400 amp service parallel conductors

[Tarzan]

As a generally accepted method in all of the NC countries an municipalities I have worked in over the past twenty years, inspectors have always passed parallel 3/0 copper conductors for a 400 amp service. However an inspector has turned down my service with the explanation that the circular mill of the 3/0 copper must equal or exceed the same as 600 kcmil copper needed to supply the ampacity of the service. All these years we been doing it wrong?

 

[ryan_618]

The circular mil is irrelevant. Ampacity is the driving factor.

 

[RUWired]

As in article 230.42(A)(1,2).

230.42 Minimum Size and Rating
(A) General The ampacity of the service-entrance conductors before the application of any adjustment or correction factors shall not be less than either (A)(1) or (A)(2). Loads shall be determined in accordance with Article 220. Ampacity shall be determined from 310.15. The maximum allowable current of busways shall be that value for which the busway has been listed or labeled.
(1) The sum of the noncontinuous loads plus 125 percent of continuous loads
(2) The sum of the noncontinuous load plus the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent device and its assembly are listed for operation at 100 percent of their rating.
Rick

 

[roger]

Show the following article from Mark Ode to the inspector and tell him to read Mark's example in the 6th paragraph.

http://www.ul.com/regulators/ode/0703.pdf

Roger

 

[jtester]

Parallel conductors almost always have an equivalent cross section area that is less than an equal ampacity single cable. Your inspector is wrong.

Jim T

 

[Smart $]

As in article 230.42(A)(1,2).

While that is one requirement, 230.42(B) specifies the ampacity shall not be less than the rating of the service disconnecting means specified in 230.79(A) through (D), which refers one back to Article 220 calculations.

The gist of 230.42(B) is that if one's service disconnect has a higher amperage rating than the minimum calculated load, the ampacity of the service entrance conductors must have an ampacity equal to or greater than the rating of the service disconnect. Example: calculated load is approximately 325A per ungrounded conductor. One opts for a 400 amp service [disconnect]. The ampacity of the service entrance conductors must be 400A or greater.

Service drops and laterals are not bound to this requirement, and their ampacity can be determined solely by Article 220 calculations.

 

[infinity]

The gist of 230.42(B) is that if one's service disconnect has a higher amperage rating than the minimum calculated load, the ampacity of the service entrance conductors must have an ampacity equal to or greater than the rating of the service disconnect. Example: calculated load is approximately 325A per ungrounded conductor. One opts for a 400 amp service [disconnect]. The ampacity of the service entrance conductors must be 400A or greater.

So one set of 500 Kcmil conductors with a 400 amp disconnect and a less than 380 amp calculated load is no good?

 

[Smart $]

So one set of 500 Kcmil conductors with a 400 amp disconnect and a less than 380 amp calculated load is no good?

That's how I read it. I'm actually hoping someone will shoot it down, but until then...

 

[suemarkp]

NEC 230.42(B) says the wires must have an ampacity of at least the rating specified in 230.79. NEC 230.79 says you need a disconnect rated at the calculated load, or a minimum load for the four different installation types listed. Then 230.90 says that service conductors must have overcurrent protection sized no larger than their ampacity (but round up of the breaker size being allowed).

So if 230.79 says you have a calculated load of 325A, but you choose to install a 400A disconnect, what is the rating of that disconect per 230.79 -- 325 or 400 amps? I would say your disconnect rating per 230.79 is 325A, and could use service conductors rated 351 amps or more. If a 350A breaker was installed, then service conductors rated at 325A or more would be OK.

Anyone differ?

 

[Smart $]

NEC 230.42(B) says the wires must have an ampacity of at least the rating specified in 230.79.

Not exactly. To restate it, NEC 230.42(B) says the wires must have an ampacity not less than the rating of the service disconnecting means specified in 230.79(A) through (D). Note it does not say "230.79" but rather its subparts A, B, C, and D. So the main text of 230.79 is excluded. Each of the subparts stipulate the minimum rating of the service disconnecting means for its particular type of service. However, the service disconnecting means specified in each subpart IS the service disconnecting means...and its rating is specified as "not less than". So whatever its rating may be, the wires cannot have an ampacity less then that rating.

After thinking on this, I'm wondering if the intent is better stated as such:

(B) Specific Installations. In addition to the requirements of 230.42(A), the minimum ampacity for ungrounded conductors for specific installations shall not be less than the minimum rating of the service disconnecting means specified in 230.79(A) through (D).

Can anyone confirm this to be the true intent?

Isn't it amazing how one word can change the entire meaning?

 

[George Stolz]

The gist of 230.42(B) is that if one's service disconnect has a higher amperage rating than the minimum calculated load, the ampacity of the service entrance conductors must have an ampacity equal to or greater than the rating of the service disconnect.

When the calculated load is less than the minimum service size, I would agree. Once you venture above the minimum size service, then you have satisfied 230.42 and 230.79.

Your service disconnecting means and service entrance conductors are not less than the minimums.

