In the past I have always run parallel 4/0 copper from the transformer to the meter on the house. Then 4/0 copper from the meter to a 400 amp disconnect in the basement on an outside wall. Someone mentioned that I could use 2/0 copper in parallel instead. Does anyone here have anything to support this suggestion? I've checked the 2014 code which my state (Mass) is using and could not find anything supporting this setup.
400 amp underground residential service question
- Thread starter jzadroga
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HoosierSparky
Senior Plans Examiner, MEP
- Location
- Scottsdale AZ
- Occupation
- Senior Plans Examiner
The '08 & '11 would not permit that as the adjusted amapcity based on 4 current-carrying is less than 350 and 35 is a standard size breaker plus there is no provision for paralleling in 310.15(B)(6).
'14, I believe, addresses this in an entirely different manner so I can't testify but I have doubts.
GerryB
Senior Member
- Location
- woodbridge, ct. USA
In Ct. the two main POCO's are UI and CL&P. UI tells you what you should get and helps you pull it in. CL&P pulls it themselves, you need to provide a rope in the conduit. Pretty sweet! I did a 400 amp for CL&P a year ago and they only pulled 4/O. They have their own rules.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
There is not a clear cut answer as there are a few variables that would effect what wire size you could use. The first is if the conductors are run in one raceway or two raceways, the second is if you want a full 400 amp service, or can go with a little less and use the "next size up" rule. you applicable code sections are 310.15(B)(16), 310.15(B)(3)(a), 310.15(B)(7) and 240.4(B) A few suggestions though that would cut costs tremendously: I would likely use aluminum conductors, and I would likely use 2 sets of service entrance conductors and two 200 panels MB panels.
kwired
Electron manager
- Location
- NE Nebraska
310.15(B) (7) was completely re-written in the 2014. Before then it had been fairly unchanged for years, and they gave you a table that specified what size conductor was permitted for x amount of amps. No real exceptions were in place to make any adjustments, we were just stuck with the sizes in the table. A 400 amp conductor according to that chart could have been a 400 kcmil copper conductor - no mention of whether one could split that into equal parallel conductors or not.
2014 it was completely re-written an the basic rule is now you calculate those conductors like any other conductor and apply necessary adjustments just like any other conductor. You are then permitted to only use 83% of what you would otherwise as long as it is a conductor supplying a dwelling per the conditions mentioned in that section. I'd say with the changes it is more clear that you could parallel conductors as long as total ampacity is still 83% of the overcurrent protection. So a pair of 2/0 copper would be acceptable. Also keep in mind you can not parallel conductors smaller then 1/0 so you are not going to be able to use this reduced capacity rule for anything smaller then about 360 amps, and the reduced capacity rules only go up to a 400 amp circuit so you will not be using it for more then 400 amps either. This leaves you with a very narrow application where you can use parallel conductors with 310.15(B)(7). Maybe the CMP did not realize it would be this way and they did not intend parallel conductors, IDK, but that is how I see it as currently written.
- Occupation
- Licensed Electrician
Nope. They do.
kwired
Electron manager
- Location
- NE Nebraska
Though most people don't do it, you may be able to increase ampacity due to low ambient temp underground. You could have an adjustment of 1.15% if ambient is 50 F or less, though that is kind of a low soil ambient soil temp for most of the US.
But the 100% level is 78-86, and I'd guess that many areas have lower soil temps then that, especially at 18-24 inch or more depth.
shortcircuit2
Senior Member
- Location
- South of Bawstin
Can you just put in 2-200amp panels? You could get by with a service lateral that is sized for the load to be served this way...
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I agree with Kwired-- Ma. under the 2014 NEC so the 83% factor would be the rule for residential work. Assuming 2/0 copper at 75C we get 175 amps. Parallel the 2/0 and we get an ampacity of 350 amps. 400 amp service at 83% is equal to 332 amps. As long as the conductor is rated 332 amps or better then it is compliant on a 400 amp service at 83%. (332 amps)
kwired
Electron manager
- Location
- NE Nebraska
One requirement of 310.15(B)(7) is that the conductor must feed the entire load of the dwelling unit. If you are supplying it with two feeds- neither feed is supplying the entire load of the dwelling so you can not use this optional ampacity selection method in that situation. Should you be supplying a two family dwelling with one feed to each, then it would be allowed.
shortcircuit2
Senior Member
- Location
- South of Bawstin
I based my service design on the exception allowed under 230.90(A)(3)...
But if the OP customer requires a 400amp main switch then my method would not be available to him.
kwired
Electron manager
- Location
- NE Nebraska
I take it you meant 230.90(A) exception 3.
I don't see that exception working for two disconnecting means supplied by two separate feeds. You could size a single feed that taps to two separate panels using 310.15(B)(7) but the taps would not be allowed to use (B)(7) because neither one supplies the entire dwelling.
The 239.90(A) exception 3 however would allow you to feed two 200 amp service disconnecting means from a single feed of any ampacity less then 200 amps provided you still have an ampacity at least equal to the calculated load. So you could have 100 amp conductor supplying 2 - 200 amp service disconnecting means if the load calc is 100 or less. Seems kind of odd, but would be NEC compliant.
shortcircuit2
Senior Member
- Location
- South of Bawstin
Yes thats what I mean...
shortcircuit2
Senior Member
- Location
- South of Bawstin
Ok...now I need to understand what you mean
What do you mean by 2 separate feeds?
2 Services?
2 Sets of Service Entrance Conductors?
kwired
Electron manager
- Location
- NE Nebraska
I guess I was thinking 2 sets of SE conductors at the time, but if you had a situation where 2 services were allowed - you still can't use 310.15(B)(7) because neither one of them is serving the entire dwelling load.Ok...now I need to understand what you mean
What do you mean by 2 separate feeds?
2 Services?
2 Sets of Service Entrance Conductors?
310.15(B)(7) applies to service or feeder conductors supplying 120/240 volt single phase 100 to 400 amps for a dwelling unit, whether it be a single family dwelling or an individual feed to a single unit of a multifamily dwelling. A feeder or service conductor that does not supply all the loads associated with the dwelling is not permitted to use (B)(7) permitted ampacity.
mwm1752
Senior Member
- Location
- Aspen, Colo
using Kwired process and assuming you are using 90 deg wire for your 2/0 -- 2 x 195 amps x 1.15 temp correction x 80% 4-6 curentcarrying cond = 358.8 amps, 240.6 applies
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