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Old 08-27-2009, 05:39 PM   #1
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Default 400 amp service county requiring 600alu for mast?

I just finished a 400 amp residential service that consisted of a 400 amp meter base that feeds two 200 amp exterior service disconnects and two 200amp interior panels. I ran a 3 1/2" pvc mast about 8 feet or so with paralleled 4/0 alu up the mast. On 310.16 excemption 4 says I don't have to derate the conductors in conduit less than 10 ft. but the county inspector failed me and is requesting 600alu or paralleled 300's up the mast? It also rates 4/0 alu at 200 amp for a service feeder application. He says it's in the IRC book but I don't have that in front of me at the moment. What's the deal? I'm of course going to change it tomorrow
morning(Just to get the inspection passed) but this just doesn't seem right to me.


Thanks for the help in advance
-James


Last edited by Lythropus; 08-27-2009 at 05:41 PM. Reason: clerical error
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Old 08-27-2009, 05:46 PM   #2
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4/0 is too small. Can't use 310.16b6 for the subpanels.

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Old 08-27-2009, 06:16 PM   #3
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310.15(B)(2)(a) Exception No. 4: Derating factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of RGS, IMC or ridig nonmetallic conduit having a length not exceeding 3.05 m (10 ft) and if the number of conductors does not exceed four.

Your installation is a mast that has parallel 4/0. You can not use this exception. You must derate.

Why didn't you just run parallel 4/0 SEU?

NolaTigaBait 310.15(B)(6) does allow you to use the conductors sized for the service-entrance as the feeder from the main disconnect to the panelboard.
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Old 08-27-2009, 06:46 PM   #4
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Quote:
NolaTigaBait 310.15(B)(6) does allow you to use the conductors sized for the service-entrance as the feeder from the main disconnect to the panelboard.
Only can use that chart if it the main feed coming in. From the drop to the line side of meter use 310.16b6 and the load to the panels use 310.15
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Old 08-27-2009, 07:21 PM   #5
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So your basically saying that even though I have 2 4/0 seu up the mast (bare removed) and a 4/0 seu cable feeding each panel I still must derate to 80% of original load capacity because I'm running between 4 to 6 load carrying conductors in a pipe?

-james

Addition: Isn't 600alu XHHW still only 385?

Last edited by Lythropus; 08-27-2009 at 07:49 PM.
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Old 08-27-2009, 07:33 PM   #6
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Originally Posted by Lythropus View Post
So your basically saying that even though I have 2 4/0 seu up the mast (bare removed) and a 4/0 seu cable feeding each panel I still must derate to 80% of original load capacity because I'm running between 4 to 6 load carrying conductors in a pipe?

-james
Yeah, I re-red the exception and it says for underground installations as well. The feeders to the panels can't be 4/0 alum. It's pretty stupid, but it is what it is.
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Old 08-27-2009, 07:53 PM   #7
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so what does everyone else use for the mast and feeders to the panels on a 400 amp?
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Old 08-27-2009, 08:01 PM   #8
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Ok I seriously didn't think I had to derate my service, I thought 310.15(b)(6) was gold like it was. And I was wrong about the exemption...hmmm
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Old 08-27-2009, 10:43 PM   #9
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310.15 (b) 2 (a) says conductors leaving the ground, yours are not, they're leaving the meter base.
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Old 08-27-2009, 11:08 PM   #10
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Quote:
Originally Posted by Lythropus View Post
I just finished a 400 amp residential service that consisted of a 400 amp meter base that feeds two 200 amp exterior service disconnects and two 200amp interior panels. I ran a 3 1/2" pvc mast about 8 feet or so with paralleled 4/0 alu up the mast. On 310.16 excemption 4 says I don't have to derate the conductors in conduit less than 10 ft. but the county inspector failed me and is requesting 600alu or paralleled 300's up the mast? It also rates 4/0 alu at 200 amp for a service feeder application. He says it's in the IRC book but I don't have that in front of me at the moment. What's the deal? I'm of course going to change it tomorrow
morning(Just to get the inspection passed) but this just doesn't seem right to me.


Thanks for the help in advance
-James



Shows you what I know.

