NEC 702 Optional Standby Generator Sizing Intent

[electricallyminded]

I am looking to add a battery backup system to my home and have been confronted with standby generator sizing requirements from NEC 702. I am looking to backup the whole home which is a 200A service containing a 20A solar system. Realistically we peak at about 7 kW of demand when the dryer is turned on, usual demand is limited to no more than 3 kW otherwise.

The backup system I am looking to add provides for 5 kW of continuous power. This would seem to be not permissible per the NEC 702 sizing guidelines which reference Art 220. What is the logic behind this? I believe in this instance if the 5 kW generating capacity is exceeded that the frequency would simply collapse and I would be back where I started - in a power outage. Why is this not a permissible scenario? What is the safety risk? Any thoughts on how to overcome this besides fudging load data?

Thanks!

 

[charlie b]

I am closing this thread in accordance with forum rules. If you are not an electrician, we are not permitted to assist you in performing your own electrical installation or maintenance work.

If the question has to do with performing installation work at your own home, as it appears to be, then forum rules do not allow an engineer (PE or not), to post the question. Actually, it is not very often that we get engineers asking DIY questions. Most of the DIY questions I have seen come from people who have no connection at all with the electrical industry. We tell them the same thing I will now tell you: If installing stuff is not your occupation, then we can’t help you install stuff. You would have had to agree with that rule during the process of registering on the forum.

The reason is that the forum’s owner does not want a person to get an answer to the question they asked, but not get answers to the dozens of other questions they should have asked and were unaware they needed to ask. An EE is less of a risk in that category. But you need to understand that what we learn in college has very little to do with what electricians learn during their apprenticeship. Examples include wiring methods, wire sizes, load limits, conduit types, wire types, supporting of conduit, conduit bend limits, extending existing circuits, GFCI requirements, AFCI requirements, derating requirements, lock-out-tag-out requirements, and post-installation testing methods. We don’t know, and can’t know, how much of this you already understand. Mike doesn’t want anyone to get injured by any incidents related to electrical installations. But he also doesn’t want to be sued by the injured party’s relatives on the basis of him not telling you enough to keep you safe.

I have been involved in the Mike Holt Code Forum for over fourteen years. It has taught me more than I can begin to describe. It is a great resource, and EEs have been a great source of information for the other forum members. So I encourage you to stick around.