Tapped or Parallel service

Status
Not open for further replies.
augie47 said:
are we saying each individual "tap" must not be smaller than the breaker supplied ?
Click to expand...

Yes. The exception covers only the 'sum' of the OCPDs, you cannot have a sum with only one breaker.

You cannot run 4/0s to a single 350 amp breaker but you could run 4/0s to supply two 350 amp breakers as long as the calculated load was below the 4/0s rating.
 
Dennis Alwon said:
Smart you always have an interesting perspective on the NEC even if I disagree with it. :D Now where does it say that one of the 2 breakers that are added together cannot be larger than the service conductors. That was my thought but I don't see except. 3 supporting that thought.

To be honest I have never seen anyone take advantage of this rule
Click to expand...
Two (or more) service OCPD's can have a higher sum than the ampacity of any service conductors. But as iwire pointed out, the exception only covers the sum. The individual OCPD's and the ampacity of the conductors supplying them must meet the same requirement as any one service OCPD, i.e. the conductors ahead of them must have an ampacity high enough to be considered protected by the OCPD.
 
iwire said:
Yes. The exception covers only the 'sum' of the OCPDs, you cannot have a sum with only one breaker.

You cannot run 4/0s to a single 350 amp breaker but you could run 4/0s to supply two 350 amp breakers as long as the calculated load was below the 4/0s rating.
Click to expand...


But the 4/0 is feeding 2 dp breakers. it feeds the 350 and the 200 amp breakers. I am not trying to argue but understand this-- I don't quite get it
 
Dennis Alwon said:
But the 4/0 is feeding 2 dp breakers. it feeds the 350 and the 200 amp breakers. I am not trying to argue but understand this-- I don't quite get it
Click to expand...

It's fine up until the tap, beyond the tap, where the 4/0s only supply one breaker is a problem.

(BTW, the oddity of these rules has not escaped me but it seems the current rules have a long history of working fine.)
 
Give me an example of the 4/0 feeding 2 breakers. Would that be 2 breakers in one panel? If so I don't see where the article addresses this.
 
Smart $ said:
...
One would think the main service conductors should be required to at least have an ampacity greater than the highest rated OCPD... :happyyes:
Click to expand...

iwire said:
But luckily they are not required to be.
Click to expand...
Are you certain about that?

As you said, the sum of the OCPD's is permitted to exceed the ampacity of the service conductors... but that in no way allows any conductor ahead of any one OCPD to have an ampacity less than its rating under the general requirement [230.90(A)]. IOW, the ampacity of the main before the taps must have an ampacity which is greater than any one OCPD. In the OP's case, the lowest ampacity of any service conductor supplying the 325A breaker is 301A when applying 240.4(B) through Exception No. 2. For the 200A breaker, all service conductors ahead of it must have an ampacity of 176A or greater. This would be in addition to each service conductor having an ampacity equal or exceeding the calculated load there upon.
 
Dennis Alwon said:
Give me an example of the 4/0 feeding 2 breakers. Would that be 2 breakers in one panel? If so I don't see where the article addresses this.
Click to expand...

Picture this, you have a calculated load of 220 amps but need lots of panel spaces so you will use two - 200 amp main breaker panels.

Use 4/0s from the weather head down to a 400 amp meter socket, from the meter socket to each panel you will need at least 3/0 as those conductors only supply a single OCPD.
 
Smart $ said:
Are you certain about that?

As you said, the sum of the OCPD's is permitted to exceed the ampacity of the service conductors... but that in no way allows any conductor ahead of any one OCPD to have an ampacity less than its rating under the general requirement [230.90(A)]. IOW, the ampacity of the main before the taps must have an ampacity which is greater than any one OCPD. In the OP's case, the lowest ampacity of any service conductor supplying the 325A breaker is 301A when applying 240.4(B) through Exception No. 2. For the 200A breaker, all service conductors ahead of it must have an ampacity of 176A or greater. This would be in addition to each service conductor having an ampacity equal or exceeding the calculated load there upon.
Click to expand...

