Using a 9 Amp contactor for 10 Amp motor?

Hi guys,

We've a project which uses a lot of small single phase (230V AC) water centrifugal pump (10 Amp) each. We've a lot of excess overload and contactors (telemac). I was wondering if we can use the 9 Amps contactors + 7 - 10Amps overload to work with the 10 Amps pump? The pump will only be start 10 times per day, each time the duration is run for 5 minutes.

Thanks for your advice.

On the side of the Contactor is a rating for AC1 which is the rating for resistive loads (heating), ... Then there is an AC3 rating for inductive loads (Motors)
I'm almost certain that you will be quoting the rating for AC1, so the short answer is that the Contactors will be to small for the job you propose.

flyers said:

Hi guys,

We've a project which uses a lot of small single phase (230V AC) water centrifugal pump (10 Amp) each. We've a lot of excess overload and contactors (telemac). I was wondering if we can use the 9 Amps contactors + 7 - 10Amps overload to work with the 10 Amps pump? The pump will only be start 10 times per day, each time the duration is run for 5 minutes.

Thanks for your advice.

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This would be fine if the motor(s) NEVER pulled more than 9 amps. But, the general rule of thumb is to size the contactor to the motor exactly, this way if the motor were to start pulling up to it's maximum amperage (10 amps), the contactor would be sufficiently sized.

shoelesscraig said:

This would be fine if ....

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NO, It would NOT be fine. Neither from an Engineering standpoint nor from a Legal standpoint. "General rules of thumb" do not apply in cases like this. Do it right. Do it to Code. Period. End of story.

theDave2 said:

NO, It would NOT be fine. Neither from an Engineering standpoint nor from a Legal standpoint. "General rules of thumb" do not apply in cases like this. Do it right. Do it to Code. Period. End of story.

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You obviously have had the privilege of working in a perfect world all your life. In the real world, you have to do what you can to keep things running with what you have. Any idiot can pick up the NEC code book and do it by reading, it takes a REAL electrician to know when and where you can get away with something to make the plant run, when necessary. NOTE: I didn't say that I would downsize contactors if doing a new installation. But in a pinch, there are exceptions.

And yes, depending upon the situation, it would be okay. I have seen plenty of situations where a machine has a 7.5 HP motor on it and pumps just water. Of course the motor nameplate reads 9ish amps depending upon the manufacturer, but this is only if the motor has to actually go to full load, using all 7.5 HP that it is capable of. I have a setup in my plant that is exactly like this. Because of the pump design and the pressures involved, the motor never pulls over 6 amps...NEVER. So we have, in the past, had to put in a smaller contactor to get us by. The overloads are still adjusted so that anything over 6 amps gets tripped out. It has been working fine for us for years and years and years.


To the original poster...like I said, it depends on your situation and the EXACT circumstances of your application. Most of the time, doing this is not a good idea, but if you are well aware of the characteristics and operation of your setup, and you know that the pumps do not pull more than "x" during a run, then it is perfectly fine. You'll find a lot of guys that can quote code to you, but a lot of them are also the ones that sit there and let the company lose money because they've got the machine locked out because the "perfect" contactor isn't in the building. What a laugh...

There is a whole lot more money to be lost if an insurance company every catches you doing that.
Get off the "Perfect World" high horse.
Right is Right.
Wrong is Wrong.
Situations aren't static either. What if the load changes in the future?
If your company cannot afford a correct contactor, then there are probably many even more serious safety violations going on.

If you absolutely must downsize a contactor or protective device, then downsize the motor/load as well.

Ahhh, another privledged worker that has always worked for a company that has deep pockets. And BTW, I have worked for about 10 different companies in my lifetime, 6 or which have been some of the largest most powerful companies in the US, and I have NEVER EVER EVER...did I say EVER?....seen or heard of an insurance company giving a &%#$ about what kind of contactor I put in a cabinet as long as the place doesn't burn down. And yes, in many many situations, things are STATIC. Sometimes, things do not, and never will change. I realize this is not always the case, but many times it is, and a real engineer/electrician/programmer can sort out those situations.

And as for safety violations, I could send you pictures that you would swear were fake. I have worked in a few places that you can't even dream of. And, heres the real kicker, OSHA (the biggest joke of all time) let fly. When the economy is terrible, and NO ONE is finding work, and you know that your employer is going to fire you if you can't make "it" happen, you find ways to get things going and running. Try going home and telling your wife an kids that you no longer have a job because you refused to use a 9 amp contactor on a motor than said 10 amps on the side of it. Point is....you do it your way, I'll do it mine. I have never ever done anything that was unsafe. You have to know the boundaries. Chances are, I'm much older than you and waaay more experienced and have seen a lot more, so if you think you can try and change my mind about it, think again.

