Measuring 240V split phase devices using only one CT?

I am interested in opinions on using just one CT for devices which have 2 wires to the breaker (other than neutral).

I have a number of devices which use 240V for power, where both sides of the 240V line go through a breaker. The cost of using a CT for each is pretty high, since the emonTx V3 only handles 4 CT's, so this would increase the number of them I would need, at about $US 91 each.

[EDIT: This was answered below and in a BB. Typically, only one CT would be needed.]

This is on a US residence, where each 240V side is ~120V to neutral. This means that a device can use different amounts of power on each half. This means that ideally I would measure current through both legs, I think. But... wouldn't the imbalance be pretty small? For example, an air conditioner uses 4kW @ 240V for its compressor, but only a few watts for the 24VAC control transformer, which might (?) be a 120 to 24V connected across one leg.

Thoughts?

Bill Thomson's picture

Re: Measuring 240V split phase devices using only one CT?

Hello John,

There's an article in the Building Blocks section that deals with what your asking about.
Here's the link: http://openenergymonitor.org/emon/buildingblocks/EmonTx-in-North-America

I'm curious, where are you sourcing CTs at 91 bucks (ouch!) a pop? (I'm in the US too)

Regards

Bill

meso's picture

Re: Measuring 240V split phase devices using only one CT?

Bill,

   Thanks for your reply.

The building blocks article does not seem to address the specific case I mentioned. I addresses how to measure whole house power on 240V. It also shows measuring 240V appliance power, but I don't know if it is correct. I hope  it is, because what they show (one CT) is how I want to do things.

 I didn't see it address devices which may use 1/2 of the 240V internally, thus creating a current imbalance. Maybe that's because no devices do that, but I suspect that some do. But... I really don't know. That's why I was asking.

The 91 bucks is for the emonTx fully configured. That works out to about $23/CT plus the cost of the CT itself, which is $12.50 from this site, for a total of about $35.50 per circuit (not counting base unit and displays).

I probably should have been more clear on my question - referencing the BB.

Regards,

John

Bill Thomson's picture

Re: Measuring 240V split phase devices using only one CT?

Hi John,

From the BB article:

If the circuit is “mixed” 120 V and 240 V, i.e. the main load is 240 V but there is a neutral connection to supply for example a timer, an indicator lamp or similar low power device, then it might be acceptable to ignore the 120 V load and treat the load as a “pure” 240 V load and the arrangement of Fig 4c can be used. There will however, be a small error.

That's equivalent to the air conditioner/control transformer combo you mentioned earlier.

But to answer your question, yes, only one CT is required to measure a 240 Volt load. Not to worry, the BB article is correct. The author is an EE.

 

meso's picture

Re: Measuring 240V split phase devices using only one CT?

Bill,

   Thanks... I missed that little paragraph. I looked at the pictures but not all the words (hmmm... I sound like a kid ;-). That exactly addresses the issue.

Regards,

John

Bill Thomson's picture

Re: Measuring 240V split phase devices using only one CT?

Hey, I've been an Electronics Tech since 1974, and I learn something new almost every time I visit the site!

You'll find some really sharp folks here. I know I have.

Have a good 1!

 

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