1

I'm installing a 200 amp automatic transfer switch but my main service entrance is a combo meter box that breaks out into two 100 amp since there are two subpanels downstream. I'm thinking of combining the hots from both of these breakers using a couple of Morris Products 97643 Multi-Cable Connectors to feed the ATS.

After the ATS I will go back to a dual 100 amp panel to feed the existing subpanels. I'm wondering if there is a better way other than upgrading the combo meter box?

Here is an image of the main combo panel:

3
  • Can you post a photo of the label on the inside front of that meter-main please? Also, why the whole-house transfer switch? Generally speaking, it's much better to have the transfer switch feed a subpanel; that way, you don't have to worry so much about accidentally overloading your genset because a load-shed contactor didn't open – ThreePhaseEel Nov 9 '20 at 3:04
  • The standby generator is 20kw. Unfortunately the two sub panels both power critical parts of the house. Not sure why it was done this way...possibly due to an addition to the main house. Thanks – powerman Nov 9 '20 at 18:32
  • Can you get us photos of the inside subpanels in addition to that label on the inside of the meter-main's cover? – ThreePhaseEel Nov 10 '20 at 1:12
2

No, you definitely can't do that with this meter-main! You have several choices.

Put only one panel on the ATS

This is the normal approach to this problem. You pick one panel and make it the ATS-operated panel. Any critical loads in the other panel, you reroute them.

You can also put the other panel on a MTS (manually operated interlock) and those are $100-ish for most panels.

Two ATS's, one for each panel

This is also allowed though at higher cost and complexity, of course.

An ATS that stacks under the electric meter.

Some ATS's have a funny plate that fits between the meter pan and the meter, thus switching the entire service. That type of ATS will work very well in this instance.

Replace the meter-main with something appropriate

Into the trash goes both the meter and the dual main breaker apparatus. Then you fit a meter-main designed to support an ATS, or a plain meter feeding the ATS which then feeds a 200A panel which has the two 100A breakers in it.

1
  • Yes, I was initially going with the two ATS approach but these are in extremely short supply right now. The meter main replacement approach is looking more likely. Thanks – powerman Nov 9 '20 at 18:36
1

@Harper gave an excellent answer as to what you should do instead. But here is the reason why you can't just implement your original plan:

Wires and connectors are rated based on current. (Voltage too, but at this level, current is the concern.) Almost certainly the wires coming from the meter to the 100 Amp breakers are rated for 100 Amp (or possibly a little more) but not for 200 Amp. It is possible that the connectors on the meter are rated for 200 Amp, but it is quite likely that they are also not rated for 200 Amp (or more likely, not designed to hold the really big wires needed for 200 Amp).

If everything were always perfect, this would not be a problem. Power would flow equally on both wires, both in and out of the transfer switch, and everything would be OK. However, there are two potential problems:

  • Differences between the wires

If the wires between the meter and the transfer switch, despite being nominally the same, have significantly different resistance, they will carry different amounts of current. The result, in a high usage (i.e., close to 200 Amp) situation, will be that one wire will carry more than 100 Amp. Since the protection is downstream of the transfer switch, nothing will prevent the > 100 Amp wire from overheating.

  • Wire break

This is the more clearly dangerous situation. If a wire in either pair breaks, the other wire will carry the full current - possibly as much as 200 Amp. The transfer switch to breakers part will be protected by the 100 Amp breaker (though this would still not be a good situation). But if one of the wires between the meter and the transfer switch breaks (or more likely, a connection gets a little loose), there is nothing to prevent all the current going on the other wire.

There are some very limited, very high current, situations where this type of paralleling is permitted, but not normally in residential construction.

1
  • Thank you for pointing these issues out. At this point I may either go with the meter main replacement suggestion or get back in the cue for an additional ATS and run in tandem. Thanks again – powerman Nov 9 '20 at 18:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.