Putting a Smart Switch on my Hot Tub heater

[JamesW]

Heat can be calculated by the formula V²/R or I²R or voltage x current.

For example, the resistance of 1,000 feet of #8 AWG wire = 0.6282 ohms.

At 10 amps, the power (heat) = 10²(0.6282) = 62.82 watts.

That heat causes the wires and switches to heat up beyond the ambient and that causes the heat to begin to be lost due to radiation, conduction and convection.

So, you have heat gain and loss and this eventually reaches an equilibrium and the wires and switches reach a steady state equilibrium.

So, if you have 10 amps going through a 1,000 foot wire at #8 AWG, the amount of heat produced is about 63 watts and the amount of heat lost will be 63 watts once the wires reach equilibrium.

The exact temperature of the wire depends on the heat loss rate, which depends on multiple factors including the ambient temperature, insulation, ventilation etc.

 

[JamesW]

If the contacts had 1 ohm of resistance, then the heat generated by 10 amps would be 100 watts, which is like an old style 100 watt incandescent lightbulb and they got very hot.

The heat generated at 1 ohm and 20 amps is 400 watts.

At 50 amps and 1 ohm of resistance, the heat generated would be 2,500 watts.

Most of the heat was radiated in the infrared vs. the visible light spectrum.

So, most of the power was wasted.

That is why an 8 watt LED is equivalent in visible lumens to an old style 100 watt incandescent bulb.

 

[Dirk]

Holy moly! How did this get into resistance of wiring?! Relay contact resistance is measured in mΩ (milliohms!). A reasonable example of the resistance for a common high-amp rated relay is 10 mΩ max resistance and 0.3 mΩ typical, not anywhere near 1 ohm. Using James' formula and the OP's heater draw of about 25 amps: 25*25(0.003) = 1.875 watts. It'd be like an LED night light, not a 100W light bulb! Come on guys, focus. In the OP's configuration, he can safely mount a 40A relay inside his hot tub's electronics box without compromising either the relay or the existing electronics due to heat generated by the relay. That was my point. It would also solve the issue of running the max wattage rating through his Sonoff.

If he wants to try and mount that Sonoff device in there, he's gunna have to figure out the wattage it consumes and then decide if that's going to generate too much heat in there, and whether it can withstand the elements of the inside of a hot tub. My suggestion was to locate the Sonoff indoors if possible, out of all the elements, and have that operate a relay inside his hot tub enclosure. There would be virtually no additional run of wire to carry the 25 amps. The wire from the Sonoff to the relay would be relatively light weight, as it only has to power the relay's coil.

Or he could mount the Sonoff in a standalone watertight box inside the hot tub, and run a short wire to the relay inside the existing electronic box. With the Sonoff only powering the relay, perhaps the heat it generates would not be as much of an issue inside a small project box.

 

[Dirk]

And for the record, a relay is not a circuit breaker. Circuit breakers do have contacts, which put up a barely measurable amount of resistance, but they also have the tripping components (bimetal trip devices) that also generate heat. You cannot compare the two devices.

 

[JoyfulNoise]

I think the setup as it currently exists is fine. The biggest problem would be the premature failure of the WiFi switch. And even if it did fail in a shorted manner, it wouldn’t matter. The only harm that would cause would be that the tub heater will draw power during hours of the day when electricity is expensive. Failing in an open-circuit state would be somewhat worse because that would mean the tubs heater would never receive power and, if it’s cold outside and no one is paying attention then you could be in danger of freeze damage.

 

[Dirk]

And even if it did fail in a shorted manner, it wouldn’t matter.

Mostly true, in terms of hot tub functionality. But that Sonoff POWR3 is about 50 bucks. For another $16 he might have a more reliable solution. As someone that has replaced more smart switches than I'd care to admit to, I know how flakey some of them tend to be, even under the best of circumstances. And it's not just the switch cost, it's the inconvenience of losing what the device controls for some number of days, and the effort required to tear things apart to put in a new one (that's worth a lot more than the cost of the device or the 16 bucks).

