Garage addition

[Razzap]

I will be wiring up a new, unattached 960 square foot garage / shop. I calculated the load on the new unit to be around 65 amps, based on lighting, air compressor, welder, etc... The owner wants a 100 amp feeder installed for future expansion. The garage / shop will be fed from a dwelling with a 100 amp 120 /240 V service. I claim that installing a 100 amp 2-pole breaker in a 100 amp service panel is not practical, or per code. The only article that I can find to support this is Art 220.83(A). The load calcs on the exiisting dwelling are around 45 amps, because most of the appliances and heating loads are gas.
A service upgrade should be done first, since this garage / shop nearly doubles the size of the load. Am I on the right track? Are there any code sections I need to review to verify my concerns?

 

[suemarkp]

There is nothing against the code in installing a 100A feeder from a 100A service (just like it is OK that the sum of breakers in a panel can add to way more than the main breaker rating). What is important is that the new total connected load still falls within the 100A service he currently has. Most people are going to find a way to use the feeder capacity you give them. If it is just one guy, a 100A service is probably enough (he can only use so many toys at once). But if its a house full of people, the main may trip. On the other hand, welders and air compressors are rather short duration loads compared to their nameplate value.

I think upgrading him to a 200A service is prudent (especially if there are expansion plans which further increase the calculated load), but perhaps not required.

 

[iMuse97]

Mark is right that it would be prudent to upgrade the service panel. However, your client is also right on target. He wants to install feeders and breakers that are large enough that they will not be overloaded in the near future. But of course, if he uses them to capacity (depending on his shop habits, this could be very likely or unlikely ie. A/C or electric heat could push him over the top), his main panel will be overloaded.