Amp draw values

[Renzo]

I stumbled on this question while filling out a form at work regarding KW values on different appliances and equipment. On a 120 volt appliance it's easy enough to find watts without any name plate data using a an amprobe on the one leg. But lacking a data plate on a 240 volt or 3 phase piece of equipment how does it add up to find watts (E X I)?
If I get readings on a 240 v unit of 10 amps on each leg would a data plate show roughly 10 amps RLA or 20 RLA? And same with the 3 phase. Would it be 10 total amps or 30 amps. For the sake of keeping it simple for me forget power factor, etc for now.
Please be gentle with me, I'm still learning

[suemarkp]

In your examples, if you measure 10A on each leg, then the nameplate FLA/RLA value will be 10 regardless if single phase or 3 phase. Assuming the amp values you measure are representative of its load, you calculate watts as follows:

Single phase watts = volts * amps
Three phase watts = volts * amps * 1.73

The amps in this case is measured on any one ungrounded (hot) wire. On residential systems, the single phase voltage values are either 120 or 240.

On three phase systems, it depends whether the load is single phase or three phase (use the equation that matches the appliance). The single phase voltage values are usually 120, 208, 277, or 480. Or, 240 if delta. The voltages can go higher, but these are the common ones.

What begins to confuse many people is if you're trying to size loads for a 3 phase system. A 208V 10A single phase appliance will be rated at 2080 watts. Three of them would have to be supplied with 6240 watts. The weird thing in this case is each phase conductor feeding the 3 phase panelboard will have 17 amps in it if each appliance is fed from one phase, even though each load draws 10. If you put each 10A load on the same phase, you'd have 30A in two phase conductors and 0 in the third. So it is important that you measure amps in the proper place, as you'll get 17 or 10 depending on how far back you measure (in the balanced load case).

So if you're trying to size 3 phase equipment, you can come up short if things aren't balanced well, and you can really come up short if you just add up amps and think you can spread them around the phases.

[Homer]

I stumbled on this question while filling out a form at work regarding KW values on different appliances and equipment. On a 120 volt appliance it's easy enough to find watts without any name plate data using a an amprobe on the one leg. But lacking a data plate on a 240 volt or 3 phase piece of equipment how does it add up to find watts (E X I)?
If I get readings on a 240 v unit of 10 amps on each leg would a data plate show roughly 10 amps RLA or 20 RLA? And same with the 3 phase. Would it be 10 total amps or 30 amps. For the sake of keeping it simple for me forget power factor, etc for now. Please be gentle with me, I'm still learning
To start with, in a 240V/120V, single phase system there are two HOT conductors and a Neutral conductor as you are probably already aware. If an appliance only uses 240V, then the Neutral is not needed and can be omitted.
If an appliance is rated at 240V and 10A on its nameplate, it would have an apparent power of 2400VA (2400W at unity power factor). The current is still only 10A at 240V. This is equivalent power wise to 20A at 120V which is where I think the confusion arises.

I think you need to think about a residential load center for a moment to get a feel for the concepts. Consider a 100A service load center protected by a double pole 100A main breaker.

A 100A service can supply 100A on each hot leg. This gives you 100A @240V or 100A @120V for each leg (total of 200A @120V if perfectly balanced). You can run any combination of 120V and 240V loads and as long as each hot leg is below 100A the main breaker won't trip. This provides a maximum power of 100A x 240V = 24,000W.

Compare the following three examples:

1) 100A @240V + 0A @120V on leg A (black) + 0A @120V on leg B (red)
Neutral current = 0A with no 120V loads.

2) 100A @120V on leg A (black) + 100A @120V on leg B (red) + 0A @240V
Neutral current = 0A since legs are balanced.

3) 50A @120V on leg A (black) + 50A @120V on leg B (red) + 50A @240V
Neutral current = 0A since legs are balanced.

All three examples 1) , 2), & 3) would use = 24,000 Watts and be using the maximum capacity of the service at 100A in each HOT conductor.

in 1) 100A x 240V = 24,000W
in 2) (100A x 120V) + (100A x 120V) = 24,000W
in 3) (50A x 120V) + (50A x 120V) + (50A x 240V) = 24,000W

For three phase power don't forget to multiply by the square root of 3 which is 1.73 as Mark pointed out in his reply.

I hope that these examples cleared up the concepts for you.

Homer