amps? We can say (and I have said) that if the calculated load is lower than the panel?s rating, then the ampacity of the feeder only needs to be enough to handle the calculated load. I am not certain where that is stated.

 

[LAYMAN JOE]

250.53(B) says they must be at least 6 feet apart. Nothing gives a maximum distance.

I know... I had to humbly act confused and inform him they need to be at least 6' apart instead of within 6' of one another.

Thats why I forsee broblems with this inspector. I see him saying I need 600 kcmil for a 400 amp service even though the calculated load is less than 380 amps. 500 kcmil is sufficient.

 

[LAYMAN JOE]

charlie,

The link I posted in, I think post # 8, explains the other side. Basically, the service conductor size can not be rated less than the service OCP.

Check it out, It really is a good read.

two senior members disagree on this subject.

 

[Mike01]

my 2 cents

my 2 cents

I have to agree with everyone else if the load is rated at less than 380A than 500Kcmil would be sufficient for a 400A service, the only reason I can see to use a larger cable is on the secondary of a transformer with a secondary OCP of 400A article 240.21(c)(2)
Transformer secondary conductors not over 3M (10 ft) long you need to comply with all of the listed provisions and b states that the ?not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent-protective device at the termination of the secondary conductors? so in this instance I guess you could run parallel 3/0 or one set of 600Kcmil, some inspectors in the area have been enforcing this allot lately, the only problem is that most manufacturers 400A circuit breakers only hold a maximum of 1 set of 500Kcmil?s so if the equipment has already been purchased than you need to provide parallel runs.

 

[don_resqcapt19]

The code sections are 230.42 and 230.90(A) Exception #3.

 

[dnem]

charlie,

The link I posted in, I think post # 8, explains the other side. Basically, the service conductor size can not be rated less than the service OCP.

Check it out, It really is a good read.

two senior members disagree on this subject.

Question ! . Question from the peanut gallery right over here !

There is a disagreement and senior member #1 points to 230.42(B) and says, "No 400 OCPD for a 500 kcmil !"

Senior member #2 points to 240.4(B) and says, "Yes 400 OCPD for a 500 kcmil !"

What is the reply from senior member #1 ?
I'm imagining something like, "230.42(B) says 240.4(B) can't be used for a service. . 240.4(B) is only available for feeder and circuit wiring."

Is that the explanation ?

Because of wording like 230.90(A)x3, I was always under the impression that OCPD was more relaxed for services than for feeders and b.circuits. . I'm missing something here.

David

 

[Dennis Alwon]

David, I read 230.42 (B) to mean that the ungrounded service conductors referred to here are for the specific installations of art. 230.79 (A) thru (D). 400 amps does not seems to fit into these categories.

Maybe I am missing something but it seems clear that 400 amps and 500mcm are compatable in this situation.