3P breaker feeding two single phase resistive loads

[iamintn]

I need some help wrapping my head around something I saw recently. In a nutshell, there was a 3P breaker (208Y/120 3PH) feeding two separate heat trace circuits. Phase A and B was wired to the first heat trace, and the second heat trace was wired B to C. The question I have is, what is the current on B phase? Looking at these as two independent single phase loads, it looks like you would have the same magnitude entering and leaving B phase for a net result of 0? Is that correct? Are there any issues with this application?

Thanks!

 

[dbuckley]

Looking at these as two independent single phase loads, it looks like you would have the same magnitude entering and leaving B phase for a net result of 0? Is that correct?

Nope, 'cos the currents entering and leaving are 120 degrees apart.

Are there any issues with this application?

There's no problem at all, it'll work perfectly. But there is the minor issue of load imbalance, which may or not be an issue.

 

[broadgage]

If the two trace heating loads are the same, then all 3 phases will carry the same current.
Despite the fact that 2 heaters connected to the B phase, this will have the same current as the A and C phases.

 

[LarryFine]

Not quite. The two lines with single loads will each see that load, but the shared line's current will be 1.732 times that of one load.

So, if each load is 10a, A and C phase will each see 10a, but B phase will see 17.32a.

 

[iamintn]

Thanks for the answers.

Larry,

So the only caveat with this application would be to size the breaker based on the heat trace load X 1.732.

Thanks. Now that you say it, it looks so obvious.

Learning quite a bit by reading this forum!

 

[ActionDave]

This set up has me intrigued. I have never thought of doing such a thing. Why would one do this?
Has anybody on this forum done the same? Is it legal?

 

[benaround]

Using Larry's example, I don't understand why the loads are not A=10 , C=10 , and B=20 ?

The heat trace are rated 208v each, correct ?

 

[LarryFine]

Using Larry's example, I don't understand why the loads are not A=10 , C=10 , and B=20 ?

For the same reason the line-to-line voltage is not twice the line-to-neutral voltage: the phases are offset by 120 degrees, and not 180.

 

[benaround]

Larry,

Thanks for the reply, sorry to say I still don't get it. Let me ask you this, if I used two 2pole

breakers for the heat trace and located them A-B and B-C in the panel, would I still get the

same A=10a, C=10a, and B=17.32 ? This is where my line of thinking is going --------------

thx,

 

[chris kennedy]

Larry,

Thanks for the reply, sorry to say I still don't get it. Let me ask you this, if I used two 2pole

breakers for the heat trace and located them A-B and B-C in the panel, would I still get the

same A=10a, C=10a, and B=17.32 ? This is where my line of thinking is going --------------

thx,

Because your 120? apart. Wait until the phase angle guru's get a hold of this one.:roll: 900 posts?

 

[LarryFine]

Let me ask you this, if I used two 2pole breakers for the heat trace and located them A-B and B-C in the panel, would I still get the same A=10a, C=10a, and B=17.32 ?

For each two-wire circuit, each circuit conductor would see just the individual load's 10a.

Their combined contribution to the panel load, and its feeder, would be 10a on two lines, and 17.3a on the shared line.

 

[benaround]

Larry,

10-4, and thanks agian.

 

[hurk27]

Larry,

Thanks for the reply, sorry to say I still don't get it. Let me ask you this, if I used two 2pole

breakers for the heat trace and located them A-B and B-C in the panel, would I still get the

same A=10a, C=10a, and B=17.32 ? This is where my line of thinking is going --------------

thx,

Nope because you have now placed the load on seperate breakers, the "B" phase buss ahead of the breakers will see the added 17.32a but each breaker pole will now only have 10 amps on it.

Think of this like a multi-wire circuit in a single phase 120/240 service, you have two 10 amp loads, the multi-wire circuit splits just before it gets to the two loads, ahead of this split the neutral will have 0 amps, but after the split each neutral will have the full 10 amp load.

 

[hurk27]

Whoops didn't see page 2:roll:

 

[benaround]

Wayne,

Thanks for that additional info, sometime the brain can be very stubborn, seems to be the

case for me. I think your example has helped this out quite a bit. thx.

 

[ActionDave]

I need some help wrapping my head around something I saw recently. In a nutshell, there was a 3P breaker (208Y/120 3PH) feeding two separate heat trace circuits. Phase A and B was wired to the first heat trace, and the second heat trace was wired B to C. The question I have is, what is the current on B phase? Looking at these as two independent single phase loads, it looks like you would have the same magnitude entering and leaving B phase for a net result of 0? Is that correct? Are there any issues with this application?

Thanks!

Are there?

 

[LarryFine]

Are there?

Not really. While phase-to-phase load balancing is ideal, it rarely happens in the real world.

This is no different than having a one-phase line-to-line load, or any not-balanced 3ph load.

 

[iwire]

It may be worth thinking about the fact there is no such thing as a 'three phase heater' all three phase heaters are made up out of single phase elements.

For example a 5000 watt heater might be made up from five 1000 watt elements and there is no way to equally balance that across three phases.

 

[LarryFine]

For example a 5000 watt heater might be made up from five 1000 watt elements and there is no way to equally balance that across three phases.

It's interesting to see people try, though. You should see some of the weird way home theater enthusiasts have tried to combine odd (in more ways than one) combinations of tactile transducers to match amplifier impedances.

 

[ActionDave]

So here's the deal- I'm working at a hair salon/day spa recently, running power to a second electric water heater to work in tandem with the existing.

The service is a highleg so I have to move a some circuits around to get two spaces together and keep the 120V loads off the high leg.

Are you telling me that all I needed to do was swap out the breaker on the existing water heater with a three pole, land my hots for both water heaters accordingly and been done before lunch?