I would take "new installations" that original post is about and not discuss that from maintenance perspective.

In new installation that is not fine to use underrated parts.

In maintenance, it has to be done sometimes.

There is a distinction that should be made between designing a system, making one work in the real world, and a practice that will jeapordize safety.

Service factors and raings are there to allow for unforseen circumstances. You shouldn't design a system that pushes into service factors or exceeds known ratings. When Murphy's law hits, and it will, your system won't function properly or will fail prematurely. Counting on service factors to cover situations you know about during design means there is no cushion left for the things you don't know about. Using a relay for a load known to be above rating is, in my opinion, bad practice.

When you are out in the field and you have to keep a system running you can use equipment on a temporary basis to get through a problem IF you thoroughly understand the application AND you understand the basis of the ratings AND you have field measurements to verify you will not cause problems. This should be a temporary measure only, under controlled conditions. The proper equipment should be installed and the marginal equipment replaced ASAP.

Safety practices should NEVER be violated, period.

In your case the probable result of running the motor on a slightly undersized contactor will be reduced life. If the underating is severe or if an unforseen field condition causes higher loading you could have catastrophic failure. How much money will you save after the cost of putting out the fire is factored in? I'd maybe let you use the contactors as an emergency field fix until the right part can be procured. If I was in charge I would not let you design the system that way.

Tom Jenkins said:

There is a distinction that should be made between designing a system, making one work in the real world, and a practice that will jeapordize safety.

Service factors and raings are there to allow for unforseen circumstances. You shouldn't design a system that pushes into service factors or exceeds known ratings. When Murphy's law hits, and it will, your system won't function properly or will fail prematurely. Counting on service factors to cover situations you know about during design means there is no cushion left for the things you don't know about. Using a relay for a load known to be above rating is, in my opinion, bad practice.

When you are out in the field and you have to keep a system running you can use equipment on a temporary basis to get through a problem IF you thoroughly understand the application AND you understand the basis of the ratings AND you have field measurements to verify you will not cause problems. This should be a temporary measure only, under controlled conditions. The proper equipment should be installed and the marginal equipment replaced ASAP.

Safety practices should NEVER be violated, period.

In your case the probable result of running the motor on a slightly undersized contactor will be reduced life. If the underating is severe or if an unforseen field condition causes higher loading you could have catastrophic failure. How much money will you save after the cost of putting out the fire is factored in? I'd maybe let you use the contactors as an emergency field fix until the right part can be procured. If I was in charge I would not let you design the system that way.

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Well put. If I was not clear before, let me be now. I am talking about making a system work in the event of a failure. I am not talking about designing a system and trying to take shortcuts to save money. If I am building a system from scratch, of course the right size "everything" goes in the system. Never underrate in a new system. But, in a pinch, sometimes things must be done to keep running. And, like you said, it must be done with extensive knowledge of the system, constant monitoring, and immediately fixed when the right part comes in, ASAP.

TurpoUrpo said:

I would take "new installations" that original post is about and not discuss that from maintenance perspective.

In new installation that is not fine to use underrated parts.

In maintenance, it has to be done sometimes.

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And you know, my apologies, because until you pointed it out, I failed to see the part in the OP's question that says "project". I didn't realize that this was a question about a NEW installation until now. My mistake.

Quite frankly, if its a telemecanique brand IEC contactor starting ten times a day then you should de-rate the contactors by about 30%, especially if you are turnning a pump/fan where the motor starts under load. So you should be looking at an 18A rated relay. Personally, I wouldn't touch one, but a lot of OEMs use them.

Alaric said:

Quite frankly, if its a telemecanique brand IEC contactor starting ten times a day then you should de-rate the contactors by about 30%, especially if you are turnning a pump/fan where the motor starts under load. So you should be looking at an 18A rated relay. Personally, I wouldn't touch one, but a lot of OEMs use them.

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I'll second that.

RE telemecanique

On the job on someone elses money I love em - job security for a maintenance electrician.

In MY equipment and on MY dime there is no way I would even look at them.

One other note
I generally oversize contactors especially small ones by 25 to 50% to gain longer life and reliability. Same thinking for wire size and loading of branch circuits - fewer callbacks / failures. Yes costs a little more up front but the life time cost is lowered.

Dan Bentler

Alaric said:

Quite frankly, if its a telemecanique brand IEC contactor starting ten times a day then you should de-rate the contactors by about 30%,

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I could not find a reference, but I thought the IEC maximum rating was for 1 start a day, 8 hour maximum run time. I do not believe that Telemecanique is the only culprit in this category. I have seen others that are just as wimpy.