 

[JoyfulNoise]

Mostly true, in terms of hot tub functionality. But that Sonoff POWR3 is about 50 bucks. For another $16 he might have a more reliable solution. As someone that has replaced more smart switches than I'd care to admit to, I know how flakey some of them tend to be, even under the best of circumstances. And it's not just the switch cost, it's the inconvenience of losing what the device controls for some number of days, and the effort to tear things apart to put in a new one (that's worth a lot more than the cost of the device or the 16 bucks).

One of the many reasons why you will not find any “smart” garbage in my home … old school analog switches with the big chunky levers … they’ll survive the EMP and zombie apocalypse. So, while you all are trying to restart your routers and get your WiFi equipment working by candlelight, I’ll be happily reading my latest edition of “Prepper’s Monthly” and “Guns ‘n Ammo” magazine under the soft glow of my incandescent filament lamp light … all powered by my 8kW diesel generator that runs days/night/rain/shine/weekdays/weekends … (solar panel inverters will blow out too when the EMP hits … make sure you have plenty of bottled water on hand as drinking from muddy puddles will give a good case of the Jersey Squirts …)

 

[Mdragger88]

One of the many reasons why you will not find any “smart” garbage in my home … old school analog switches with the big chunky levers … they’ll survive the EMP and zombie apocalypse. So, while you all are trying to restart your routers and get your WiFi equipment working by candlelight, I’ll be happily reading my latest edition of “Prepper’s Monthly” and “Guns ‘n Ammo” magazine under the soft glow of my incandescent filament lamp light … all powered by my 8kW diesel generator that runs days/night/rain/shine/weekdays/weekends … (solar panel inverters will blow out too when the EMP hits … make sure you have plenty of bottled water on hand as drinking from muddy puddles will give a good case of the Jersey Squirts …)

We’re coming to your house in the “end” ! You’ve got room right?

 

[Mdragger88]

@JoyfulNoise you know @Dirk is really a hardwired kinda feller, I can see him setting something like this up but having an ethernet cable coming out the access door (in an asthetic way of course)
the next owner of his house will have quite the time figuring out what all the cables are for!

 

[JoyfulNoise]

Very true. My favorite and most often used bit of @Dirk “wisdom” is to always run an additional 8 miles of CAT8 Ethernet cable in every conduit that is laid down, even if the conduit is not meant for Ethernet cable. You never know when you’ll need.

My survival bunker has at least 17 miles of extra cable in it … everything from 22AWG 4-wire cable to 3 different gauges and 4 colors of THHN wiring. The high frequency RG-6 cable was hard to pull along with all the rest but a little bit of chicken fat helped grease it all up …

 

[Forest_Spa]

Heat can be calculated by the formula V²/R or I²R or voltage x current.

For example, the resistance of 1,000 feet of #8 AWG wire = 0.6282 ohms.

At 10 amps, the power (heat) = 10²(0.6282) = 62.82 watts.

That heat causes the wires and switches to heat up beyond the ambient and that causes the heat to begin to be lost due to radiation, conduction and convection.

So, you have heat gain and loss and this eventually reaches an equilibrium and the wires and switches reach a steady state equilibrium.

So, if you have 10 amps going through a 1,000 foot wire at #8 AWG, the amount of heat produced is about 63 watts and the amount of heat lost will be 63 watts once the wires reach equilibrium.

The exact temperature of the wire depends on the heat loss rate, which depends on multiple factors including the ambient temperature, insulation, ventilation etc.

View attachment 544387

Wow, those maximum current values don't meet current US electrical code. I am not an electrician, but AFAIK, 14 AWG gets a 15 amp breaker and a 12 AWG gets a 20 amp breaker. Maybe this is uninsulated wire on knob and tube wiring.

 

[Dirk]

My favorite and most often used bit of @Dirk “wisdom” is to always run an additional 8 miles of CAT8 Ethernet cable

I am currently nursing pulled muscles in my back from another trip to the attic last week, adding 7 more runs of Ethernet cable. If only I had followed my own advice. I had originally left two extras up there, just in case. But I already used those!! Ugh. Last count: 43 cables. I kid you not. That was the last time I go up, I swear!

the next owner of his house will have quite the time figuring out what all the cables are for!

