Feeder Conductor Sizing

A feeder is just a smaller service, and I size it the same way: run the load calc, add the required margin, then pick a conductor that can carry it and a breaker that protects it.

I have opened up plenty of subpanels fed with wire two sizes too small because somebody grabbed what was on the truck. That wire is browning off before you ever notice.

When it applies

Any conductor running from the service or source to a downstream panelboard is a feeder, and it gets sized by calculation, not by whatever spool is on the truck. You run the load calc, size the overcurrent device, then size the conductor so it carries the load and coordinates with that breaker.

Common mistakes

• Sizing the feeder to the breaker only and forgetting the calculated load and continuous-load factor.
• Ignoring voltage drop on a long run to a remote sub-panel.
• Undersizing the feeder equipment grounding conductor for the breaker ahead of it.

In the field

On a strip mall I size feeders for the tenant load plus growth, because the guy who finishes out the space next year will thank me instead of pulling new wire.