Feeding a Second Load Center

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RSquirrel

Member
Location
Watsonville, CA, USA
Occupation
Mechanical Engineer (retired/consulting)
I am replacing an obsolete Zinsco ML12 panel (9-3/4” wide) with two “stacked” Square-D QO816L100DF panels (8-7/8” wide). Two panels are needed in order to provide enough circuits and to fit into the available space (14-1/2" wide panels are too large) without making structural changes to a load-bearing wall. The 100-amp feed will come into the bottom of the lower panel and all of the circuit feeds will exit out of the top of the upper panel. The upper panel will only contain 15- and 20-amp breakers.

My questions concern the best method to feed power from the lower panel to the upper panel.

Method 1: Install an appropriately-sized 2-pole breaker (e.g. QO260) in the lower panel, feeding power via 6-AWG THHN’s to the upper panel (in effect, the upper panel would be a sub-panell to the lower panel).

Method 2: Connect the two hot and the neutral lugs of the lower panel with those of the upper panel.

With either method, the two panels will be bonded together.

As it relates to method 2, I checked the diameters of the lug holes and they should be able to accept both the 3-AWG primary power feeds and the piggy-backed 6-AWG feeds to the upper panel.

Q1. Are both of the methods acceptable per the current NEC?

Q2. Other than avoiding the cost for the QO260 breaker, is there an advantage to one method vs. the other?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If by piggy-backed your implying that you'll stick two conductors under a single terminal then no that is not code complaint.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Lugs can only be used with one conductor unless specifically identified for use with more than one. 110.14(A)
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
I don't know if there is sufficient room in those panels for this solution. You can run your main feeder to the first panel's lugs, then use an insulation piercing (bite-on) connector to feed the second panel. You could also use Polaris style connectors.
 

RSquirrel

Member
Location
Watsonville, CA, USA
Occupation
Mechanical Engineer (retired/consulting)
Hi infinity and don_reqcapt19,

Since you both cited/implied the same code reference, it looks like method 1 is the only feasible one. I assume since you didn't state otherwise, that method 1 is acceptable. Thanks for your prompt responses . . .
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Yes you can feed the second panel from the first. There are many ways to do this based on what your loads will be.
 

RSquirrel

Member
Location
Watsonville, CA, USA
Occupation
Mechanical Engineer (retired/consulting)
Dear Infinity,
You state that "there are many ways to do this based on what your loads will be." The loads in the upper (sub-) panel would be: three 15-amp lighting circuits (95% LED lighting), three general purpose 20-amp receptacle circuits, one dedicated dishwasher 20-amp circuit (each via four QOT tandem breakers) and two 20-amp/208v baseboard heater circuits (both via two 2-pole QO breakers). Other than the two methods I cited (one of which is nixed), what other methods would be feasible, or is my 'Method 1' the "best"?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
First, I would nipple the two panels together, perhaps 1" or 1-1/4", rather than cables between them. Then, I would land the incoming #3 wires in three 3-position Polaris-style blocks and feed both panels with #3 pigtails. This will give you maximum ampacity and breaker space in both panels.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I would use a back-fed 100 amp CB to feed the first panel and then 3-#3's in a nipple to the second panel from the main lugs in the first panel. Now both panels will maintain their 100 amp rating.
 
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