Subpanel to subpanels load capacities


Old 03-28-19, 12:02 PM
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Subpanel to subpanels load capacities

At the main electrical panel outside my house, I have two meters, one for the residence and one for ag. In that panel, there are two separate 200 amp double pole main breakers.

One main breaker, marked “res,” serves as the overcurrent protection for the main subpanel in the house (i.e., the subpanel that distributes all the circuits in the house). The other main breaker, marked “ag,” protects the feed to the ag subpanel installed right next to the main electrical panel. Currently, the ag subpanel contains a set of 50 amp breakers that protect the feed to a subpanel installed at a pump pad in a vineyard near the house, and a set of 70 amp breakers that protect the feed to another subpanel installed in a barn near the house. We are in the process of building a shop building about 60’ away from the ag subpanel. My question is, can I feed a new 100 amp shop subpanel from the ag subpanel without upgrading the 200 amp main breaker to the ag subpanel? When I’m done, the ag subpanel will house three sets of breakers rated at 220 amps total.

Also, in case it is useful to know, the actual loads served by the pump pad and barn subpanels total considerably less than 50 amps and 70 amps, respectively, and they are very seasonal/intermittent. For example, the loads at the pump pad are rarely engaged in winter or spring, and the loads at the barn are rarely engaged in fall or summer.

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Old 03-28-19, 12:53 PM
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You size the panels based on actual load, not breaker handle size. There is a formal calculation called "demand load calculation" to know for sure. But based on your description of rare/occasional agricultural use with no time overlap, I see no problem powering the new equipment from the existing service.
Old 03-28-19, 03:23 PM
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Thanks. I figured it didn’t matter if the main breaker to the subpanel was 200 amp versus a combined breaker amperage of 220 for the 3 feeds from the subpanel. I know that the loads from all 3 feeds won’t likely ever exceed 200 amps.

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