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Author Topic: How do you measure current on your home service / generator?  (Read 559 times)
N1AUP
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Posts: 246




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« on: July 06, 2019, 06:38:18 PM »

How would you measure AC current flowing from your generator into your house?

The genset is putting out 220 VAC on a four prong plug.  I assume that I have Hot1, Hot2, Neutral and ground.  220 reads between the two hots, while 110 between either hot and neutral.

I heard that you can't put a current transformer over the entire genset cord because the two sides of the 220 will cancel each other out.

Do you have to measure current on each leg of the 220, and add them?  Or if I have a 100 amp service on the house, does that mean that 50 amps runs on one 220 leg and 50 amps on the second one?

Looking at one of those Chinese digital power meters with the current transformer to monitor what's going on with the generator.

Thanks

C
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W9IQ
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Posts: 3217




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« Reply #1 on: July 07, 2019, 02:53:29 AM »

You are correct that you need to clamp the ammeter on individual hot leads to take the readings. Your 100 amp service means you can draw a theoretical maximum of 24 kW (240 volts x 100 amps) at any one time. This could consist of a 100 amp 240 volt load, a 200 amp 120 volt load split equally between the two legs or any other combination that totals less than 24 kW and draws less than 100 amps per leg.

Your total household load will consist of 240 volt and 120 volt loads. So you will need to measure the current in each hot leg and add them together to get the total amperage at 120 V. If you need to compute the power in watts, multiply this total amperage by 120 volts.

The above technique will correctly account for the 120 volt and the 240 volt loads. Don't make the mistake of measuring the current in the neutral wire as this will not properly account for most 240 volt loads. Although if you have no 240 volt loads, this technique will allow you to use a single clamp on ammeter. The safety ground wire should not carry any current under normal operating conditions.

For the sake of safety, do not use shunt type ammeters in this application. Stick with clamp on ammeters that do not require you to remove any wire insulation or make any electrical contact with your generator circuit.

- Glenn W9IQ
« Last Edit: July 07, 2019, 03:15:30 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N1AUP
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Posts: 246




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« Reply #2 on: July 07, 2019, 06:54:25 AM »

Thanks Glen.  Excellent explanation.

Do electricians plan where breakers go for various circuits to balance both sides of the load center?

I would assume that they wouldn't put a stove, a water heater and a hot tub on the same side of the LC.

C
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N2SR
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Posts: 1176




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« Reply #3 on: July 07, 2019, 09:54:06 AM »

Thanks Glen.  Excellent explanation.

Do electricians plan where breakers go for various circuits to balance both sides of the load center?

I would assume that they wouldn't put a stove, a water heater and a hot tub on the same side of the LC.

C


Yes, that is typically what is done.  And the higher current loads are placed closest to the main breaker of the panel.    After I added my 100A subpanel, and was adding circuits to it, the electrician in work told me that "wire is cheap," so when you add a circuit, use enough of the wire (hot, neutral, ground) so that you can move the breaker anywhere in the panel.   So, whenever I add a circuit, I just run the "extra" down to the bottom of the panel box, and then back up to the new breaker.   

BTW, in most cases it is legal use a wire nut inside the panel box to extend the wire if some of the old circuit wiring is not long enough.

Tom, N2SR
 
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If no one is doing it that way, there is a probably a very good reason.
W9IQ
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Posts: 3217




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« Reply #4 on: July 07, 2019, 11:03:34 AM »

As Tom stated, it is best to try to balance the load on the two legs but this isn't always practical. The total of your individual circuit breakers rating in each leg can far exceed the service rating as long as the actual total leg load does not. This can help with reducing nuisance trips at the expense of investing in more breakers and panel space.

Don't forget to use a proper cut-over circuit for your generator. Most other techniques put you and the lineman at risk of electrocution.  You could also blow up your genset.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KD0REQ
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Posts: 2364




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« Reply #5 on: July 09, 2019, 10:55:19 AM »

branch circuit breakers protect the wire from overheating, thus preventing the framing from overheating, thus preventing fires.

the main breaker protects the utility company wire from overheating, thus burning up the trees, thus preventing burning down the block.

breakers never protect loads. they protect wires. if you need to protect loads, you want a whole-house surge arrestor, which installs at the absolute closest bus location to the main breaker, and as short a wire as possible to the ground buss.
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KB6DYA
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Posts: 84




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« Reply #6 on: July 09, 2019, 04:27:11 PM »

You will need a isolation ( transfer ) switch that separates the generator from the utility service. if not you run  the risk of back-feeding the utility service. Or you have to make sure the the main breaker from the utility service is turned off BEFORE starting the generator.  BE SAFE and do not take chances!!
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KJ4RWH
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« Reply #7 on: Yesterday at 05:25:23 AM »

On my genny to measure both legs current draw I run the neutral lead through a current transformer and read the total draw directly.
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WA8NVW
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Posts: 18




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« Reply #8 on: Yesterday at 07:59:32 PM »

On my genny to measure both legs current draw I run the neutral lead through a current transformer and read the total draw directly.

Measuring current in the neutral wire gives you the DIFFERENCE between the two phases, not the sum.  As an example, you have two 100 watt light bulbs, each one connected between a phase (leg) and neutral.  Each lamp draws a current of 0.83 amperes [100 W/120 V].  The load on the generator is 200 watts and the current in the neutral lead is zero.  To measure the current you need to meter each phase separately, but you don't add the numbers together.  Neither leg is allowed to exceed the rated output current.
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