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I have a relay from SparkFun (https://www.sparkfun.com/products/10924), and a pump that draws ~13 amps. Can I use this relay with the pump?

Also, I don't understand why this relay says NO20A/NC10A, anyone know? The datasheet seems to suggest it is 20A, but the NC10A worries me.

Is there typically a difference between amperage ratings at 120 and 240V, or are they usually the same? I don't quite understand what it is that determines the relay's rating.

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    \$\begingroup\$ That relay does not have a specification for horsepower. It just says max current. I would be hesitant to use it for a 13A pump because the startup current is likely in excess of the max relay current. The ideal thing would be to find out the motor rated horsepower and then select a relay that has a horsepower rating as large or larger. Many relays designed to switch motors will have a horsepower rating. You can also use a contactor. This is basically a relay designed to switch a motor. \$\endgroup\$ – mkeith Oct 23 '15 at 20:54
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That relay is rated for 20A @240V, but the [crappy] datasheet doesn't quite drive home the usual message that that's for resisitive loads. For inductive loads [motor] you need to derate to 10% or 20% of that, which won't be enough for your 13A load assuming "~13A" is the continuous/steady-state load current. Without any additional protection circuitry your relay will have its contacts fused soon enough by the much higher start and stop currents.

enter image description here

(A suggestive diagram found in more wordy relay application guide.)

It sounds like you're using a 2hp (=1500W) motor. I suggest you buy a real contactor indended for motors rather than a relay for hobby projects. From this catalog for example, you can see that a 2hp 120V (one phase) motor needs a relay (contactor) rated for around 40A-50A. But I suggest you get something intended for motors rather than a random relay (even one rated that high) because these have been tested by the mfg. and some by UL to actually work a specific number of cycles with that motor load.

enter image description here

Note that the same story applies for SSRs:

Solid State Relays and Solid State Contactors that have been evaluated by a safety agency or regulatory body as “motor controllers” carry motor power ratings in Horse Power or HP, making the controller selection process for any given application much simple because the HP rating is coordinated by UL or IEC standards with both LRA and FLA ratings. Therefore motor rated Solid State Relays and Solid State Contactors are often preferred because the necessary coordination calculations have already been made and validated by the safety agencies.

And a 2hp, 120V SSR also needs to be able to do around 50A resistive, as you can see from another catalog

enter image description here

These motor-rated things (EMR or SSR) are designed to take a big surge for a brief period, e.g. the aforementioned SSR can take 800A for brief moment:

enter image description here

The UL/IEC standards have a big margin of safety for certification purposes. The random-fire ("instantaneous turn on") version of this thing is rated for 2 hp motors, while the zero-crossing version only for 1 hp.

It's also instructive to look for comparison at a 50A continuous-rated SSR from the same manufacturer, but which [SSR] is not UL/IEC-certified for motor operation. The peak current this one can handle is (about 20%) less:

enter image description here

Beware that SSRs are basically never multipole, you need to buy two if you want to cut the neutral to your pump too (for some reason).

Also for comparison purposes, I dug up the datasheet for the first (EM) contactor (2hp) I mentioned. Alas it doesn't have a nice graph, but its surge rating is actually higher than the SSRs above; 380A at 1s vs ~200A for the SSRs at that time mark.

Also be aware that there are clones this LC1D25 being from a rather famous mfg and alibaba-style shops sell the clones under this name as if it were the original (but you can tell from image that it's not the same thing). You can also buy more honest clones/knockoffs of this that come with UL certifications; they'll usually have similar but not identical name and reference this as equivalent.

enter image description here

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  • \$\begingroup\$ So, when the contacts fuse...will that mean it is stuck in the 'on' position, leaving the pump on? \$\endgroup\$ – wordsforthewise Oct 26 '15 at 3:23
  • \$\begingroup\$ @wordsforthewise: Yes, it will get stuck 'on'. It may not happen immediately, but from the other question linked where someone used dozens of these it does happen. Also, "fused" was probably not the best choice of word [on my behalf]. "Welded" would have been more appropriate. \$\endgroup\$ – Fizz Oct 26 '15 at 3:27
  • \$\begingroup\$ What about using a triac and optocoupler? For example, the BTA41-800B (st.com/web/en/resource/technical/document/datasheet/…) \$\endgroup\$ – wordsforthewise Jan 28 '16 at 9:26
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NO20A/NC10A is one rating for the normally open connection 'NO' and one for the normally closed connection 'NC'.

If you're only using the NO connection then you don't need to worry about the NC rating. (And you don't need a changeover relay in the first place, but that's a minor issue)

I can't see that NO20A/NC10A bit in the datasheet I'm reading from that page, so I don't know if I'm looking at the same relay. Generally, if the relay's OK at 240VAC it will have no worse a rating at 120VAC.

There are often two ratings quoted, one for resistive loads and one (much lower) for motor loads.

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I've been working with relays for quite some time now. I'll tell you this from my experience, avoid mechanical relays like the ones you've linked to when it comes to inductive loads. They have this issue while turning the motor on/off, a "spark" is generated when they click and disconnect which will cause a huge rise in heat momentarily. This will therefore deteriorate your relay mechanically causing the resistance to increase, which will affect your overall rating overtime. Infact there are hardly any mechanical relays to control inductive loads for 4A rating and above.Usually those kind of relays are rated at 1/2HP @ 120V and 1HP @240V, You'll need a relay where its rated between 2-3HP. You will need something called a "random fire solid state relay", these are specifically designed for inductive loads but they come with a huge price. A typical 20A random fire SSR would cost you about 30$ a piece. And an additional investment in heat sinks and thermal paste. I even contacted spark fun for a mechanical relay that can handle high inductive loads, they did respond saying they don't have any.

There are even other options like "Contactors" which are basically relays but are designed for high loads (resistive and inductive) but they are really expensive.

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