Can I run a 40 amp central ac unit and a 30 amp ac unit on a subpanel protected by a 60 amp breaker? There are also lower level outlets and an outside gfci outlet with two 15 amp breakers in the subpanel.enter image description here

This is Name plate informaton on the Goodman central ac unit. enter image description here![enter image description here](https://i.stack.imgur.com/R3hUv.jpg

The last picture is the setup of the breakers that will work in this sub panel. Thank you to all who have replied to my question. You have been a great help to this project. enter image description here

  • Why do you have 2 separate AC units here? How many receptacles does this subpanel serve? Can you post photos of the nameplates on the AC units, or even just a make and model? Aug 19 '19 at 0:24
  • The 2nd ac is in a motorhome. I am running 30 amp service from a sub panel that has a 40 amp breaker for central ac
    – Bukasensei
    Aug 19 '19 at 2:25
  • Is this motorhome a 30A/120V unit? (TT-30) Also, what's the nameplate load of said central AC? Aug 19 '19 at 2:32
  • Yes. I will check tomorrow.
    – Bukasensei
    Aug 19 '19 at 3:17
  • I've deleted several "answers" posted below for not being an answer. SE is a Q&A site, so we have different policies from forums. See why and how are some answers deleted for more information.
    – BMitch
    Aug 19 '19 at 23:52

You really cannot go off breaker sizes for ac units. The reason I say this is because this is one place code allows larger breakers than the wiring can support. How can code allow larger breakers than the wiring can support? It is because of the starting current, this can and many times is 5x the running value of the unit (with conventional ac systems) on a newer system with a variable frequency drive they don’t have as high starting currents and vary the speed to save $.

Based on what you have I would bet you could use this panel for both but would want to look at the name plate fro each unit. If it was close and I did hook it up I would then want to test with both systems running at 100% and actually measure the load on the system. I use a mastech ms2180 it is an inexpensive ac/dc amp clamp meter with voltage, ohms, diode and capacitance measuring abilities I think it will run for 10 minutes recording peak values and this can help you decide if the feeder is large enough for your loads. If you add up the breakers it is common to have a much higher total for several reasons. The breakers on the even numbered slots are on 1 leg and the breakers on the odd slots are on the other leg, if all single pole breakers 60 + 60 = 120 so to get a better idea you would just add the breakers on each leg. Now just adding 1 leg the total can still be over the total for several more reasons including a 20% safety margin and usually the total load on a circuit is less than 80% of the breaker size except on startup. I have found every home I check many are running less than 50% of the main breaker size many in the 30% range because of load diversity.

There is a fairly large safety factor built into wiring standards and the short peak period of both units do start at the same time will probably still be ok because breakers are inverse time rated, this means they can supply current above their rated value for a short time, most breakers do not immediately trip at their set value but 10 seconds or more after reaching this value based on their trip curves. The higher the current the faster they trip so your actual load could be 150 amps for a few seconds with out tripping the 60 amp breaker since it is only a few seconds the wiring doesn’t have time to heat up and damage the insulation this is the key. When everything is up to speed and cycling normally we like the load to be at 80% .

This got a bit long but to provide an example I just added a new branch circuit to a panel that by the total breaker value was 320a even after adding this new load (30 amp double pole) my measured draw for 30 days did not exceed 70% (larger measurement system used) this measurement shows I could safely add another double pole 15 amps to that 150 amp panel (grand total of 410 single pole value) and still be fine.

  • Thank you for your reply. An electrician told me I could not add to this panel. A master electrician told me he was connecting my 30 amp circuit to this panel box. I am happy to lean it can be done.
    – Bukasensei
    Aug 19 '19 at 16:01
  • I am an electrician as are several others that respond here that is a quality panel I can’t read what the buss is rated for but most of the 12 lug 12 space are 100 amp and if it is a 12/24 panel it could have a 125a buss but you are limited by the wire size can you read the printing on the cable or wire that is the limiting factor here unless larger wire is installed.
    – Ed Beal
    Aug 19 '19 at 16:24
  • There is nothing printed on the wire.
    – Bukasensei
    Aug 19 '19 at 16:51
  • It looks like a large enough cable. The 60 amp breaker is a two pole. Does that m.
    – Bukasensei
    Aug 19 '19 at 23:18
  • The 60 amp breaker is a two pole. Is each stab rated for 60 amps or the entire box?
    – Bukasensei
    Aug 19 '19 at 23:21

A bit of balance helps things greatly here

Right now, with the 26.4A maximum load your AC pulls, along with receptacle loads, you have enough room (albeit not by too much) for your 30A/120V RV. However, your breakers are arranged in a way that unbalances the load, making it so that the AC at max power, a full RV load, and a full receptacle load puts more than 60A on one leg of the feeder, while leaving capacity on the other leg unused.

