Sizing grounded conductor for 4000A service

philly

Senior Member
We are looking at replacing an existing 4000A service entrance switchboard and feeders and am trying to determine the best approach to sizing grounded conductor (neutral). I'm not sure if the existing facility has any 277 L-N loads which leaves me debating weather nor not I need to pull a full size neutral or just size the neutral per requirements of 230.24 (C).

If I have a situation where I don't have any L-N loads I know that 230.24(C) tells me that I can reference Table 250.102(C)(1) to determine size of grounded conductor which will be a smaller size than the phase conductors. However if I have L-N loads then I need to consider the grounded conductor as a neutral conductor carrying neutral current and need to size the neutral to carry unbalanced current. Without knowing the existing L-N loads that may exist (or may be added in future) do I need to consider this to be a full size neutral or is there a provision that allows me to used reduced neutral size (I know provision exists but cant put my finder on code section).

If the neutral is considered a current carrying conductor in a service do the requirements of Table 310.15(B)(3)(a) apply for adjusting cable ampacity for having 4 current carrying conductors in a raceway. Assuming the neutral is a CCC any applying adjustment factors we come up with 10 Sets of 4/C 750MCM for service entrance. Existing installation had 11 Sets of 4/C 500MCM which indicates that they pulled full neutral but did not de-rate for 4th CCC.

I was hoping to hear thoughts from others on approach here for replacement of service.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
What does the existing service have for their neutrals? If I didn't know then I would just pull full size however, I would be surprised with a 4000 amp service if there were anywhere near 4000 amps L-N
 

philly

Senior Member
What does the existing service have for their neutrals? If I didn't know then I would just pull full size however, I would be surprised with a 4000 amp service if there were anywhere near 4000 amps L-N
Existing service has full rated neutral w/11 sets of 4/c 500 MCM (3phase + neutral each set).

With a full rated neutral does that have to be considered a 4th current carrying conductor and this require de-rating of phase conductors based on 310.15(B)(3)(a)?
 

kwired

Electron manager
Location
NE Nebraska
Existing service has full rated neutral w/11 sets of 4/c 500 MCM (3phase + neutral each set).

With a full rated neutral does that have to be considered a 4th current carrying conductor and this require de-rating of phase conductors based on 310.15(B)(3)(a)?
Only if majority of the line to neutral load of a three phase wye system is non linear - most often applies to data centers, which could also be a reason to increase neutral size compared to if same load amount but no non linear loads involved.

Otherwise it only carries the unbalance of the phase conductors and heating in raceway/cable is about the same as it is for three conductors.


General comment and not knowing much about your installation but presuming no major portions of load is non linear and connected to 277 volts, I'd guess there is either a lot of 480 volt three phase load, maybe some 277 volt lighting but not a major part of the total load, probably even some transformation to 208/120 that the 480 side will see as line to line loads - unless you have some 277 volt heating units there isn't a much else that is common line to neutral @ 277V. This means only the max expected unbalance of the 277 volt load will be the neutral load, and is likely low enough that minimum size grounded conductor per 250.102(C) is going to be sufficient. If you were dealing with 208/120 there would likely be more neutral load to have to deal with to figure max unbalance.

First thing is to look at what is line to line and what is line to neutral - somewhat my philosophy that if (in your application of 4000 amps) you have 2000 amps of line to line load - then right there is 2000 amps that you can automatically ignore when calculating maximum neutral current in most cases. Good chance there is still low amount of unbalanced neutral in the remaining 2000 amps available though, if the 277 volt loads are somewhat minimal and your neutral calculation will be less than minimum neutral required.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Existing service has full rated neutral w/11 sets of 4/c 500 MCM (3phase + neutral each set).

With a full rated neutral does that have to be considered a 4th current carrying conductor and this require de-rating of phase conductors based on 310.15(B)(3)(a)?
You can reduce the neutral size based on the calculated load but for parallel conductors it cannot be smaller than #1/0 in each raceway. In your installation the neutral would not count as a CCC.
 

drktmplr12

Senior Member
Location
South Florida
Occupation
Electrical Engineer
Isn’t a SE switchboard required to be 4W+G with the grounded conductor (neutral bus) bonded to ground bus with main bonding jumper in switchboard?
a 4W+G switchboard has a complete neutral bus nearly identical to the ground bus. a 3W+G switchboard will have no place to land a neutral and a complete ground bus. a SE rated 3W+G switchboard will have an insulated lug to land your neutral and means to bond it to the ground bus.
 
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kwired

Electron manager
Location
NE Nebraska
You can reduce the neutral size based on the calculated load but for parallel conductors it cannot be smaller than #1/0 in each raceway. In your installation the neutral would not count as a CCC.
Yes with parallel conductors you can never go less than 1/0 per parallel element, with general light and power circuits anyway.
 
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