Oh come on. You know better than that. Here's just a corner of my schematic (Adobe Illustrator). It's for me, because I forget things, and for the next guy. Not shown: extensive key for deciphering all the symbols and colors.



Tip'o'the day: I use Monoprice cables and always order different colors. When I have to duplicate a color, I use a Sharpie and draw stripes on each end of the cable: 1, 2, 3, 4 stripes, etc. For video cables, I write the location of the cam on the cable.

We’re coming to your house in the “end” !

Sorry, by the time you get there, I will have "acquired" Matt's gennie and it will already be running at my place, in an undisclosed location. Remember, It's not who you know, but what you know... about where he hides his stuff!

 

[Dirk]

PS. Sorry @wayner. It's winter and we get bored and sometimes a little sidetracked!

 

[JoyfulNoise]

View attachment 544431

Dude that wires data centers for a living be like …

 

[JamesW]

I will have "acquired" Matt's gennie and it will already be running at my place.

You have to get past "Fluffy" his new guard dog.

 

[JoyfulNoise]

You have to get past "Fluffy" his new guard dog.

View attachment 544456

“… genetically-enhanced, new guard dog …”

If you’re wondering what caused the zombie-apocalypse to start, it has absolutely nothing to do with my experiments on Fluffy

 

[RoyR]

You don’t switch AC with relays…you need to switch ar zero cross-over, use something like this SSRL with SCR technology.

Solid State Relays with AC or DC Control Inputs | Omega

The solid state relays used to control large resistance heaters with temperature controllers, up to 100 Amps, Control ac Lines to 660 Vac.

www.omega.com

 

[Dirk]

You don’t switch AC with relays

Please explain. Why not? Are you talking about AC on the relay coil, or the load?

And which type of relay uses more watts to close?

 

[ajw22]

Please explain. Why not? Are you talking about AC on the relay coil, or the load?

And which type of relay uses more watts to close?

I think he needs to qualify his statement a bit more that SSR can handle higher loads (amperage) then mechanical contact relays.

However this does not come without some other complexities. You need to read the fine print...

Solid state relays are SPST,... Three-phase loads can be controlled using 2 or 3 SSR’s. Use 3 SSR’s for Y or star 3-phase loads using a neutral line. Two SSR’s will control “delta” loads with no neutral line. Three solid state relays are also used when there is no neutral load to provide redundancy and extra assurance of control.

To dissipate the heat, an SSR must be mounted on a finned heat sink or aluminum plate. An SSR should be located where the ambient temperature is relatively low, since the current switching rating is lowered as the temperature increases.

Another SSR characteristic is a small leakage current across the output when the relay is open. Because of this, a voltage will always exist on the load side of the device.

It is essential that a properly rated, fast blowing I2T fuse be installed to protect the load circuit.

 

[Dirk]

You don’t switch AC with relays…you need to switch ar zero cross-over, use something like this SSRL with SCR technology.

Solid State Relays with AC or DC Control Inputs | Omega

The solid state relays used to control large resistance heaters with temperature controllers, up to 100 Amps, Control ac Lines to 660 Vac.

www.omega.com

From the spec sheet (not exactly selling points for this application):

These SSR’s are of the twin SCR type, inherently more reliable and capable of higher overloads before failure than triacs. Heat is developed in a solid state relay due to the nominal voltage drop across the switching device. To dissipate the heat, an SSR must be mounted on a finned heat sink or aluminum plate. An SSR should be located where the ambient temperature is relatively low, since the current switching rating is lowered as the temperature increases.

Another SSR characteristic is a small leakage current across the output when the relay is open. Because of this, a voltage will always exist on the load side of the device.

In comparing SSR’s with mechanical contactors, the SSR has a cycle life many times that of a comparably priced contactor. However, SSR’s are more prone
to failure due to overload and improper initial wiring. Solid state relays can fail, contact closed, on overload circuits. It is essential that a properly rated, fast blowing I2T fuse be installed to protect the load circuit.

Finned heat sinks are anodized fabrications that come complete with tapped mounting holes and screws. See thermal rating curves and ordering instructions for proper selection.