320 amp continuous, 400 amp max upgrade


  #1  
Old 07-05-04, 06:43 PM
ratboy69
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320 amp continuous, 400 amp max upgrade

Want to upgrade underground service from 150 amp to above. Ground transformer to meter base is 60 feet. Present interior loadcenter is back-to-back from meter base. Planning on runnin' parallel 3/0 copper wire through 3" steel conduit to meter base and double lug both line and load sides of the meter base. Will place one 200 amp loadcenter where present one is and second 200 amp loadcenter 3 feet from 1st loadcenter in the attached garage. Is it code compliant(Tampa Bay, Fl. area) to run the 2nd loadcenter's 3/0 copper feeders through the 1st loadcenter? The loadcenters will be connected with 2" steel nipple. Is it code compliant to just use the interior loadcenters' main disconnects rather than exterior disconnects? The GEC will be 1/0 copper.Do you keep both loadcenters' ground and neutrals bonded together? Thanks ---Billy
 
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Old 07-05-04, 07:30 PM
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For the service entrance conductors, from the transformer to meter base, you will need 400 kCMIL copper or 600 kCMIL AL conductors.
 
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Old 07-05-04, 07:46 PM
ratboy69
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Thanks Handy Ron and what about answers to the other questions if you may be so kind. ---Billy
 
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Old 07-05-04, 07:48 PM
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There should be only one point where the neutral (the "grounded conductor") is attached ("bonded") to the grounding conductor. If the neutral is attached to the ground in the 2nd panel, then any current flowing on the neutral between unbalanced loads and the transformer will also flow on the ground wire between them. You don't want that to happen.

Will you be replacing the existing panel with a new 400 amp panel so it has the capacity to handle both its own load, as well as the 2nd panel (which could simply be the existing panel moved over to the side with its neutral ground bonding disconnected)? If you want to run both through the first panel's main breaker, it needs to have the capacity. And even if you just want to double lug it and have 2 mains (NEC allows up to 6 in the same location), the capacity from the lugs back to the transformer has to be rated for that current, including the meter and meter base.
 
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Old 07-05-04, 08:45 PM
ratboy69
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Skapare, The existing 150 amp panel will be thrown away. There will be no 400 amp panel only two 200 amp panels. The individual meter base/socket will be rated for 320 amp continuous and 400 amp peak flow. The feeders for the 2nd panel that is located 3 feet from 1st panel will not connect to anything in the 1st loadcenter. The 2nd loadcenter's feeders will just use the 1st loadcenter as a "conduit" if you will and pass through the 1st loadcenter , through the nippled 2" steel conduit that connects the two loadcenters and then to their final destination-- the 2nd loadcenter. Please comment further. Thanks ---Billy
 
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Old 07-05-04, 09:02 PM
ratboy69
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The NEC allows up to 6 disconnects in the "same location" THe way i'd wire the disconnects would be to use the main disconnects that are present in the two loadcenters. The loadcenters are only separated by 3 feet. Is 3 feet in the "same location"? ---Billy
 
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Old 07-06-04, 06:19 AM
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ratboy69,

IMHO, your question goes far beyond what could be fully answered in a discussion board such as this. You are going to need to involve the local inspector and the utility, and are going to have to take the time to read the appropriate sections of the NEC and a handbook on the NEC. There are lots of details to deal with, and you will get different answers from different electricians...and boards such as this include non-electricians who know code quite well (such as myself); but even professionals will have problems and different interpretations on your issues.

My take:

You have decided to use parallel conductors rather than single large conductors. This is permitted. However table 310.15(B)(6) identifies specific conductor types and sizes that are permitted to be used for dwelling services, and does not include the use of parallel conductors. So to use parallel conductors, IMHO you need to go to table 310.16. Additionally, you are running these parallel conductors in a raceway, so you need to derate for the number of conductors.

You need 200A per conductor, and you have 4 current carrying conductors in the raceway, so you need to derate to 80% of normal capacity. You need conductors with at least a 200A 75C rating, and at least a 250A 90C rating (for derating), and thus you need to parallel 4/0 Cu with 90C insulation. If your _calculated_ load is less than 360A, then you could use parallel 3/0 conductors...but for 400A you need the parallel 4/0 Cu. 4/0 has an area of 212kcmil, so IMHO you are probably better off simply pulling the suggested 400kcmil conductors...but dual 4/0 might be easier to find or an easier pull.

For the feed to the disconnects, you will again need to use 4/0, unless you use _separate_ conduit from the meter base to the disconnects, in which case you can use 3/0. This is because of the thermal derating of having 4 conductors in a single conduit.

If you have appropriately rated lugs on the first panel, then you could run 400kcmil from the meter to the first panel, then 3/0 from the first panel to the second. This sort of tapping is generally done incorrectly, and will always raise a red flag with the inspector...but if you are careful to get properly rated lugs, and keep the information to show the inspector, you should be able to do this.

In general, you _can_ run conductors through panels, using the panel as a raceway. However you have to be careful to make sure that you meet wire bending space requirements and that the panel is large enough to be used as a raceway for these conductors. Doing this figuring is enough of a pain that it is probably easier (and more likely to be done correctly) for you to run a separate conduit from the meter to each panel, or to run a conduit from the meter to a raceway, and then separately connect each panel to this raceway.

Grounding in this case is difficult to get right. I believe that because each panel is a service disconnect, that you actually _must_ bond both panels, even though this will explicitly introduce a 'parallel path'. This is acceptable because ground bonding at service panels is very strict, limiting any ground system voltage to a minuscule amount, while at the same time the size of the ground conductors is quite large. IMHO this is exactly like the grounding of separate service panels in an apartment building. But on this point you really should check with the inspector!!

-Jon
 
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Old 07-06-04, 06:59 AM
ratboy69
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Jon, Appreciate your thorough replies to the above situation ---Billy
 
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Old 07-06-04, 09:56 AM
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Originally Posted by winnie
ratboy69,
Grounding in this case is difficult to get right. I believe that because each panel is a service disconnect, that you actually _must_ bond both panels, even though this will explicitly introduce a 'parallel path'. This is acceptable because ground bonding at service panels is very strict, limiting any ground system voltage to a minuscule amount, while at the same time the size of the ground conductors is quite large. IMHO this is exactly like the grounding of separate service panels in an apartment building. But on this point you really should check with the inspector!!

-Jon
Yes, there is definitely a parallel path if the neutral is bonded to the ground in both panels. This is why I plan to avoid this kind of setup for my future house (which will exceed 42 circuits). But I had put some thought into designing it before I rejected the idea. Among the ideas was to make sure the grounding conductor is fully rated for the entire 200 amp current it could end up carrying in the worst scenario (fully unbalanced 120 volt load at 200 amps plus the real neutral comes loose which would not be detected in this case). Another option I considered (and I never found out if this would comply with code) was ground the two panels to separate ground rods. The potential problem with that is that some small amount of current would still be forced between those ground rods (the very low resistance of the real neutral taking most of the current). But in the end I rejected the idea, and my current plan is for a panel rated for whatever my load calculations come up with (not time to do that part, yet) feeding subpanels (one beside the main panel, and probably 2-3 others elsewhere, depending on house floor plan I end up choosing).
 
 

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