TT plugged in at home?

Dick_B wrote:
On another Forum it was determined that the startup current of a Dometic 15K A/C is 13.6 amps with a 2.8 amps run current. That would imply that you can run the 15K A/C off the 20 amp circuit; but don't push it with other `stuff' on the same circuit.

Sorry, but that is so incorrect. RV AC units are typically around 15 amps running and starting (momentary inrush) current (technical term is locked rotor amps) is around 5-6 times the running current. The data listed in this Duo-Therm by Dometic Instructions shows that for a 15K BTU unit, the running current is 15.4 amps and the starting current is 77.0 amps. These figures are for running at the equipment's normal operating voltage and for the Dometic units is 115 volts, not 120 volts.

Note that the MINIMUM specified wire size for all Dometic AC units (13.5 or 15K BTU) is #12 gauge and the specified breaker size is 20 amps. This is the same for other manufacturer's AC units as well. Again, note that this is for running at 115 volts (at the terminals on the unit itself). There is also a specified max. run of 24 feet of #12 wire. The breaker is sized independent of wire size to prevent nuisance tripping of the breaker when starting. Wire is sized to maintain appropriate voltage and to comply with code.

"Nominal" AC line voltage is N. America is standardized at 120 volts. Power Utility companies are all on an interconnected grid and voltage tolerance boundary has to comply with ANSI standard C84.1 and the permitted variation from 120 volts is +/- 5 percent. However, if you include the voltage drop allowed by the NEC for a customer's premises, the permitted total variation allowed is + 6 percent and minus 13 percent. At the worst case scenario of a 13 perent, the voltage at the point of use on a customer's premise can be 104.4 volts and still comply with all codes and standards. The figure of 104.4 volts allows for voltage drop in a customer's wiring system up to the point of use. Pacific Gas and Electric has a very good explanation here.

If voltage at a CG is down to 104.4 volts, you have no code or standard you can point your finger at to blame the CG. However, if the voltage is indeed that low, it can in the majority of cases, have been higher because the NEC does not adequately provide for the quantity of 50 amps RVs in use today plus it allows for substantial downsizing feeder cables on an overall total basis of 41 percent for more than 36 pedestals.

The voltage rating found on some equipment is not 120 volts. The reason for this is that a manufacturer assumes and takes into account that the equipment will normally be operating at less than 120 volts. Equipment is also required to be able to run at +/- 10 percent of it's rated voltage. There will be a NEMA standard that covers this and the PG&E info. above makes mention of this. Motorized equipment in particular can have rated voltages less than 120 such as 110, 115 & 117 volts. A Dometic AC unit with a voltage rating of 115V should be able to operate satisfactorily at 103.5 to 126.5 volts and without incurring damage.

Of interest here is that the two common EMS units shut power off at 102 volts (Surge Guard) and 104 volts (Progressive Industries). IMO, 102 volts is too low. The figure of 104 volts could very well be based on the minimum allow voltage under ANSI & NEC (104.4). On this basis alone, I would not buy a Surge Guard EMS.

Anyone attempting to run an AC unit on a 15 amp circuit or are occasionally or frequently doing this are not meeting the min. requirements specified by the AC manufacturer. The fact that you CAN get your AC unit to run does not mean that all is well.

SoundGuy wrote:

To suggest that a standard 13,500 BTU roof mount A/C cannot be successfully started and run by a 15 amp breaker protected circuit is simplistic nonsense.

This also is incorrect. I'm an EE and I will bet my degree on it. Damage to an AC unit is not necessarily instantaneous. It can be cumulative over time. If you are running an AC unit on a 15 amp circuit, you are likely reducing it's life. Looking at the Dometic data, the normal running current at 115 volts for a 15K unit is 15.4 amps and for a 13.5K unit ranges from 13.5 to 14.6 amps (depending on model no.). By code (CEC or NEC), the min. wire size just for one AC unit running (and no other loads in the RV) would require a 20 amp circuit (by applying the 80 percent safety factor). On top of an AC unit's running current, you have to add any other running loads and at a min. can have the converter/charger and fridge drawing a few amps.

One thing that is unique to AC units is that they draw more current as the voltage goes down compared to just about anything else (resistive loads like say a heater, toaster or coffee maker and motors). And for those loads that have outputs that do go down as voltage goes down, their output drops as the square of the voltage and at 104 volts for example, output would be 69 percent of the rating at it's listed operating voltage of 120 volts nominal.

Say you are running just a 13.5K or 15K AC and no other loads and the voltage goes down, you will definitely be overloading the wire and exceeding the required breaker size. At some point as the voltage goes down, the unit's current will be high enough at starting to trip a breaker. It get's complicated to predict with other loads running at the same time. To an AC unit's required breaker size, you would typically add the running current of everything else. But with current of the AC going up when voltage goes down, there is no manufacturer's data available with which you can use to do a calc.

It is important to understand that an AC unit draws much more current during the initial startup of a few cycles or so of up to around 5-6 times the running current. In the case of a Dometic 15K unit, you are looking at 77.0 amps (5.7 times running current). So here you would have a 15 amp breaker and #14 wire at home trying to support 77 amps at startup. The result is a very high momentary voltage drop. If you do manage to get your AC unit running, every time it starts up, it suffers some degree of damage. It may not significant at each start up event, but over time it is cumulative and you can significantly reduce the life of an AC unit. Now, if the voltage is low and the running current is high, the starting current will be much higher than at 115 volts. This is one reason why an AC may start and seem to be running okay but will not restart.

Some homes will only have a 15 amp breaker and #14 wire. As well, a 15 amp recept. can have other loads connected on the same circuit. In the US, houses now have a 20 amp breaker and #12 wire feeding multiple recepts. and you *may* have a better chance of getting an AC unit to start and run satisfactorily. Receptacles on a 20 amp circuit use a "T-slot_ configuration that will accept a 15 amp or 20 amp plug so if you see a T-slot recept., it will be supplied by a 20 amp breaker and wire. At some point down the road your AC unit can fail and you will be left wondering how the heck it happened. I have looked and looked and have not been able to find data showing current draw versus voltage for RV AC units but it is an accepted fact that the current does go up with dropping voltage. Well pumps are one example where wire size can be increased to allow for the distance from the panel to the well and down to a submersible pump (can be hundreds of feet down) so that it will reliably start every time and so that damage will not be caused to it.

Another thing of interest is that 30 to 15 amp adapters are only rated at 15 amps and will be CSA listed as such. With just an AC unit running and everything else off inside an RV, the adapter will be operating over it's rating. So if voltage drops and you also have other loads running, the adapter can really be over-taxed. Add plugging in with a live circuit all the time (which damages plug and recept. contact surfaces), you have a recipe for damaging the adapter and causing a meltdown. The puck style adapters are the worst for heating up as they have a short path for heat to be transferred compared to a pigtail adapter.

Bottom line is that if you want your AC unit to run at home satisfactorily all the time, do not want adapters and extension cords to melt down and don't want to cause damage to the AC unit, don't plug into a 15 amp circuit/receptacle, don't use a 15 amp rated adapter and make sure the wire size (house panel to recept. on house and from the receptacle to the panel inside an RV is adequately sized. I'm not sure off-hand if and any 30/15 adapters have a 20 amp rating on the plug end, but if you have a T-slot receptacle at home you could make up your own adapter.