IotaWatt - SCT019 not measuring correctly

Hardware
1

Hey all,

I am currently testing my IotaWatt unit, (within Canada : 120V)
I’ve calibrated my 6VAC Voltage to work correctly and used the SCT030 preset which has worked on 10 CTs. Now testing both my SCT019 I am seeing smaller values. I tried the preset and also added a generic with 6000 turns /3 phase, same result. Does anyone have insight on the reason for this issue?

Hair Dryer (Low)
345 Watt - Kill-a-Watt
340 Watt - SCT030
93 Watt - SCT019
90 Watt - SCT019 (3.8 times smaller)

Hair Dryer (High)
1157 Watt - Kill-a-Watt
1150 Watt - SCT030
337 Watt - SCT019
328 Watt - SCT019 (3.5 times smaller)

Christmas tree lights
126 Watt - Kill-a-Watt
123 Watt - SCT030
21 Watt - SCT019
21 Watt - SCT019 (5 times smaller)

Thanks!

2

Can you post your config.txt file?
You can access it through the file manager tool. Click on config.txt in the left column file list. The file will display in the editor window. Copy and paste to the forum. Obfuscate any EmonCMS credentials.

3 
{
	"format": 2,
	"timezone": 0,
	"device": {
		"name": "IotaWatt",
		"version": 3,
		"channels": "15",
		"burden": [
			0,
			24,
			24,
			24,
			24,
			24,
			24,
			24,
			24,
			24,
			24,
			24,
			24,
			24,
			24
		]
	},
	"inputs": [
		{
			"channel": 0,
			"name": "Voltage",
			"type": "VT",
			"model": "generic120V",
			"phase": 2,
			"cal": 15.75
		},
		{
			"channel": 1,
			"name": "Input_1",
			"type": "CT",
			"model": "SCT013-000",
			"phase": 3,
			"turns": 2000,
			"cal": "83.33"
		},
		{
			"channel": 2,
			"name": "Input_2",
			"type": "CT",
			"model": "SCT013-000",
			"phase": 3,
			"turns": 2000,
			"cal": "83.33"
		},
		{
			"channel": 3,
			"name": "Input_3",
			"type": "CT",
			"model": "generic",
			"phase": "3",
			"turns": "1860",
			"cal": 77.5
		},
		{
			"channel": 4,
			"name": "Input_4",
			"type": "CT",
			"model": "generic",
			"phase": "3",
			"turns": "1860",
			"cal": 77.5
		},
		{
			"channel": 5,
			"name": "Input_5",
			"type": "CT",
			"model": "generic",
			"phase": "3",
			"turns": "1860",
			"cal": 77.5
		},
		{
			"channel": 6,
			"name": "Input_6",
			"type": "CT",
			"model": "generic",
			"phase": "3",
			"turns": "1860",
			"cal": 77.5
		},
		{
			"channel": 7,
			"name": "Input_7",
			"type": "CT",
			"model": "generic",
			"phase": "3",
			"turns": "1860",
			"cal": 77.5
		},
		null,
		null,
		null,
		null,
		null,
		{
			"channel": 13,
			"name": "Input_13",
			"type": "CT",
			"model": "generic",
			"phase": "3",
			"turns": "6000",
			"cal": 250
		},
		{
			"channel": 14,
			"name": "Input_14",
			"type": "CT",
			"model": "SCT019-000",
			"phase": 2,
			"turns": 6000,
			"cal": "250.0"
		}
	],
	"outputs": [],
	"server": {
		"type": "emoncms",
		"node": "IotaWatt",
		"postInterval": 10,
		"bulksend": 1,
		"url": "http://emoncms.org",
		"apikey": "xx",
		"userid": "xx",
		"outputs": [
			{
				"name": 1,
				"script": "@0",
				"units": ""
			},
			{
				"name": 2,
				"script": "@1",
				"units": ""
			},
			{
				"name": 3,
				"script": "@2",
				"units": ""
			},
			{
				"name": 4,
				"script": "@3",
				"units": ""
			},
			{
				"name": 5,
				"script": "@4",
				"units": ""
			},
			{
				"name": 6,
				"script": "@5",
				"units": ""
			},
			{
				"name": 7,
				"script": "@6",
				"units": ""
			},
			{
				"name": 8,
				"script": "@7",
				"units": ""
			},
			{
				"name": 9,
				"script": "@8",
				"units": ""
			},
			{
				"name": 10,
				"script": "@9",
				"units": ""
			},
			{
				"name": 11,
				"script": "@10",
				"units": ""
			},
			{
				"name": 12,
				"script": "@11",
				"units": ""
			},
			{
				"name": 13,
				"script": "@12",
				"units": ""
			},
			{
				"name": 14,
				"script": "@13",
				"units": ""
			},
			{
				"name": 15,
				"script": "@14",
				"units": ""
			}
		]
	},
	"update": "MAJOR"
}
4

