Conductor size for 400 amp 120/240V 3ph delta panel main feed.

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RME INC

Member
Location
Santa Rosa, CA.
I have a 30 plus year old building with a 400 amp 120/240 volt delta power system. With old cloth style 3/0 or 4/0 aluminum wire that the insulation is coming off at the weather head.
I need to replace the conductor from the weather head to the panel.

Existing conductor is in 3" conduit approx. 45'.
There are two hots and one neutral with the larger size conductors and the center buss has a smaller size conductor.

Conductor comes in at the top of the panel to the buss bars then goes to the meter then to the main breaker.

I am having trouble determining the size conductors I need for this application.

I refer to table 310.15b and looks like size 500 THHN copper.

Need assistance Help.

Question: Why were the old conductors sized smaller and the smaller stinger leg conductor?
 

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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I have a 30 plus year old building with a 400 amp 120/240 volt delta power system. With old cloth style 3/0 or 4/0 aluminum wire that the insulation is coming off at the weather head.
I need to replace the conductor from the weather head to the panel.

Existing conductor is in 3" conduit approx. 45'.
There are two hots and one neutral with the larger size conductors and the center buss has a smaller size conductor.

Conductor comes in at the top of the panel to the buss bars then goes to the meter then to the main breaker.

I am having trouble determining the size conductors I need for this application.

I refer to table 310.15b and looks like size 500 THHN copper.

Need assistance Help.

Question: Why were the old conductors sized smaller and the smaller stinger leg conductor?
The conductor size may well have been determined by the utility and they usually size conductors for what they see the real load as likely to be rather than the calculated load. It is also possible the panelboard was repalced at some point.

My guess is that someone figured there would be less current on the "stinger" leg so sized the conductor accordingly. It is not an unreasonable assumption to make.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Years back, it was not uncommon in this area, for a 240/120 delta service to have a reduced size "high" leg, but the service normally fed a fused main with the B phase fuse reduced to the service conductor ampacity.
If you are supplying a main breaker panelbaord, I see no Code provision for you to reduce the high-leg. Load calculation permitting, you can use 240.4 (B) "next size up" rule but you will still be limited to a 500 Cu or 700 AL IMO
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Years back, it was not uncommon in this area, for a 240/120 delta service to have a reduced size "high" leg, but the service normally fed a fused main with the B phase fuse reduced to the service conductor ampacity.
If you are supplying a main breaker panelbaord, I see no Code provision for you to reduce the high-leg. Load calculation permitting, you can use 240.4 (B) "next size up" rule but you will still be limited to a 500 Cu or 700 AL IMO

depending on the load calculation it might be cheaper to install smaller wire and change out the main breaker.

just curious if the inspection authority is going to allow you to change the wire size only to the weatherhead.
 
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RME INC

Member
Location
Santa Rosa, CA.
Original panel.
Have not check with the local building officals yet.
I am looking into the possibility of reducing the breaker size, to accomodate a smaller conductor.
I was hoping there was some code section I missed that allows for the smaller conductors on a main feed.
 
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