Single phase vs 3 phase efficiency

[joshfromoshoto]

I had a discussion with a co worker today, say there's a 1000w heater, hooked up to single phase 480v, amp draw would be roughly 2 amps. Now would the same exact heater, hooked to 480v 3 phase have a lesser amp draw? I've heard people say that 3 phase power is 40% more efficient but I don't know how calculate it other than 1000w÷480v.

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[LarryFine]

Now would the same exact heater, hooked to 480v 3 phase have a lesser amp draw?

Yes, because a given power is being supplied by a greater number of conductors.

I've heard people say that 3 phase power is 40% more efficient but I don't know how calculate it other than 1000w÷480v.

For 1ph, yes; for 3ph, it's 1000 / 480 / 1.732.

1000 / 480 = 2a (ish)

1000 / 480 / 1.732 = 1.2a (ish)

But, more efficient? Depends on how you define it.

They're both still 1kw of load.

 

[jaggedben]

By themselves, neither watts, amps, or volts at the point of consumption will ever really tell you anything about efficiency.

 

[tortuga]

I think all electric resistance heating basically 100% efficient, no more no less.

I had a job a few years ago swapping out 1000W electric resistance baseboard heaters becasue the customer thought they bought more a 'efficient' model that mounted on the wall. The only thing I could think of was more efficient was the fancy efficient ones had more thermal mass, so they stay warm longer, but the wall mount model was about 1/2 the wattage. I think it would take the same kwh to heat the room.

I had a discussion with a co worker today, say there's a 1000w heater, hooked up to single phase 480v, amp draw would be roughly 2 amps. Now would the same exact heater, hooked to 480v 3 phase have a lesser amp draw? I've heard people say that 3 phase power is 40% more efficient but I don't know how calculate it other than 1000w÷480v.

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One way to think about it is ask yourself how many ohms of heating element you'd need for each configuration?
Check my math but its something like
To get 1000W out of 480 3 phase you'd need three 691 Ohm elements connected in a delta configuration (333W each)
To get 1000W out of 480 single phase 2 wire you'd have a 230 Ohm element (1000W).
Or something like that

 

[electrofelon]

say there's a 1000w heater, hooked up to single phase 480v, amp draw would be roughly 2 amps. Now would the same exact heater, hooked to 480v 3 phase have a lesser amp draw?

It would be exactly the same, as there would be no terminal for the the 3rd phase to land on.

 

[LarryFine]

It would be exactly the same, as there would be no terminal for the the 3rd phase to land on.

We obviously took the question to mean the load would be re-distributed equally among the three lines.

 

[electrofelon]

We obviously took the question to mean the load would be re-distributed equally among the three lines.

he said "exact same heater".

 

[jaggedben]

Yeah...

say there's a 1000w heater, hooked up to single phase 480v, amp draw would be roughly 2 amps. Now would the same exact heater, hooked to 480v 3 phase ...

Stop right there, because that's not making sense.

 

[Hv&Lv]

I had a discussion with a co worker today, say there's a 1000w heater, hooked up to single phase 480v, amp draw would be roughly 2 amps. Now would the same exact heater, hooked to 480v 3 phase have a lesser amp draw? I've heard people say that 3 phase power is 40% more efficient but I don't know how calculate it other than 1000w÷480v.

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I’m not sure where the 40% came from unless someone is using the 43% less current for a three phase vs single phase motor. For the heater it doesn’t work like that.

To calculate the efficiency difference between a three-phase and a single-phase system for a 1000 watt load, you have to consider factors like power transmission losses, equipment efficiency, and system characteristics.
Without dissecting the electrical system with wire sizes, lengths, etc let’s simplify it a little

For single-phase system let's assume a typical efficiency of around 90% for a single-phase system.
For a three phree-phase system, transmission losses are lower, there is smoother power delivery, so let's assume a slightly higher efficiency, say 92%, for a three-phase system.

So.
For single-phase system power loss= 1000 watts * (1 - 0.90) = 100 watts
For three-phase system power loss, 1000 watts * (1 - 0.92) = 80 watts

The difference in power loss between the two is only 20 watts.

So for this simplified 1000 watt load, the 3 phase system might be approximately 20 watts more efficient than the single-phase system.

For the percentage efficiency we can calculate the efficiency improvement
(100 watts - 80 watts) / 100 watts = 0.2
0.2 * 100= 20% efficiency

In your 480 single phase heater it is connected L-L, and as noted in other posts, the amps are 2.08A.
This 480 heater in a three phase system is connected in delta and the amps are again, as noted in other posts, 1.20

See the single phase relationship to the three phase system with the calculations in other posts? There is the 1.732 relationship.

