Technical - Second Power Inlet


Last Changed 11/4/2014
Even though we have a longer than normal power cord for Tige, we find we often have to use an extension cable as more and more RV parks that we use are geared for motorhomes rather than trailer. RV trailers tend to have their power inlet at the rear street side corner of the trailer. Motorhomes tend to have their power inlet located in the middle of the street side of the motorhome. We wound up with a spare 50 amp power inlet from an incident last year. We decided to add a second power inlet near the front street side of Tige. The power inlet is one of the expensive parts of this project. The second expensive part was the wire (#6-3 with ground) was $3.26 a foot and we used 18'. Finally, the Automatic Transfer Switch was the third expensive item (about $100). Automatic Transfer Switch (ATS) When you have two power inlets, you might think you just combine the wire from each inlet to the electrical system of the RV. This is not a smart thing to do for two reasons. First, the places the wires would be attached probably can't hold two #6 wires. More importantly, connecting the two power inlet in parallel can be extremely dangerous. When The power cord is in one inlet, the other inlet's blades would be live with 120/240 volts. Though not probable, the possibility is there for an inquisitive child to open that mysterious cap on the unused power inlet. Therefore fore safety sake, we used a ATS. An ATS has two relays inside that connect only one of the two inputs to the output. Thus the unused leg is isolated from electricity. The automatic switching happens by monitoring the input legs of the ATS. When there is voltage on the secondary input, the that relay is energized and the primary relay is opened. For example, when we start the generator, when the generator voltage is present, then the power cord is disconnected.
	This is a schematic of what our input power would be like. We already had a ATS to switch between the power cord and the generator. We will be adding an ATS on the power cord input of the existing ATS to switch between the two power inlets.
	Here is the ATS that we bought for this project. It is a Progressive Dynamics 50 amp Transfer Switch Model PD52. We bought ours on eBay.
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	You can see the two relays that with connect only one of the inputs to the output. In this ATS, the right relay is connected to the existing power inlet. The left relay is connected to the new power inlet. A small circuit board does the sensing of voltages on the inputs to determine which relay should be energized.
The Installation
	This is the existing ATS that switches between the power cord and the generator.
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	The existing ATS also has two relays. There are some extra wires in this ATS as there is a delay when the generator comes on line. This allows the generator to stabilize before the load is applied.
	We were constrained by the lengths of the existing wiring. In a perfect world, the current ATS would have been relocated. Unfortunately, the ability to stretch wire is limited. So we had to deal with the current ATS were is was and added the new ATS where we could. Also, #6-3 Romex is fairly stiff and does not make sharp bends. We moved the current power inlet wire from the top of the existing ATS and connected to the new ATS. Then a cable from the output of the new ATS was fed into the correct ATS power cord input.
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	This is the area just behind the cargo door on the street side. There is a panel I removed for access. We had mounted the new power inlet when we have the trailer repaired so the new inlet would be painted to match the trailer.
	We ran the #6-3/ground Romex through non-metallic conduit for protection of the Romex. We had to have a bend in the wire so it could be attached to the power inlet. The power inlet core is pulled out to attach the wires.
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	This is the new power input located just behind the street side cargo door. Now when the park power post is near the front of the trailer, this is the inlet we will use.
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This is a tiring job as the #6-3 Romex takes a lot of manhandling to get it in place. Then getting the #6 wires into the relay connectors takes a fair amount of hand force. Other than that, this is not a complicated job.
Update
We had a small problem where the ATS between the two power inlets didn't always pick a couple of times. We contacted the manufacturer and found we had missed one line in the installation manual about mounting the ATS so the relays were horizontal (the pivot shafts of the relays being horizontal). We had mounted the new ATS in the vertical position. We did not relish the task of reinstalling the ATS in the horizontal position
	Since this was the second time, the wiring came out neater than the first installation. Still difficult to work with the stiff #6 wires.
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	We removed the blue non-metallic conduit from one of the power cables to make tighter turns. The cable is protected by the new ATS.
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