I thought you size service conductors based on the calculated load.
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Old 08-28-2009, 11:04 AM   #11
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Originally Posted by NolaTigaBait View Post
Only can use that chart if it the main feed coming in. From the drop to the line side of meter use 310.16b6 and the load to the panels use 310.15
The more I read 310.15(B)(6) the more I see that is says you can use the conductor sized for the service-entrance all the way through to the panel board. This is a residential job not a commercial job. In residential you are allowed 4/0Al for a 200a service.

I have always installed residential services that way and never had any issues.

Correct me if I am wrong.
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Old 08-28-2009, 05:22 PM   #12
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Quote:
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The more I read 310.15(B)(6) the more I see that is says you can use the conductor sized for the service-entrance all the way through to the panel board. This is a residential job not a commercial job. In residential you are allowed 4/0Al for a 200a service.

I have always installed residential services that way and never had any issues.

Correct me if I am wrong.
You are right, if the calculated load isn't more than what 4/0 aluminum is rated for at the 60 degree chart.
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Old 08-28-2009, 10:07 PM   #13
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I don't have the calcs in front of me but the house didn't even need a 400 amp service we just like to make sure they have plenty of room to grow on larger homes, and we split up large rooms alot into lighting loads and rec loads so we end up with alot of circuits...
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Old 08-28-2009, 10:30 PM   #14
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so what does everyone else use for the mast and feeders to the panels on a 400 amp?
3/0 copper paralleled twice. Around here the power company will run the riser, but most of the time the electrician installs it. Is it that way in other parts of the country?
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Old 08-30-2009, 11:14 AM   #15
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Originally Posted by heel600 View Post
Shows you what I know.

I thought you size service conductors based on the calculated load.
Did you exceed the four conductor limit?
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Old 08-30-2009, 05:11 PM   #16
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310.4 (a) say that I can parall 4/0 And 310.15 say that parlled 4/0 can be used for a 400A dwelling service, 310.15(4)) say that a neuteral that carries only the unbalance current from other conductors of the same circuit shall not be required to be counted Also in 310.15(B)(6) chart says that paralled 4/0 is the same as 600 mcm any way the wire will carry the same load

Last edited by Little man; 08-30-2009 at 05:17 PM.
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Old 08-30-2009, 06:22 PM   #17
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310.4 (a) say that I can parall 4/0 And 310.15 say that parlled 4/0 can be used for a 400A dwelling service, 310.15(4)) say that a neuteral that carries only the unbalance current from other conductors of the same circuit shall not be required to be counted Also in 310.15(B)(6) chart says that paralled 4/0 is the same as 600 mcm any way the wire will carry the same load
yes but paralleled 4/0 alu up a mast is 6 conductors 4 of them load carrying which he is saying I have to derate according to 310.15(b)(2)(a) at 80% with between 4 to 6 load carrying conductors
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Old 08-30-2009, 06:23 PM   #18
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they make us run the riser
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Old 08-30-2009, 09:06 PM   #19
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I don't know why the inspector failed you here. Table 310.15 (B)(6) specifically states 4/0 Aluminum is good for 200 amps, so why wouldn't the same amperage apply in parallel if you have followed the guidelines laid out in 310.4? Derating should have nothing to do with it if you don't have more than three current carrying conductors in each conduit.

You did run 2 separate conduits on the line of the meter right?
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Old 08-30-2009, 09:14 PM   #20
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Quote:
I ran a 3 1/2" pvc mast about 8 feet or so with paralleled 4/0 alu up the mast.

So you have all (6) conductors up one conduit? Now I know why your inspector failed you. That's 6 CCC's (current carrying conductors) in 1 conduit. Table 310.15 (B)(2)(A) ranks those 6 conductors to be derated by 80% of the ampacity listed in table 310.16, not 310.15 (B)(6).

The ampacity of 4/0 aluminum per 310.16 is 150 amps.

150 amps X .80 = 120 amps.

120 amps X 2 = 240 amps.

So what you thought was a 400 amp service is really only good for 240 amps. You need to run a 2nd conduit so you don't have to derate and can actually use table 310.15 (B)(6) for your 400 amps.

Good luck.

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