Los angeles
What are the maximum size heaters that should be used to protect a 40 horsepower 460 three phase motor with a service factor of 1.5 and a nameplate rating of 52.0a?
Do I use service factor rule anything 1.15 or greater is 125%
Or do I use nameplate rating multiply bye 1.15%


Senior Member
On eutectic overloads you MUST follow the manufacturers instructions.and all the tables to the letter. It’s about ten pages. It is manufacturer specific and even then different for different models. And they are being obsoleted because of RoHS.

For less than the cost of one set of heaters I can sell you a bimetallic. The bimetallic will cover a few motor sizes. No need to keep all those heaters around. For about the SAME price I can make it electronic. This allows three relays to cover every size motor and is more accurate than either bimetallic or eutectic.

With BOTH types the catalog lists the amp range and usually HP. Use the amps in the NEC table not the name plate and if you can pick an OL where that number is in the middle. So on a 5 HP motor with 6 A pick the 4-8 A overload not 2-6 for example: By using the NEC number you guarantee the same OL will work with any motor of the same HP rating. By using the middle of the range you can often substitute larger or smaller motors if needed.

There is also the issue of the tabs lining up if you jump brands. On large motors we don’t wire directly through the OL anyway. We use a CT and an adapter that the relay connects to. This can be used for adapting to different brands, too.

To remain competitive even AB is selling these.

I’ve only had one case in recent memory where this would not work. Customer had a huge number of 3 HP motors with half height MCC buckets. Everything was jammed in so tight even though the OL relay itself went bad I had to just replace exactly the way it was.

With both of them you just set the dial to the same FLA as the name plate.

As far as service factor use name plate. The chemical industry among others insists on longer motor insulation life (but still destroys them far sooner) so the motor industry responded with 20 year insulation motors instead of 10 years and marked the difference with a service factor. Similarly compressors have high starting torque requirements so they need oversized motors but lower continuous mechanical load so they rerate larger motors with a big service factor.

Multiplying by service factor does two things. First the heater tolerances are decreased and second you are negating all the reasons the service factor was used in the first place. And you are not following manufacturers instructions unless they tell you to do it. Which violates Listing. Your job is to set it up according to FLA.

Even if you did try to relate it yourself every motor except PM types has two currents: flux and torque. Torque currents scale with HP but flux doesn’t. The service factor is a multiplier on HP, not FLA. so multiplying FLA by SF overestimates FLA.

if you really need a bigger motor, buy one.


Senior Member
NE (9.06 miles @5.9 Degrees from Winged Horses)
Electrical Contractor
Most of the time we don’t sell the motors. We connect and set the overloads. Many equipment manufacturers depend on running into the SFA, cheaper upfront cost.

We had a brand of grain dryers that depended on the burner being on to keep the amps low. That took awhile to figure out.


Staff member
San Francisco Bay Area, CA, USA
Electrical Engineer
We cannot answer this for you, other than to say RTFM (Read The F****** Manual). Some OL relay manufacturers build-in a 1.15 factor and you adjust up for using the SF, some assume you will have / use SF and start off with a 1.25 factor then you adjust DOWN for 1.0SF, some do nothing and expect you to do it all yourself.

F****** = Factory... what did you think?