[Tee Jay]

Hello,

In attached picture, if I want to measure total amp on this 240V, do I place my amp meter clamp on:
a. Wire A
b. Wire B
c. Both, and the larger value is the total amp
d. Both, and add the two amp values up for total current

Thank you.

 

[seharper]

It's complicated and depends why you are measuring it.
For metering purposes you add and x120V.
For sizing on static loads, you take the larger but that wouldn't work on a whole house due to the diversity of loads.
For provisioning, you disregard the reading and follow NEC Article 220 for the calculated load based on your square footage, circuits and built-in appliances.

 

[dcwired]

Answer: D but this is assuming all 120 volt loads. 240. Volt loads read amperage on both sides, but that load is not added up such as a 20 amp 240 volt circuit is still only 20 amps, not 40.

 

[SW Dweller]

You have to measure one phase at a time.
The phase reading the highest number is the load on the panel.
Do not have any other wire in the current transformer, just the phase wire.

This measurement is close enough for most residential applications.
Remember that the measurement you take is the load at that moment. Possibly not the highest load.

Or you can contact the POCO and get the highest KWH from the last year by month.
A little math and you can get amps.

 

[chandler48]

Our POCO website allows us to see current usage over a time period. It is up to date within 15 hours. See if yours has such a feature. It would be interesting to know why you are wanting to measure current like that.

 

[dcwired]

Smart meters have a reading on them giving max. KW used for the month.

 

[Tee Jay]

Thanks all. My intent is to be able to measure wattage during "essentials only" usage minus the high power devices like AC, range and dryer. I have a 6500W generator so I want to figure out if that's enough.

Here's my transfer switch set up. So if want to measure the wattage during generator use with a clamp-on amp meter, is this correct formula?

Total watts = (current on red wire + current on black wire) x 120V

 

[chandler48]

That may work with generator running, all paddles on generator and loads placed on each circuit.

I have one concern. I see shadows on the transfer switch. Is it outside? Your 10 gauge wire needs to be in liquidtite from the inlet to the transfer switch, and the wires leading to the panel should be in a conduit as well.

 

[Tee Jay]

Good observation; it's actually inside in my basement. Here's the final set up. That 10 gauge wire goes out to my garage where it is in a conduit before going to receptacle on my garage outside wall.

 

[SW Dweller]

Where is the required interlock for the main breaker.
how did you feed the power in from the plug you put outside.
The cover you have outside is only weather proof when it is closed.

 

[chandler48]

With that transfer switch, you don't need an interlock.

@Tee Jay just keeping you straight !!

 

[viper]

Thanks all. My intent is to be able to measure wattage during "essentials only" usage minus the high power devices like AC, range and dryer. I have a 6500W generator so I want to figure out if that's enough.

Here's my transfer switch set up. So if want to measure the wattage during generator use with a clamp-on amp meter, is this correct formula?

Total watts = (current on red wire + current on black wire) x 120V

Sorry, with AC loads, you cannot simply multiply amps and volts to attain the watts, without considering power factor. My recommendation is to see what your power company has available, reading through their meter, and use that info. Otherwise you are just guessing. Their meter is going to be FAR more accurate.

Locally our POCO will provide snapshots down to 15min so simply run a 15min test load and retrieve that data online. Just as an example here.

 

[Tee Jay]

Where is the required interlock for the main breaker.
how did you feed the power in from the plug you put outside.
The cover you have outside is only weather proof when it is closed.

Like @chandler48 said, no interlock needed for transfer switch. Each circuit on it has three-way switch that selects where power comes from, main-off-generator, so there's no chance for backfeeding the generator power to the main.

As for the receptacle, it's rated for outdoor use, in fact, it's illegal to install indoor.

 

[zanydroid]

Thanks all. My intent is to be able to measure wattage during "essentials only" usage minus the high power devices like AC, range and dryer. I have a 6500W generator so I want to figure out if that's enough.

Here's my transfer switch set up. So if want to measure the wattage during generator use with a clamp-on amp meter, is this correct formula?

You should definitely be able to power all that with 6500W, even with startup surge, on 120V/20A loads. I'd be more worried about AC waveform quality. Personally I think your time is better spent doing empirical testing with the generator and specific appliances you want to run, to see if there are unhappy interactions.

