how big does a relay have to be before it is a contactor?

how big does a relay have to be before it is a contactor?

As far as I can tell that is a very scientific and esoteric calculation based primarily on how much Starbucks has been consumed by the marketing staff before the meeting.

Tom Jenkins said:

As far as I can tell that is a very scientific and esoteric calculation based primarily on how much Starbucks has been consumed by the marketing staff before the meeting.

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har har.

I took a poll in the office today after posting this, the conclusion was if it fits in a socket or not, maybe.

Most contactors are relays, but not vise-versa.
I generally think of 'contactors' as having a specific purpose i.e. Motor Contactors or Lighting Contactors. etc.

Over here it's called a contactor if it's main contacts are in the powering circuits, e.g. a line contactor, a wye contactoe, a delta contactor etc.

If it has only contacts in the logic control circuits we call it a relay.

Not related to size. The difference is the construction of the contact mechanism.
If you have a large arc with a relay, the contacts can weld.

With a contactor, we expect the contacts to occasionally weld.
The contacts are spring loaded and floating. Most of them have a double set for each pole. If one side welds, the contact will move at an angle and break itself loose.

If it scares you when it opens or closes it is a contactor

I thought that when a relay hits puberty it changes.

Good to know, keithkyll .

Correct Keith - apart from the small European ones that pivot - they call them contactors. Not in my mind - has to be double break.

It is not the size, it is how you use it.

As it was mentioned above, a contactor is supposed to have some kind of an arc-quenching feature so it can reliably open an inductive load without arcing and welding the contacts. And yes, the border between a contactor and a relay is a bit murky but this ability is the key.

Well, it seems to me that, in the US, under NEMA standards, anything that operates or is rated over 600 volts, or over 10 amps, can't have a relay contact rating at that level. (A600; N600). So, one might conclude that NEMA does not consider anything that handles over 600V and/or over 10AMPS to be a relay.
Although, I just searched my NEMA standards, and it does not come out and say this, specifically.

There is no rule or "official" definition. But in the industrial controls world, you won't see anything described as a "relay" if it is over 40A, yet you will see "contactors" that go as low as 9A.

As a gross general rule, the real difference is when switching inductive motor loads. With a 600V rated "relay", even if it is rated 35A for example, it's inductive motor rating is only at 240V max., and in this case, 5HP single phase, even if you have a 3 pole relay. With a "contactor" rated at 32A thermal, it can switch a 10HP 480V motor.

It's not really the current rating per se, its all about the arc quenching capabilities. Relays are not intended to quench a sustained arc that occurs when interrupting an induction motor circuit, where the back EMF from the motor attempts to keep the current flowing across the gap as the contacts separate. Contactors are SPECIFICALLY designed to do that, regardless of the current rating. That's why a 9A "contactor" will cost you more than a 20A "relay".

redbarron55 said:

if it scares you when it opens or closes it is a contactor

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+1

Here is everything you wanted to know.
Basically...as said...it is power related.

But now you can be up on all types of relays and contactors.