To fix this, you'll want to turn the feeder breaker off, and then swap the 30A breaker in the subpanel with the 15A breaker above it; this puts the two receptacle circuits on one leg with the RV on the opposite leg, and thus balances the maximum feeder loads, keeping the feeder from exceeding its ampacity even with the AC running and the receptacles and RV maxed out.

  • I understand what you are saying. That would put 70 amps on each stab? Right now there are 85 on one side and 55 on the other?
    – Bukasensei
    Aug 19 '19 at 23:33
  • 1
    @Bukasensei the AC only counts for its 26.4A nameplate load, not the 40A breaker handle rating, since it's a fixed load Aug 19 '19 at 23:58
  • @Bukasensei -- I would since you don't know what might get plugged into the serviceperson's receptacle in particular (that's the kind of thing where power tools might get plugged in at) Aug 20 '19 at 0:27
  • Ok. I will switch those two breakers. Thank you very much for your help.
    – Bukasensei
    Aug 20 '19 at 0:29

This is a matter of provisioning.

If these were general receptacle circits which summed up to over 100A, then there's no trouble doing this, since we get to assume it's highly improbable that all will be heavily loaded at the same time. So you can provision 4.5 amps per circuit or whatever, as appropriate.

However, when we have two loads of the same class (be them A/C units, heat pump emergency heat, BitCoin miners, grow lights, table saw + dust collector) and we know they will be running at the same time... Then it's a different deal. I In that case you must provision the actual loads.

So you need to provision what the units' instructions tell you, or if they don't say, 125% of their actual VA draw. If none of that information is available, then you can go with the breaker values of 40+30.

Obviously you can overlap loads which are mutually exclusive, like heat pump emergency heat and A/C.

If you must rewire the feed to this subpanel, think about other future loads.

  • Should I increase the breaker to 80 amps. The main panel is 100 amps.
    – Bukasensei
    Aug 19 '19 at 2:28
  • 2
    @Bukasensei you cannot increase breaker sizes. Ever. So no. A breaker can only be increased as part of increasing wire size in this case, and as part of other work also in other cases. Aug 19 '19 at 2:41
  • I think what he means by provisioning is that they (should) be mechanically or electrically interlocked so they can't start at the same time.
    – Greg K
    Aug 19 '19 at 3:50
  • 1
    @Bukasensei What size and material is the feeder wire? Aug 19 '19 at 5:53
  • 1
    @Bukasensei the writing appears to be embossed not printed. Can you read it and tell us what it says? It should be repeated every 12" or so. Aug 19 '19 at 14:40

Probably not. While the 60A breaker's primary job is to protect the wiring between the panel and the subpanel, you can have breakers totaling more than 60A in the subpanel. But with a 40A AC unit and a 30A AC unit, you will almost certainly have both running at once and that will almost certainly overload the circuit and pop the 60A breaker.

If, however, the startup load of the combined units are less than or equal to 60A, it should work.

  • The starting currents of both units combined will definitely be more than 60 amps.
    – JACK
    Aug 19 '19 at 0:10
  • @jack Thats not a problem for short durations. AC units commonly have larger breakers than the wire size. This is allowed by code. This is why we cannot go by the breaker size but need the name plate information.
    – Ed Beal
    Aug 19 '19 at 16:25
  • @EdBeal Thanks Ed. I was just referring to the statement "If, however, the startup load of the combined units are less than or equal to 60A, it should work".
    – JACK
    Aug 19 '19 at 16:37
  • That’s not what I am trying to say. The combined start up loads may be 150 amps and it could be fine breakers are inverse time most will allow 5x their rating for 10 seconds without tripping for example Snyder shows a 20a breaker will allow 100a (5x the rating) for 6 seconds this is common. The 60 amp breaker feeding this could support 300 amps for 6 seconds based on that. I mention 10 seconds because that is the class I use . The common classes are 5, 10, 20 there are longer ones yes and even programmable breakers but these are usually on large industrial loads.
    – Ed Beal
    Aug 19 '19 at 16:54

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