I don't think power factor is that important. Yes, Power Factor can be pretty terrible on 120V appliances since very few (if any) of those have required power factor correction. But there is plenty of headroom on this generator.

For 240V appliances, the 6500W may have some trouble with large motors. I was under the impression that they tend to have power factor correction too.

 

[viper]

You should definitely be able to power all that with 6500W, even with startup surge, on 120V/20A loads. I'd be more worried about AC waveform quality. Personally I think your time is better spent doing empirical testing with the generator and specific appliances you want to run, to see if there are unhappy interactions.

I don't think power factor is that important. Yes, Power Factor can be pretty terrible on 120V appliances since very few (if any) of those have required power factor correction. But there is plenty of headroom on this generator.

For 240V appliances, the 6500W may have some trouble with large motors. I was under the impression that they tend to have power factor correction too.

PF will be all over the map, depending on what's running. For instance, a water well pump can hover at .6-.7PF, while an HVAC compressor can run close to unity due to the run cap.

But you are correct about doing some testing. Really, I think he could nearly ignore the "'watts" and focus on amps. At the end of the day, that generator can only supply so many amps.

But I share my story commonly that I have a 5000w generator and a friend has a 6500w. They turned out to be the EXACT SAME generator!!!! Same engine, same generator head. Talk about a scam!!! those bargain barn screamers 100% will NOT deliver rated watts without severe degradation of voltage and frequency......but people will continue to buy them.

 

[seharper]

Thanks all. My intent is to be able to measure wattage during "essentials only" usage minus the high power devices like AC, range and dryer. I have a 6500W generator so I want to figure out if that's enough.

Here's my transfer switch set up. So if want to measure the wattage during generator use with a clamp-on amp meter, is this correct formula?

In that case, the larger of the two numbers x 240V.

Alternately, divide by 240 to get the generator amps.

Imagine your loads were 5A on one wire and 30A on the other wire, that would average to 4200W and you'd go "5000W generator (21A) is OK then!"

Don't forget the enormous fudge factor needed for motor startup.

 

[zanydroid]

PF will be all over the map, depending on what's running. For instance, a water well pump can hover at .6-.7PF, while an HVAC compressor can run close to unity due to the run cap.

But you are correct about doing some testing. Really, I think he could nearly ignore the "'watts" and focus on amps. At the end of the day, that generator can only supply so many amps.

But I share my story commonly that I have a 5000w generator and a friend has a 6500w. They turned out to be the EXACT SAME generator!!!! Same engine, same generator head. Talk about a scam!!! those bargain barn screamers 100% will NOT deliver rated watts without severe degradation of voltage and frequency......but people will continue to buy them.

I don't 100% understand it yet, but I think apparent and active power will have different impact on the generator (and the impact will vary based on whether it's got an inverter stage or directly coupled to the coils). Inverters are sometimes rated for both VA and for W

That's why you test it . You need to know what kind of crazy AC waveform will come out, under light load and the heaviest startup load you have. There are different failure modes for different kinds of generators.

One thing I'm working on next is using generators to charge up home backup battery, since we've had tons of power outages this year in Northern California. In that case you have a lot of interesting dimensions to play with, engineering wise. For instance you can try to size your system to charge the battery at the optimal point of the generator; this also has the advantage that you don't have to babysit/protect from weather a portable generator for as long as if you were directly powering the house. If the generator has really yucky AC quality or does funky stuff under load, you can try more lenient AC chargers to DC couple it into the system. Better battery inverters can team up with a PSW generator that creates a clean AC, albeit with some deficiency in total power or surge power, to start up more difficult appliances. They do this by locking onto the generator's AC and running in parallel with it, adjusting the inverter parameters to insure that they push out their fair share of power to the load.

 

[Tee Jay]

My POCO has my usage summary like this from the website. Quite convenient for me.

 

[chandler48]

There ya go !!

 

[seharper]

Except those are hourly usage averages. Interesting for billing, but useless for provisioning.

According to this data, a 4KW generator will run all your loads for the whole house. Nope.