Connect 240V to 9V AC REF port?

On the IoTaWatt base (North America), is it possible to connect the “9V AC REF” port directly to a 240V circuit breaker? This would be easier for me than installing an AC outlet near the circuit breaker box to plug in the 9V AC transformer.

Definitely not. You need a transformer to get galvanic isolation for safety. The IoTaWatt is ETL tested to UL standards when used with a UL Listed transformer.

That said, there are UL listed transformer that you can connect directly to breakers in a panel. You probably wont find one with 9VAV output but 12VAC is common and as along as the no-load voltage doesn’t exceed 14.9VRMS it will work fine. If the voltage does exceed 14.9V, you can add an external resistor in series to make it work.

I recently tested a 277VAC/12VAC transformer to use in a three-phase 277V/480V industrial installation. These transformers mount through a 1/2" knockout in the load center and can connect directly to a breaker. The secondary needs to be spliced to a wire with a 5.5mmx2.1mm barrel jack.


This one is made by Elco and available from (or on eBay). They also have a 120V model. I believe the 277V volt model would work fine for 240V.

These are heavy transformers, weighing in at about 2 lbs, so the phase shift is very low. The one I tested was about 0.18 degrees at 240V.

My house has slightly different voltages when measuring between phase A + neutral, compared to phase B + neutral. So, I figured I could just give it 240VAC from phase A + phase B, to make it better accurate, but I need to step down to 9-12VAC as you recommended. I see there are Chinese transformers on eBay that might work, however they are not UL listed.

I just noticed that my circuit breaker box already has a 120VAC to 20VAC transformer to power the door bell. Could I add a resistor to go from 20VAC to 9VAC?

Slight differences are normal and really don’t have much effect on accuracy. I would recommend that you install a 120V plug and use the VT in the NA bundle. You will also need a USB supply (comes with NA bundle) so you need a receptacle anyway and a standard duplex 120V will serve both purposes.

Consider using a box with integrated duplex GFCI receptacle. I use these.

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On 5/31/2019 3:45 PM, Agnes wrote:
Do you think this will work as a 9V AC reference? It is UL listed.

Edwards Signaling 590Y 240V AC Primary 10V Secondary Transformer

-----Original Message-----
From: overeasy
To: Agnes
Sent: Sat, Jun 1, 2019 9:00 am
Subject: Re: [IoTaWatt User Community] [Support] Connect 240V to 9V AC REF port?

Should work, couple of things:

You will need to calibrate.
Phase shift is unknown (although should be low).
It’s very expensive.

On 6/1/2019 10:44 AM, Agnes wrote:
I bought the last one yesterday for $7.28, sold by services. Now there is a 3rd party seller ripping people off for $60. The manufacturer’s list price is $23.90. I’ll try to find that phase shift number.

-----Original Message-----
From: overeasy
To: Agnes
Sent: Sat, Jun 1, 2019 11:53 am
Subject: Re: [IoTaWatt User Community] [Support] Connect 240V to 9V AC REF port?

I doubt you will find any kind of phase shift spec for that unit. That’s not something that transformer manufacturers typically measure and publish. It’s irrelevant for most uses. Moreover, the method I use to determine shift is specific to the unit as used in the IoTaWatt.

In general, only light duty transformers have significantly large shift. Units weighing a pound or more with 1A or more output are usually low shift, and higher weight/output it becomes almost insignificant. This transformer is the 5VA model which would be about 500mA. I suspect it will weigh in at less than a pound and probably has fairly significant shift. I could be wrong, but the whole spectrum of “bell-transformers” are just not that impressive.

The 230V transformer I currently sell (Euro plug) weighs a little over a pound, is 10VA, and has shift of about .15°. I’m about to change to using a lighter model that has about 1.5° due to the 25% China tax and international shipping cost. IoTaWatt does a good job of correcting the shift, as long as it knows what it is, and sometimes greater shift matches up better with CTs that have similar shift.

On 6/1/2019 12:31 PM, Agnes wrote:
If I try to input 120V to the 240V transformer, would the phase shift measurement still be accurate? The issue is that each channel on the oscilloscope is tied to a common ground, so it’s not possible to measure the full 240V mains power on a single channel of my scope (it would fry the scope). So, I would try the phase shift measurement for one 120V leg with reference to neutral. Or, is there a better oscilloscope out there that can probe the difference between two 120V legs?

It will provide a voltage reference. At 5VA that’s 500mA. Most lightweight transformers have a lot of phase shift. IoTaWatt will compensate for the shift if you configure the amount to compensate. Typically I measure that for the transformers in the tables using an IoTaWatt and a special zero-shift reference that I have.

Low shift isn’t necessarily better. What’s best is when the VT and CT have similar shift so very little correction is applied. I suspect that VT will have high shift. If so, consider using high shift CTs like the YHDC products.

My rule of thumb is that the shift is inversely proportional to the weight of the VT. A VT that weighs a pound or more is typically low-shift.

This VT appears to be a lightweight bell transformer. What is the reluctance to using a 120V wall transformer? I know I said that a 240V transformer could be marginally better than a 120V, but I don’t think it’s worth getting this far off the reservation with a lightweight uncalibrated transformer.

I’m putting everything in a sprinkler control box and there may not be enough room for a transformer because I also want to fit a small UPS inside the control box too. We have a whole house generator, and I want to use IoTaWatt to record the transition from mains power to generator power, when we lose mains power.

Does an 8V AC transformer have enough voltage for the IoTaWatt 9V AC ref port? Edwards makes larger 10VA and 20VA transformers for 240V, however they only convert to 8V/16V/24V, not 10V.

Yes. 8V would be fine.

I installed this AC reference transformer in my circuit breaker box and the IotaWatt is working fine. I’m monitoring the mains plus 12 other circuits.
Edwards 592Y Class 2 Transformer; 240 Volt AC Primary, 8/16/24 Volt Secondary, 10/10/20 VA

After calibration, the IotaWatt’s status page voltage matches the Kill-A-Watt voltage of about 123.0 VAC. The output of my AC reference transformer is about 9.10 VAC, with an input voltage of about 246 VAC.

That’s a good way to monitor mains voltage in a split phase system. By definition your reference voltage will always be the average of the two legs and your 240 volt appliances will have the exact 240V reference provided you calibrated when the two legs were balanced.