2.08A/1.732 = 1.20A

Clear as a mud puddle now?

 

[LarryFine]

he said "exact same heater".

Yes, he did, and I have pointed that out myself in the past.

 

[garbo]

One watt produces 3.412 BTU 'S of heat per hour no matter what the voltage is or feed by single or three phase. If you had a large heater it would be cheaper to install three conductors and using three phase then two larger conductors and only using single phase . I would be more concerned on construction of wall mounted baseboard r!ectrical heaters and wall hung larger wattage unit heaters with forced air.

 

[winnie]

I had a job a few years ago swapping out 1000W electric resistance baseboard heaters becasue the customer thought they bought more a 'efficient' model that mounted on the wall. The only thing I could think of was more efficient was the fancy efficient ones had more thermal mass, so they stay warm longer, but the wall mount model was about 1/2 the wattage. I think it would take the same kwh to heat the room.

If you are using any form of resistance heating, then to put the same number of BTU into a room you need to consume the same kWh. Doesn't matter if it is a baseboard, a big wall mounted thing, or a small parabolic IR heater. There is no difference in efficiency.

Where you _might_ see a difference is in 'efficacy', in terms of producing the desired result of room occupants feeling comfortably warm. It might be the case that a different type of heater needs to put fewer BTU into the room for the occupant to feel comfortable. The big radiant panel, for example, might make people feel comfortable while leaving the air cooler.

-Jonathan

 

[kwired]

Less amps means nothing when it comes to efficiency aside from resistive losses in conductors, but that is kind of negligible for short runs of conductors sized per their rated ampacity.

If what you are targeting as "efficiency" is operating cost then watt-hours is the main thing you are concerned about and not just amps. But 1000 watts is 1000 watts whether it is at 24 volts, 120 volts, 480 volts, or single phase vs three phase. The amount of current drawn is going to be different for each one of those voltages and will also be different for single phase vs three phase.

 

[NoahsArc]

Just a point of clarification, when we are comparing a 3ph vs 1ph 480V connection, are we comparing it as 2-legs vs 2-legs or 3-legs vs 2-legs.
I am not following that a 2-leg vs 2-leg 480V comparison is any different.

 

[electrofelon]

In practice you will find a three phase load of the same KW will require less conductor. This is because ampacity is not linear with conductor size. A three phase load will have smaller conductors, and even though there are 50% more conductors, the total area will still be less. For example consider a 48 KW load. At 240 single phase this would be 200 amps or 3/0 CU, for a total circular mail area of 335600. For three phase that would be 115 amps , #2 CU, for a total circular mail area of 199080. Of course one could theoretically parallel smaller conductors for the single phase case, but there is of course the NEC 1/0 limit for paralleling and even above that in many cases it would not be done.

 

[Jpflex]

Yes, because a given power is being supplied by a greater number of conductors.


For 1ph, yes; for 3ph, it's 1000 / 480 / 1.732.

1000 / 480 = 2a (ish)

1000 / 480 / 1.732 = 1.2a (ish)

But, more efficient? Depends on how you define it.

They're both still 1kw of load.

Wouldnt the single phase heater be hooked up to 2 legs or line wires out of the 3 legs from the 3 phase system and therefore still have those same number of wires? Stoves placed on a 3 phase service are done this way and equally distributed.

 

[electrofelon]

Wouldnt the single phase heater be hooked up to 2 legs or line wires out of the 3 legs from the 3 phase system and therefore still have those same number of wires? Stoves placed on a 3 phase service are done this way and equally distributed.

We would need some more details about what exactly is meant by hooking the "exact same heater to three phase". A heater with 3 (or multiples of three) elements could be rewired into a delta or y configuration for three phase operation, presumably that is what the OP meant.

 

[Jpflex]

We would need some more details about what exactly is meant by hooking the "exact same heater to three phase". A heater with 3 (or multiples of three) elements could be rewired into a delta or y configuration for three phase operation, presumably that is what the OP meant.

Ok I see what you mean. There can be single phase heaters wired from 2 legs of a three phase source or a three phase heating element wired in either delta or wye from all 3 phases?

 

[Hv&Lv]

Ok I see what you mean. There can be single phase heaters wired from 2 legs of a three phase source or a three phase heating element wired in either delta or wye from all 3 phases?

Think of this as three single phase heaters connected together.

 

[Jpflex]

Think of this as three single phase heaters connected together.

That diagram could also be a 1 phase heater connected to 2/3 phase legs or connected as “single phase.”

Three single phase heaters connected to a 3 phase source I think would resemble a delta-delta diagram.

Yes three single phase heaters connected connected together as you shown could form a delta