Amps/watts/kilowatts

[Greg1707]

I apologize for my ignorance when it comes to electrical calculation.

1,000 watt 240 volt baseboard heater. If I clamp my amp meter on to one leg of the circuit in the electric panel it should read 4.16 amps. For electrical usage the heater is consuming 1 kilowatt?

Let us suppose I clamp my amp meter on to one leg of the SEC and it reads 28 amps. If I clamp the meter on the other leg of the SEC and it reads 15 amps--how many kilowatts am I consuming?

 

[Carultch]

Question 1: yes. The heater consumes 1 kW of electricity and produces 1 kW of thermal energy. All heaters are 100% "efficient", because there is no other form where the energy could go, that doesn't ultimately end up as heat. The only losses, would be heat sent to an undesired destination instead of the space you want to heat.

Question 2:
For a 120/240V 3-wire system, the total power is calculated by:
current on black wire * black wire voltage to neutral
plus
current on red wire * red wire voltage to neutral

So with your data, that would be 28*120 + 15*120 = 5160 Watts, or 5.16 kW.

In your example, there will be 28A on the black wire, 15A on the red wire (or vice-versa), and the imbalance of 13A is carried back to the source on the neutral.

 

[Carultch]

I will add to my response on Question 2. This all assumes we have unity power factor, where voltage and current waveforms are perfectly synchronized. Resistive heating is an example of a load with unity power factor.

If voltage and current were not synchronized, we'd have a discrepancy between the apparent product of voltage and current (called apparent power in units of kVA), and the real power in units of kW. The kW would be less than the kVA.

 

[Little Bill]

Question 1: yes. The heater consumes 1 kW of electricity and produces 1 kW of thermal energy. All heaters are 100% "efficient", because there is no other form where the energy could go, that doesn't ultimately end up as heat. The only losses, would be heat sent to an undesired destination instead of the space you want to heat.

Question 2:
For a 120/240V 3-wire system, the total power is calculated by:
current on black wire * black wire voltage to neutral
plus
current on red wire * red wire voltage to neutral

So with your data, that would be 28*120 + 15*120 = 5160 Watts, or 5.16 kW.

In your example, there will be 28A on the black wire, 15A on the red wire (or vice-versa), and the imbalance of 13A is carried back to the source on the neutral.

How is the imbalance carried on the neutral if the load doesn't use a neutral?

 

[Carultch]

How is the imbalance carried on the neutral if the load doesn't use a neutral?

Then there can be no imbalance for a single phase load that doesn't use a neutral. Current has to add up vectorially to zero among all conductors meant for carrying current. If there is an imbalance, and no neutral brought to the load, then you have a ground fault on the circuit for that particular load.

I'm assuming that the OP is talking about the service conductors in the aggregate from the center-tapped service transformer, in the example with different current values on the two line conductors.

 

[jaggedben]

How is the imbalance carried on the neutral if the load doesn't use a neutral?

Seems like the OP starts off talking about a 240V baseboard heater as an example but then switches to talking about service entrance conductors (SEC) instead? I agree that one shouldn't be getting different line readings on a 240V appliance.

 

[Little Bill]

Then there can be no imbalance for a single phase load that doesn't use a neutral. Current has to add up vectorially to zero among all conductors meant for carrying current. If there is an imbalance, and no neutral brought to the load, then you have a ground fault on the circuit for that particular load.

I'm assuming that the OP is talking about the service conductors in the aggregate from the center-tapped service transformer, in the example with different current values on the two line conductors.

I was just going by the 240V load of the heater, which doesn't use a neutral.

 

[Greg1707]

Seems like the OP starts off talking about a 240V baseboard heater as an example but then switches to talking about service entrance conductors (SEC) instead? I agree that one shouldn't be getting different line readings on a 240V appliance.

OP Here. My question was about determing cost of operation by taking readings and then converting to WATTS. So, I understood the uage per hour for the baseboard heater but was not sure how to calculate usage for the entire service. After metering the baseboard heater I metered the two legs of the SEC to determine the load for the entire house.

 

[ggunn]

OP Here. My question was about determing cost of operation by taking readings and then converting to WATTS. So, I understood the uage per hour for the baseboard heater but was not sure how to calculate usage for the entire service. After metering the baseboard heater I metered the two legs of the SEC to determine the load for the entire house.

Is it load you are trying to determine or is it usage? Load on a service in Watts (W) is constantly changing and isn't very useful information except if you are concerned with peak load, i.e., the most power that will be drawn from the service in any particular instant in time. If it's the electric bill you are concerned with, it's usage in kiloWatt hours (kWh). An analogy is speed vs. distance; Watts is like speed - how fast you are driving in mph at any point in time, which can be constantly changing. kWh is like distance - how far you have traveled in an hour.