Example: calculated load is approximately 325A per ungrounded conductor. One opts for a 400 amp service [disconnect]. The ampacity of the service entrance conductors must be 400A or greater.

I agree with the outcome, but 230.42(B) has no bearing on this determination. 230.90 exception 2 is what gives us this information. If we selected a 350A breaker in your example, then we could use conductors sized for 325A according to exception 2 of 230.90.

If you cannot stay within the confines of the exception, then you must follow the rule, which says that "Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor."

So one set of 500 Kcmil conductors with a 400 amp disconnect and a less than 380 amp calculated load is no good?

That's how I read it. I'm actually hoping someone will shoot it down, but until then...

I disagree. Unless the load calculation comes to less than 350A (the last size fuse or circuit breaker), then this would be compliant.

Any load calculation between 351A and 380A for the setup Trevor described would be compliant, IMO.

 

[hillbilly]

"inspectors have always passed parallel 3/0 copper conductors for a 400 amp service".

"inspector has turned down my service with the explanation that the circular mill of the 3/0 copper must equal or exceed the same as 600 kcmil copper needed to supply the ampacity of the service".

Table 310.15(B)(6) lists the conductor size for a 400A dwelling service and feeders as 400 kcm copper or 600 kcm aluminum.

Did you mean that the inspector is requiring 600 kcm copper to feed a 400A service?...or 400 kcm copper?... or 600 kcm aluminum?

Table 310.16 lists the ampacity of 3/0 copper as 200A (75C).
Parallel 3/0 copper is large enough for a 400A service. (your point)

....or.....

Per 310.15(B)(6)....a 400A service requires 400 kcm copper.
Parallel 3/0 is only 336 kcm and is not large enough. (inspector's point)

...or.....

Forgetting 310.15(B)(6), the inspector is requiring 600 kcm copper (420A @ 75C) to feed a 400A service?

I can see the inspector's reasoning on the first point, not enough copper (336 kcm).
I can see your reasoning too, parallel 3/0 cu is rated 400A.
It's in the interpretation...I wonder who will win?

IMO....If the inspector is disregarding 310.15(B)(6), and requiring 600 kcm copper for a 400A residential service, he's wrong.

I hope this makes sense.
steve

 

[George Stolz]

Steve,

• Even if this is a dwelling unit, the installer is not required to use 310.15(B)(6).
• According to Table 310.16, two runs of 3/0 CU has an ampacity of 400A.

How is this an interpretation issue?

If circular mils were the driving force, then Table 310.16 would be configured according to circular mils, don't you think?

 

[iwire]

I also ask Steve if he has forgoten 240.4(B)?

 

[hillbilly]

Steve,
"Even if this is a dwelling unit, the installer is not required to use 310.15(B)(6)."
Correct...it is the minimium. If the installer does use it the inspector should accept it.

"According to Table 310.16, two runs of 3/0 CU has an ampacity of 400A.
How is this an interpretation issue?"
Apparently it is a intrepretation issue for the inspector....he turned it down.
Got any Ideas?

"If circular mils were the driving force, then Table 310.16 would be configured according to circular mils, don't you think?"
I'm trying to identify inspector's reasoning, I didn't say that I agreed with it.

steve

 

[hillbilly]

I also ask Steve if he has forgoten 240.4(B)?

Nope...didn't forget it.
steve

 

[Tarzan]

Got it straightened out Friday, chief inspector passed the 3/0 for the 208/120 three phase service as I had expected, my OP was done kinda tongue in cheek. Maybe the main lug SE panel with five mains threw the inspector. In two weeks he gets to inspect a 600 amp rated main lug SE rated panel with four mains ,with a 414 amp calculated load, feed with parallel 4/0 cu. Interesting comments and interpretations on the subject also, thanks guys, that kinda thinking keeps us on our toes.

 

[george t. everett]

If the Inspector is enforcing the International Residential Code, he is correct.
Check out Table E3503.1 of the IRC.

 

[George Stolz]

I'm trying to identify inspector's reasoning, I didn't say that I agreed with it.

I apologize, I didn't mean to do that to you.

 

[Smart $]

George,

Regarding your Post #11, I cannot say I disagree with your assessment. However an assessment such as yours is an interpretation of 230.42(B). All is fine if you interpret 230.42(B) as:

(B) Specific Installations. In addition to the requirements of 230.42(A), the minimum ampacity for ungrounded conductors for specific installations shall not be less than the minimum rating of the service disconnecting means specified in 230.79(A) through (D).​

That one word changes the subject from the rating of the service disconnecting means to the minimum rating of the service disconnecting means. This is a prime example of the difference between implicit and explicit language, or perhaps at best, a simple and inadvertent omission. Are we experienced electrical professionals to just put on blinders and say we know better? Shall we take all the rest of the code to be written in implicit terms? We can then choose which requirements best serve our interests and ignore others that we interpret to have no bearing. Of course that'll lead to AHJ's doing the same thing!