You have totally lost me.

To sum up, your point is that if I had 100 amp service conductors that I could connect six 100 amp breakers to them but I could not install one 125 amp and five 100s.

If so I am not buying it yet buy may be swayed.
 
iwire said:
Picture this, you have a calculated load of 220 amps but need lots of panel spaces so you will use two - 200 amp main breaker panels.

Use 4/0s from the weather head down to a 400 amp meter socket, from the meter socket to each panel you will need at least 3/0 as those conductors only supply a single OCPD.
Click to expand...
Now change the calculated load to say 160A. Still 200A panels.

The way I understand your previous posts, you're saying we can drop 2/0@175A to the socket, but yet we still have to run 3/0's to each panel. I'm saying we have to drop at least 3/0's because the highest rated OCPD is 200A.
 
Dennis: Also think about multi-family dwelling units before modular meter cabinets became popular. A wireway with service conductors tapped to (6) meters & disconnects.
 
iwire said:
You have totally lost me.

To sum up, your point is that if I had 100 amp service conductors that I could connect six 100 amp breakers to them but I could not install one 125 amp and five 100s.

If so I am not buying it yet buy may be swayed.
Click to expand...
Consider the highest rated OCPD (125A). Where in the requirements or exceptions does it permit any service conductor supplying it to be rated 100A or less.
 
iwire said:
It's fine up until the tap, beyond the tap, where the 4/0s only supply one breaker is a problem.
Click to expand...
I can buy that.. that's normally the situation I see also.

iwire said:
(BTW, the oddity of these rules has not escaped me but it seems the current rules have a long history of working fine.)
Click to expand...
Apparently. A MLO switchboard with (6) 400 amp breakers with (3) parallel 500s as a supply and similar installs has bugged me for decades but I've yet to see my first one melt :)
 
augie47 said:
I can buy that.. that's normally the situation I see also.
Click to expand...
It's not fine. The highest rated breaker is 325A. All service conductors ahead of it must have an ampacity considered to be protected by that 325A OCPD.


Apparently. A MLO switchboard with (6) 400 amp breakers with (3) parallel 500s as a supply and similar installs has bugged me for decades but I've yet to see my first one melt :)
Click to expand...
In this case it is fine. The ampacity is 380A ea. x 3 sets = 1140A combined, and that's greater than any one OCPD.
 
iwire said:
In my opinion exception 3 covers it when combined with others.
Click to expand...
Say you do have one 125A breaker and five 100A breakers, 100A main conductors. Disconnect the five 100A breakers. Is the remaining installation compliant?
 
Smart $ said:
Now change the calculated load to say 160A. Still 200A panels.

The way I understand your previous posts, you're saying we can drop 2/0@175A to the socket, but yet we still have to run 3/0's to each panel.
Click to expand...

IMO we could drop to 2/0s to the socket. (I think there is a section telling us the individual runs don't have to be larger than the supply, but I might be thinking of feeders)


I'm saying we have to drop at least 3/0's because the highest rated OCPD is 200A.
Click to expand...

I am not seeing that.
 
iwire said:
Of course not, it also no longer fits the exception.
Click to expand...
But its not in the exception to begin with. The exception only excepts the sum relative to the ampacity of the conductors ahead of it. It does not except the individual OCPD and the conductors ahead of it.

Disregard Exception No. 3 for a minute... as if it's never been there. Now apply 230.90(A) to the 125A and five 100A breakers. What is the least ampacity the service conductors can be.
 
View attachment 480V Model (1).pdf So the installation would be code compliant even though no OCPD is protecting the cabling from the transformer to the tap and then to each of the service entrance main breakers? I have a problem with the tap located within the 480V panel before both main breakers of the panels being fed. I am not a fan of taped split-bolt connections either.

please take a look at the .pdf if that helps

P.S. Maybe I should put this in the 'what's wrong this picture' contest.
 
Last edited:
Status
Not open for further replies.
Top