30A switch vs contactor

[jdudek]

Hi All,

My understanding is that when building a power on switch for a large heating element (30A), a low voltage switch is used along with a 30A contactor.

I was wondering if there are any drawbacks to using these:
https://www.auberins.com/index.php?main_page=product_info&products_id=260
it's what i currently use on my 20A controller. I am converting to 240/30 and would like to simply add 1 more block to my switch rather than getting an extra component (not a lot of space in the box).

Any reason a contactor is a better solution?

thanks

 

[doug293cz]

Hi All,

My understanding is that when building a power on switch for a large heating element (30A), a low voltage switch is used along with a 30A contactor.

I was wondering if there are any drawbacks to using these:
https://www.auberins.com/index.php?main_page=product_info&products_id=260
it's what i currently use on my 20A controller. I am converting to 240/30 and would like to simply add 1 more block to my switch rather than getting an extra component (not a lot of space in the box).

Any reason a contactor is a better solution?

thanks

The switch used to control the contactor is usually a low(er) current switch, not a lower voltage switch (although it can be lower voltage as well.)

Most of the Auber, and similar from other sources, switches have contacts rated for 10A max, so they cannot be used to switch circuits carrying more than 10A.

If you want to switch directly (without going thru a contactor) you can use a Leviton 3032. It looks like an old fashioned wall light switch, but is double pole (so it can switch both hot lines in a 240V system) and rated for up to 30A.

There are several reasons you might want to use a switch + contactor for switching the high current circuits within a control panel:

• Looks better than an "ugly" 3032 type switch.
• Allows multiple switches in the power control loop (such as main power switch, and an emergency power off switch.)
• Required if you want to implement a "safe start" interlock that prevents main power from coming on if any of the load switches (pumps, elements, etc.) are on. This can prevent unpleasant surprises, or even damage, when powering on the system.

Here's a schematic for a single pump, single element control panel that shows how a safe start interlock is implemented.

Brew on

 

[jdudek]

Apologies that was a brain fart. I meant low current switch, not voltage as you pointed out.

the link I sent is for a parallel switch extender. I am using one of those at the moment to support 20A. I can add a third one to support 30A. So the auber switch will support 30A via 3 parallel 10A paths.

I do have a main switch for the box that’s one of those big breaker switches. So the switch I am talking about is already a second independant element switch on.

but yeah I can’t do the safe start. I will give this a think. Thanks as always.

 

[doug293cz]

...

the link I sent is for a parallel switch extender. I am using one of those at the moment to support 20A. I can add a third one to support 30A. So the auber switch will support 30A via 3 parallel 10A paths.

...

This is not good practice, and I suspect it would violate the NEC (although, I am not a code expert, so can't be sure.)

Brew on

 

[jdudek]

What is the reason it’s not good practice, just for my education?

 

[Bobby_M]

Apologies that was a brain fart. I meant low current switch, not voltage as you pointed out.

the link I sent is for a parallel switch extender. I am using one of those at the moment to support 20A. I can add a third one to support 30A. So the auber switch will support 30A via 3 parallel 10A paths.

I do have a main switch for the box that’s one of those big breaker switches. So the switch I am talking about is already a second independant element switch on.

but yeah I can’t do the safe start. I will give this a think. Thanks as always.

The reason why it's not good practice to try to squeeze out more current capacity by stacking NO contacts is that if any one of the blocks fails (which would definitely be more likely when running at full 10a capacity), the remaining blocks will now share the 30 amps running 15 through each. The switch will melt down.

A 30 amp contactor can be had for $12. That's the right way to do it. Use your current switch to switch the 240v that would feed the contactor coil. In addition, a 2 pole contactor will switch both legs of the 240v supply ensuring that there is no voltage present at the output when off.

 

[jdudek]

The reason why it's not good practice to try to squeeze out more current capacity by stacking NO contacts is that if any one of the blocks fails (which would definitely be more likely when running at full 10a capacity), the remaining blocks will now share the 30 amps running 15 through each. The switch will melt down.

A 30 amp contactor can be had for $12. That's the right way to do it. Use your current switch to switch the 240v that would feed the contactor coil. In addition, a 2 pole contactor will switch both legs of the 240v supply ensuring that there is no voltage present at the output when off.

Thanks makes sense. Contactor it is then.

 

[doug293cz]

Found some information on switches in the NEC - Article 404-14:

"404.14 Rating and Use of Switches. Switches shall be used within their ratings and as indicated​

in 404.14(A) through (F).​

...​

(A) Alternating-Current General-Use Snap Switch. A form of general-use snap switch suitable only for use on ac circuits for controlling the following​

(1) Resistive and inductive loads not exceeding the ampere rating of the switch at the voltage applied​

{2} Tungsten-lament lamp loads not exceeding the ampere rating of the switch at 120 volts​

{3) Motor loads not exceeding 80 percent of the ampere rating of the switch at its rated voltage​

(B) Alternating-Current or Direct-Current General-Use Snap Switch. A form of general-use snap​

switch suitable for use on ether ac or dc circuits for controlling the following​

(1) Resistive loads not exceeding the ampere rating of the switch at the voltage applied.​

(2) Inductive loads not exceeding 50 percent of the ampere rating of the switch at the applied voltage. Switches rated in horsepower are suitable for controlling motor loads within their rating at the voltage applied​

(3) Tungsten-lament lamp loads not exceeding the ampere rating of the switch at the applied voltage if T-rated."​

By my reading, 404.14-(A)-(1) and 404-14-(B)-(1) prohibit the use of multiple switches in parallel to carry a load current higher than the switch ratings.

Brew on