@heppell

The config looks fine, but it was a concise way to understand your configuration without asking a lot of questions. There are two things that I’d like to try:

First, I’m assuming that the two SCT019 outputs that you indicated in your initial post were from the two SCT019’s, clamped on the same wire as the SCT013, at the about the same time. That being the case, it’s unlikely that the problem is caused by the SCT not fully closed, but nevertheless still possible. The SCT019’s can be hard to fully close (and reopen). Also, the connection piece can only go one way (one side is a tongue, the other a groove). At the risk of underestimating your capabilities, can you verify that they are being installed completely closed?

Next, I’d like to see what happens if you configure one of those SCT019’s on a one of the channels 1-7. If that gives the expected results, then again on one of the channels 8-10. If that gives the expected result, try 11 and 12.

Finally, can you try either the SCT013-000 or one of the other SCT013-0xx (previously voltage type) in the channels 13 and 14?

5

Hey Bob - thanks for your help.

I’ve deleted all the inputs and re-added them using the presets for SCT 019 (200amp ct) and 013 (100amp ct)

113.9 Volts
82 Watts (input 1 : SCT019-000) – Same result on input 7 and 8
316 Watts (input 14 : SCT013-000)
316 Watts (Kill-a-Watt)

I’ve verified SCT 019 which is fully closed (there’s no gaps on the side).
I’ve added a picture of my “test bench” which clamps on a wire that is connected from the power to the blow dryer, I have a switch to cut the power, and my KAW to measure against as well.

SCT019 SCT013 IotaWatt test
SCT019 SCT013 IotaWatt test480×640 110 KB

6

@heppell

OK, that rules out hardware problems in the IoTaWatt. The problem is that you have SCT019-200 CTs, which are voltage type CTs. They have an internal burden of 10 ohms, which combined with the 24 ohm burden in the IoTaWatt yields an effective burden of 7.06 ohms. That works with the hair dryer on high numbers in your initial post. Understand that with that small a burden, the CT will not do well with small loads.

To prove this out, you can change the burden specification in your IoTaWatt (in the configure device section). If you change those channels with the SCT019-200 to 7.06, a reasonable power should result, given a modest load of say 1000W or more.

I’d be curious to know if you got those CTs from the OEM shop, as that would require some follow up to avoid a repeat of this issue. If you manage to figure out how to remove the burden from the SCT019, I’d be interested. I have a few of those hanging around that are of little use to me right now.

Let me know how you make out.

7

That is disappointing about using SCT019-200 CTs, I removed the burden from the SCT030, but SCT019 seems trickier to do so. These are some I had here that I’ve used with emonTX in the past which I’m reusing for this project. They weren’t purchased at the OEM Shop.

Input 1 SCT013-000 (Burden Setting: 24)
Input 14 SCT019-000 (Burden Setting: 7.06)

Kill-a-Watt: 1173 W, Input 1: 1192 W, Input 14: 1149 W (98% Accuracy)
Kill-a-Watt: 338 W, Input 1: 335 W, Input 14: 306 W (91% Accuracy)
Kill-a-Watt: 43.5 W, Input 1: 43 W, Input 14: 0-2 W (2% Accuracy)
Kill-a-Watt: 13.8 W, Input 1: 10 W, Input 14: 0-1 W (4% Accuracy)

I’ve attached an image of both SCT019 and SCT013 showing my mains legs. The SCT013 wouldn’t fit anyway, but theoretically I’d still rather use the 200A CT. In order to calculate KWh using SCT019-000 it will potentially be inaccurate counting during lower load times - I’ll have to work on a solution for that one.

SCT019 vs SCT013 (no fit) Mains

8

I don’t recognize Kill-a-Watt as a standard of accuracy, but it’s encouraging that they agree somewhat.

The SCT019-000 (200A:33ma) works very well with IoTaWatt.

I’ve also recently tested the SCT023R which has a lower turns ratio, and pretty good phase-shift at 1.5. The hole is 23mm (vs 119mm on the SCT019). It’s rated to 300A but is good up to 150A as configured in IoTaWatt. They are also a little easier to install, being a hinged design.

I’m going to mark this solved.