Tesla gateway tap feeders?

[jwoody]

I've been inspecting the field installations of energy storage systems (powerwalls) and I'm wondering if the feeders for the tesla gateways (ATS) need to be rated as tap feeders. There are double mechanical lugs in the gateway (ATS) for each phase fed from the incoming gateway feeders. If it is a double lug and if 2 sets of feeders are from them, each one feeding a separate bus, then shouldn't they be rated as tap feeders? I've done dozens of these inspections and not one of them is rated for tap feeders but I can't seem to understand how they are not considered tap feeders. Any feedback appreciated

 

[wwhitney]

The conductors need to be protected per 240.21. They will have an OCPD upstream of them (e.g. the main disconnect), and so if they are sized to be properly protected by that, they comply with 240.21(A). But if they are sized smaller, then they need to comply with the tap rules in 240.21(B).

Examples:

200A residential main disconnect, 2/0 copper conductors to Gateway (83% residential rule), 2/0 conductors on each pair of load side lugs, complies with 240.21(A).

Same as above, but one set is smaller. The smaller set is a feeder tap and needs to comply with 240.21(B).

Same as above, factory supplied jumpers to internal MLO panelboard used as a generation panel, not sure what size the jumpers are, and whether they need to be 2/0 or can be smaller since they are factory supplied.

Note that 705.12(B)(2) requires that that any feeder or feeder tap to a load panel have a main breaker, or the feeder/tap be sized based on the sum of the grid OCPD and the inverter OCPDs (rather, 125% of the inverter output current, although for Powerwall 2, that's 30A either way).

Cheers, Wayne

 

[jwoody]

Thanks for your input Wayne.

Based on 240.21(B)(1)(1) under 10'- Isn't that saying that the tap needs to be sized for the combined loads, meaning the ampacity on the tap could be much more the ocpd protecting it? (hypothetical) Example: 200A main w/ a double lug bus tap, each load rated 150A. Wouldn't the bus need to be rated at 300amp?

 

[jaggedben]

@jwoody

Broadly speaking you are correct that 240.21(B) will often apply to these installations, and in my (albeit somewhat limited experience), this isn't something that Tesla trains people very well on. In particular they recommend using one set of lugs for a 'generation panel', and the conductors feeding that generation panel are often tap conductors, meaning that the panel needs to have a main breaker per 240.21(B), something that's often left out. It's less often that the tap conductors actually need to be sized larger, since the generation output will usually require conductors larger than the requirements of 240.21(B). But the rule does often apply and needs to be complied with.

Add to this Wayne's point 705.12(B)(2), and it is certainly the case that whenever there are two sets of conductors coming off the gateway, both panels are likely to need a main breaker.

As for this part...

Based on 240.21(B)(1)(1) under 10'- Isn't that saying that the tap needs to be sized for the combined loads, meaning the ampacity on the tap could be much more the ocpd protecting it? (hypothetical) Example: 200A main w/ a double lug bus tap, each load rated 150A. Wouldn't the bus need to be rated at 300amp?

It's not so clear cut. What you have described seems to be two sets of 150A rated tap conductors. Each one cannot have a calculated load greater than 150A, but they can be less. For example if the calculated load on each tap on is only 100A, then the upstream 200A main feeder is fine, and so are the taps. This isn't so unlikely because many residential electricians just oversize stuff rather than do precise load calculations. Also keep in mind it's possible that with demand factors the total calculated load may be less than the sum of the load on both feeders. It's theoretically possible, perhaps even likely, that you could have calculated loads of 130A on each of those feeders but a total calculated load together of less than 200A.