Can a 150 amp service panel feed a 100 amp sub service panel without causing potential problems?

Question

I have a newly built 1700 sq ft home with 150amp service panel. It has 100 amp CB feeding a 100 amp service panel in a detached 1200 sf shop garage. This concerns me of frequent overload on the house.

Answer 1

As long as the combined maximum demand of both the house and the garage does not exceed 150A, it should be fine. You have a 100A circuit breaker in the garage to stop the shed exceeding 100A . Unless you are using the shed as a production facility or something else that continually runs then you should not reach that amount of consumption. Also is it 100 amp single phase or 100A per phase for 3 phase. An electrician uses an instrument called a tongue or clamp meter which can measure the amount of current draw at the busiest time of day. It would cost $50-$70 to get him for an hour. Well worth it.  

Answer 2

as long as the 150 amp panel isn,t over loaded, or at 50 % capacity,  

Answer 3

If you have not been experiencing any problems to date, you probably will not at anytime in the near future.

Just because the current may be available to use, it is not going to "go" anywhere without any utilization equipment drawing power. Look at it this way, by Code; you can put any number of receptacles you want to on a residential general lighting & appliance branch circuit. The circuit may be rated at 15 Amps, but if all you have on it is a table light with a 75 Watt lamp burning, that's all the current it will be using. You could plug in 19 more of the same before you began to come close to having a problem. You have 15 Amps of available current, but the 75 Watt lamp uses only 5/8 of an Amp.

The same applies for the garage/shop area. Having 100 Amps available doesn't mean it is being used. In the case of a huge fault occurrence in the shop, you have the breaker supplying the feeder panel in the garage as your first source of overcurrent protection (it is actually the second, since each circuit in the garage has its own overcurrent protection, the third if there is a breaker serving as the disconnecting means for the garage panel). Should a fault occur that was in excess of 100 Amps, the panel breaker would open up, and there should not be any affect to the Main Panel.

I doubt you have experienced any overloading of any of the circuits in the home. A 1700 S.F. home would have to have about 10 1/2 H.P. appliance motors, plus an electric dryer and an electric oven to come close to using 150 Amps of current. (And, don't forget that you actually have 300 Amps of current being supplied to you, one on each leg of the service).

Of greater concern would be questions regarding the installation of the feeder panel in the garage. Since it is an accessory structure with more than 2 circuits, is there a grounding electrode installed at the structure (and bonded to the panel enclosure with a listed lug and correctly sized grounding electrode conductor)? Is there a listed means of disconnect for the garage? Has the bonding jumper been removed from the panel? If a Main Lug Only (MLO) panel was installed, this disconnect will be separate from the Panel; unless a double pole circuit breaker was "back-fed" to use as both overcurrent protection, and as the disconnecting means for the garage feeder circuit. If a "back-fed" breaker was installed in the enclosure, has it been bolted in place with the listed parts, as specified by the manufacturer? Are all of the 15 and 20 Amp receptacles GFCI protected?

If you are losing sleep over these issues, hire a licensed electrical contractor to do some investigating. Even if he finds nothing wrong, you will have gained some peace of mind.

Answer 4

That 100A. panel in the shop is sized for motor surges from your

shop equipment, not for actual load.

Breakers have thermal trip for overloads, and will ride through

short term overloads without tripping or dangerous overheating of the wires.

Bigger wires won't overload anything. Unless you put some very

large equipment in that shop, ... no problems.