Are Panel Load Calculations done in kVA or kW?

[winnie]

An extremely common practice is to use the 'add the kVA' to size the loads on individual busses in a 3 phase panel.

So for any 3 phase load you take the total load kVA, divide by 3, and assign that value to each bus.

For a single phase L-L load, you take the load kVA, divide by 2, and assign that value to each connected bus.

For a single phase L-N load, you take the load kVA and assign it to the connected 'hot' bus.

Question are there any corner cases where this would result in an underestimate of the individual phase load?

 

[wwhitney]

Question are there any corner cases where this would result in an underestimate of the individual phase load?

Yes, if I understand your procedure correctly. It assigns a VA to each bus, but you don't specify how to turn that VA into a current rating for the bus. I'm going to assume you intend to divide the VA by the bus's L-N voltage (for the 4-wire wye case). To concretize all this, let's just consider a 208Y/120V system; other systems just different by a scale factor.

For one single phase L-N load, your procedure seems obviously correct--assign the VA to one bus, divide the VA by 120V, and you get the current on that bus. Likewise for one balanced 3 phase L-L-L load, if the current is I, the VA is 3 * 120V * I. So to compute I from VA, you divide by 3 and further by 120V. That matches your procedure of assigning 1/3 the VA to each bus.

But for one single phase L-L load, if the current is I, the VA is 208 * I. You are going to assign 104V * I VA to each bus. But you should be assigning 120V * I to each bus, so that when you divide by 120V at the end, you get I as desired. In other words, you need to assign 1/sqrt(3) of the VA to each bus, not 1/2 the VA to each bus.

[Note that for a balanced set of 3 L-L loads, the vector addition of currents will give you cancellation so that you can just assign 1/2 the VA to each bus for each load. In other words, you can split your L-L loads into balanced sets plus the unbalanced remainder(s); then use the 3-phase rule for the balanced sets, and the 1/sqrt(3) rule for the unbalance remainders.]

With that correction, the procedure will not underestimate the total phase current (at least for linear loads). It accurately gives you the magnitude of the phase current for each load, and the total phase current will be the vector sum of those individual load phase currents. Then the magnitude of the sum of the vectors is always no more than the sum of the magnitudes of the vectors.

Cheers, Wayne

 

[jaggedben]

An extremely common practice is to use the 'add the kVA' to size the loads on individual busses in a 3 phase panel.

So for any 3 phase load you take the total load kVA, divide by 3, and assign that value to each bus.

For a single phase L-L load, you take the load kVA, divide by 2, and assign that value to each connected bus.

For a single phase L-N load, you take the load kVA and assign it to the connected 'hot' bus.

Question are there any corner cases where this would result in an underestimate of the individual phase load?

Your description of the method seems incomplete. Am I to assume this applies to wyes, and that a 1.73 factor is applied to 3-phase loads, and that the L-N voltage is used at the end to get amps?

If so, then at the very least, I believe it underestimates the amps for single phase L-L loads by a factor of 208/240. Which strikes me as a lot worse than a 'corner case.'

 

[jaggedben]

What Wayne said, while I was typing.

 

[charlie b]

Reading the original post, I agree. kW + kW always gives correct total kW, but I agree that the number is incomplete for sizing the system.

-Jon

That was what I intended to convey.

I think we are pretty much on the same page now.

 

[ggunn]

Yes, if I understand your procedure correctly. It assigns a VA to each bus, but you don't specify how to turn that VA into a current rating for the bus. I'm going to assume you intend to divide the VA by the bus's L-N voltage (for the 4-wire wye case). To concretize all this, let's just consider a 208Y/120V system; other systems just different by a scale factor.

For one single phase L-N load, your procedure seems obviously correct--assign the VA to one bus, divide the VA by 120V, and you get the current on that bus. Likewise for one balanced 3 phase L-L-L load, if the current is I, the VA is 3 * 120V * I. So to compute I from VA, you divide by 3 and further by 120V. That matches your procedure of assigning 1/3 the VA to each bus.

But for one single phase L-L load, if the current is I, the VA is 208 * I. You are going to assign 104V * I VA to each bus. But you should be assigning 120V * I to each bus, so that when you divide by 120V at the end, you get I as desired. In other words, you need to assign 1/sqrt(3) of the VA to each bus, not 1/2 the VA to each bus.

[Note that for a balanced set of 3 L-L loads, the vector addition of currents will give you cancellation so that you can just assign 1/2 the VA to each bus for each load. In other words, you can split your L-L loads into balanced sets plus the unbalanced remainder(s); then use the 3-phase rule for the balanced sets, and the 1/sqrt(3) rule for the unbalance remainders.]

With that correction, the procedure will not underestimate the total phase current (at least for linear loads). It accurately gives you the magnitude of the phase current for each load, and the total phase current will be the vector sum of those individual load phase currents. Then the magnitude of the sum of the vectors is always no more than the sum of the magnitudes of the vectors.

Cheers, Wayne

How would you approach four equal L-L loads on a three phase system? That's a real world situation for me at the moment. I tried getting them to drop one load or add two more but they didn't go for it.

 

[wwhitney]

How would you approach four equal L-L loads on a three phase system?

Didn't we cover that in the responses to this post of yours?

EV Charging Transformer Sizing Calculations

Have to wonder if many of the utility companies will have generation plants, transmission lines & hard to get transformers installed within the next 15 years. I'd love to know what the plan is for truck stops. Current production chargers are one megawatt. Trucks will need to sit and charge...

forums.mikeholt.com

 

[winnie]

I would use the approximation of saying 4 loads means 8 terminals, assigning 1/2 of a single load kVA to each terminal.

Then I would apportion the 8 terminals to the supply as 2,3,3.

So one leg of the supply sees 1 full load, and 2 legs see 1.5 full load.

Note: for purposes of calculating current in the above approximation you need to use 1/2 L-L voltage (as pointed out NY @jaggedben ) not the L-N voltage.

Jon

 

[wwhitney]

Note: for purposes of calculating current in the above approximation you need to use 1/2 L-L voltage

Doing that is the same as naively adding up the L-L currents up. E.g. if each load is 10A L-L, you get currents of (20A, 30A, 30A) for 4 loads.

If you split the load into its balanced portion, and use your 3 phase formula for it, and then do the above just for the unbalanced load, you'd get (17.3, 27.3, 27.3). Which gives you the exact current for the balanced portion (17.3, 17.3, 17.3), and just adds the current of the unbalanced portion of (0, 10, 10) to it.

Or if you do the vector addition of the current, you get the sharp answer of (17.3, 26.5, 26.5).

Cheers, Wayne

 

[ggunn]

Didn't we cover that in the responses to this post of yours?

EV Charging Transformer Sizing Calculations

Have to wonder if many of the utility companies will have generation plants, transmission lines & hard to get transformers installed within the next 15 years. I'd love to know what the plan is for truck stops. Current production chargers are one megawatt. Trucks will need to sit and charge...

forums.mikeholt.com

We discussed it, yes, but the perspective of this thread is a little different so I thought I would bring it up here.