Help me understand electricity - watts amps etc

[bdog]

I know volts time amps equals watts. I am trying to size a generator and am getting confused on the watts for 240 things vs 120.

So if I have a 10 amp load on 120 that is 1200 watts correct? If I have 10 amps on 240 that is 2400 watts?

Is there an easy way to measure the actual amps/wattage used? My trailer which I want to power has a mix of 120 and 240 things in it. The high draw items are 240. Plugged in to the 240v outlet in my shop it will occasionally blow the 40 amp breaker if everything is on. Not always but has done so 2-3 times other times it will run fine all day with everything on. So does that mean I need 9,600 watts or say a 10k rated to be safe?

 

[CobyRupert]

I know volts time amps equals watts. I am trying to size a generator and am getting confused on the watts for 240 things vs 120.

So if I have a 10 amp load on 120 that is 1200 watts correct? If I have 10 amps on 240 that is 2400 watts?

Is there an easy way to measure the actual amps/wattage used? My trailer which I want to power has a mix of 120 and 240 things in it. The high draw items are 240. Plugged in to the 240v outlet in my shop it will occasionally blow the 40 amp breaker if everything is on. Not always but has done so 2-3 times other times it will run fine all day with everything on. So does that mean I need 9,600 watts or say a 10k rated to be safe?

So if I have a 10 amp load on 120 that is 1200 watts correct? -Yes (Actually it's 10 "volt-amps" (VA), but let's ignore the difference in the 2 terms for now).
If I have 10 amps on 240 that is 2400 watts? -Yes
....so, on a "3 wire connection" (+Ground = 4 wires actually) (Let me explain: 3 wires: 2 hots and 1 neutral, where there's 240v between the 2 hots, and 120V between each hot and the neutral)(known as a 120/240V).

If you have both of your above mentioned loads turned on together this would mean you have 20 amps running down 1 hot wire (10 amps from your 240V load + 10 amps from your 120V load) and 10 amps down the other hot wire (the 10 amps of your 240V load) (& 10 amps returning on the neutral wire from your 120V load).

Not an easy way to measure amps without an amp meter (ammeter). Other than to calculate it if you know the load's wattage
The other issue with generators is if you're trying to start motors and the genny's ability to handle the motor's starting surge (which may depend on what else is already running). Your shop's breaker also has to deal with this same issue (to a lesser degree) too.

Generally speaking, the Code says don't load breakers more than 80% of there amp rating.

 

[CalG]

So if I have a 10 amp load on 120 that is 1200 watts correct? -Yes (Actually it's 10 "volt-amps" (VA), but let's ignore the difference in the 2 terms for now).
If I have 10 amps on 240 that is 2400 watts? -Yes
....so, on a "3 wire connection" (+Ground = 4 wires actually) (Let me explain: 3 wires: 2 hots and 1 neutral, where there's 240v between the 2 hots, and 120V between each hot and the neutral)(known as a 120/240V).

If you have both of your above mentioned loads turned on together this would mean you have 20 amps running down 1 hot wire (10 amps from your 240V load + 10 amps from your 120V load) and 10 amps down the other hot wire (the 10 amps of your 240V load) (& 10 amps returning on the neutral wire from your 120V load).

Not an easy way to measure amps without an amp meter (ammeter). Other than to calculate it if you know the load's wattage
The other issue with generators is if you're trying to start motors and the genny's ability to handle the motor's starting surge (which may depend on what else is already running). Your shop's breaker also has to deal with this same issue (to a lesser degree) too.

Generally speaking, the Code says don't load breakers more than 80% of there amp rating.

You have the math right!

Now get an "Amp Clamp" and know your loads for SURE!.

I never knew how useful an amp clamp would be until I got one. Now I use it frequently!

A reference to "something", no comments about durability. I use an Amprobe.

 

[dickfoster]

Here's a great meter that doesn't cost much. It has a clamp on that will measure both AC and DC current without the need to break the circuit. It's really handy for both home and auto use. Of course it has the usual volts and ohms along with temperature etc.

Sears.com

AC clamp on amp meters are common but DC clamp ons are not so common but are very handy. This one has nice ranges for home and automotive use.

 

[MossRoad]

I know volts time amps equals watts. I am trying to size a generator and am getting confused on the watts for 240 things vs 120.

So if I have a 10 amp load on 120 that is 1200 watts correct? If I have 10 amps on 240 that is 2400 watts?

Is there an easy way to measure the actual amps/wattage used? My trailer which I want to power has a mix of 120 and 240 things in it. The high draw items are 240. Plugged in to the 240v outlet in my shop it will occasionally blow the 40 amp breaker if everything is on. Not always but has done so 2-3 times other times it will run fine all day with everything on. So does that mean I need 9,600 watts or say a 10k rated to be safe?

The way I understand it is 10 amps at 120 is 5 amps at 240.

If you're tripping a 40amp 240V breaker, that's sort of the equivalent of 80amps of 120.

But let's just call it 40 amps of 240v....

What are you running in the trailer that pulls that many amps?

 

[CalG]

What are you running in the trailer that pulls that many amps?

Must be a dead short, or one H#LL of a lot of heat ;-)

 

[bdog]

It has two roof top AC units and battery chargers for some oilfield equipment. The chargers are these big racks and will charge 96 batteries at once from completely dead to charged in three hours. The chargers run on 240v.

 

[MossRoad]

It has two roof top AC units and battery chargers for some oilfield equipment. The chargers are these big racks and will charge 96 batteries at once from completely dead to charged in three hours. The chargers run on 240v.

YIKES!!! I was thinking camper.... :laughing:

 

[vsteel]

I would look at 12K on up. Really I would suggest 14K to be safe. 12K would put you at 50 amps and if you figure an 80% load, that would put you right at the 40 amps you are pulling. Though I would suggest you even go bigger than that to just make sure that you don't fry the generator. Your 40A breaker might be popping low or high so you don't know exactly where you are at current wise and you won't want to run a generator very long above 80% output.

 

[bdog]

Thanks guys. I ordered a clamp on meter to see what it is using. I know a 10k will power it because that is what I am using now but I am looking to replace it as it is a 3600rpm unit and is loud. I want something 1800 rpm and quieter. I was eyeing an 8kw unit and was wondering if it would be enough.