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Last update 12/09/2010

John's Time Delay
Generator Loading Relay


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It is a good idea to start a generator and allow it to warm up without any load. Indeed some generators will not come up to speed with any significant load attached. Bogging the generator is not only hard on the generator, it is also very hard on loads such as air conditioners which suffer under low voltage, low frequency, high current conditions. The Onan AJ series generator in my 82 Itasca motor home is an example of a generator that will not start under significant load.

It is a pain to have to go around and turn off all the major loads before cranking the generator. This is particularly annoying when one wants to start the generator while underway in order to use the roof air conditioner to aid in cooling the vehicle. I decided to do something about it.

It is also nice to have the power transfer automatically from shore power to generator power when the generator is started.  Like, for instance, when the power goes off during a storm in the middle of the night.  Just crank the genny, roll over and go back to sleep.  Sure beats tromping out in the weather to move the power cord from the shore power connector to the generator connector.

A system to automatically delay the application of the load until after the generator is up and running and warm is an appropriate solution. Fortunately this solution is simply implemented, costs under $50 and involves only two components.

Already-manufactured generator delay/automatic transfer boxes are available.  They're more expensive than my design plus they're usually physically larger which makes finding a mounting place more difficult.  Besides, I like to roll my own.

My solution uses a Potter and Brumfield PRD series 30 amp single pole definite purpose contactor and an HVAC-oriented time delay relay.   A double pole contactor is necessary because the neutral and the hot lead must be switched to make sure GFI protectors work properly.

The particular relay I used is the PRD-11AGO-120, 30 amp contacts, 120 volt coil. For 50 amp service use the PRD-11AFO-120.  The data sheet for the PRD series of relays can be downloaded here.  This relay should cost no more than $20 and usually half that. 

To implement the time delay, a simple, inexpensive time delay module is used. This module is a delay-on-make relay. It is also known as an anti-short-cycle relay because its most common use is to prevent short cycling (energizing too soon after turning off) the compressor in an air conditioner by delaying the power to the compressor by a set number of minutes.

The relay I selected is a MARS (Motors And Armature Service) #32391. This relay can be set for a delay from between 0.1 minutes to about 8 minutes. The relay is a 2 terminal device that simply blocks the passage of current for the set number of minutes. Many companies manufacture a compatible relay; the MARS unit was selected because my friendly local electric motor repair shop stocks it.  To the right is a  photo of the relay.

The 30 amp relay should cost from $15 to $50 depending on whether or not you buy it wholesale and the time delay relay should cost from between $12 to $24. In addition to electric motor repair shops, these components can be had from appliance parts warehouses, HVAC supply houses and HVAC contractors as well as direct sales operations such as Graingers.

The simple schematic for this device is shown at right.. The hot wire from the generator is connected to the Normally Open (NO) side of the contactor. Shore Power goes to the Normally Closed (NC) side. The house wiring is connected to the other. Power is supplied to the 120 volt contactor coil from the generator lead through the time delay relay (TDR).

How it works

When the generator is started, 120 volts AC appears on the generator "line" lead. This powers the time delay relay. After a set amount of time, the TDR energizes, passing the 120 volts to the contactor coil. This energizes the contactor which passes the 120 volts to the house wiring. When the generator is turned off, as soon as the voltage drops significantly, the TDR de-energizes, dropping the contactor and cleanly cutting off the power to the house. This keeps trash power from the slowing generator out of the house wiring.  When the generator is off, shore power is fed to the RV via the normally closed contacts.


The contactor and the relay was mounted in a PVC plastic electrical pull box, available from Home Depot. This box is waterproof. The leads were brought out through a strain relief and caulked with silicone RTV. The entire assembly is waterproof. The box is mounted in the generator compartment away from any heat generating sources such as the exhaust. Set the TDR to about 30-45 seconds and the job is complete.


Simply start the generator. Everything else is automatic. After the time delay you set, the house will power up normally.


This article and graphics copyright 1999 by John De Armond. All rights reserved. Private, personal use permitted. Other uses only by prior explicit written permission. I may be contacted via the email button to the left

Compliments of Potter & Brumfield


Generators loaded since 05/23/07