Residential 3-Phase Calculation

[Blink]

Greetings- I was hoping to get some help. I am currently just starting the Mike Holt courses and don't have much experience sizing 3-Phase applications. My primary work is installing residential generators. I am currently working to size a generator for a residential property located adjacent to a church (rectory). The house is actually fed off of the church and is 120/208 3-Phase. However, there are no 3-phase appliances, everything is single phase 120 or 208V. My question is this- how is the calculation different since the service is 3-phase? Where do I enter the PF and 1.73 if everything is single phase in the house?

Thanks in advance,
-Newbie

 

[charlie b]

Convert everything to VA. Do not attempt to add amps to amps to get amps, as this will not work for combinations of 120V and 208V circuits. Take the currents associated with the 120V loads and multiply by 120. If the power factor for any given load is not 1.0, then divide the result for that item by the PF. Then take the currents associated with the 208V loads and multiply by 208. Once again, if the power factor for any given load is not 1.0, then divide the result for that item by the PF. Add all that up, and you will get the total load in units of VA. To calculate the equivalent current for the 3-phase service, divide that total by 208, then divide that value by 1.732.

 

[david luchini]

Take the currents associated with the 120V loads and multiply by 120. If the power factor for any given load is not 1.0, then divide the result for that item by the PF.

This part doesn't sound quite right.:?

 

[Blink]

None of the appliances list a PF? Also where does the .8 PF of the service/generator get placed?

 

[Smart $]

None of the appliances list a PF? Also where does the .8 PF of the service/generator get placed?

Load calculations do not include power factor. Using power factor essentially removes reactive amperes (the portion that goes back and forth from source to load without getting consumed) from total amperes. Load calculation is only concerned with total amperes to properly size conductors and OCP.

 

[Iron_Ben]

I think charlieb's explanation and methodology is correct in every way including accounting for power factor. However for residential loads, one can safely ignore power factor and just assume it's 1.0 (it's likely in the range of 0.9 - 1.0). Also, are you sure this is a three phase four wire wye service, 208Y/120? It may be, but we served a few houses here and there in special circumstances from a 208Y/120 source. We always fed them with two hots and a neutral. In that case, the volt-amp approach is still arithmetically valid, but you'd want to amp the conductors (including the neutral) and look at the panel closely because it's not likely to be balanced equally across the two legs and the neutral might surprise you.

 

[david luchini]

I think charlieb's explanation and methodology is correct in every way including accounting for power factor.

I think Charlie may have erred in accounting for power factor. Multiplying a load's current times its voltage will give VA. Dividing VA by PF gives nothing useful.

 

[Iron_Ben]

True. Dividing watts by pf gives volt amps by definition though. He didn't say that, but I inferred it. Incorrectly!

 

[lielec11]

None of the appliances list a PF? Also where does the .8 PF of the service/generator get placed?

Most generators are listed with both kW/kVA ratings.

For example 500kW/625kVA. Depending on what your nameplate says, use the 0.8 to get to VA.

 

[Blink]

Ben, I am 110% sure it's 120/208Y service. 3 hots plus a neutral.

So by reading all if this, it sounds like I do my calculation exactly like I would for single phase 120/240, assuming everything in house has a PF of 1, right? I'm used to converting everything to KW, not KVa because I usually work with only single phase units. That said, the KVA and KW should be the same here given a PF of 1?

 

[Blink]

...same meaning not insert a PF or 1.732 anywhere. Or should I not convert any of the appliances to KW individually, yet add up all of the amps for the house then divide by 208 and 1.732?

 

[Smart $]

Ben, I am 110% sure it's 120/208Y service. 3 hots plus a neutral.

So by reading all if this, it sounds like I do my calculation exactly like I would for single phase 120/240, assuming everything in house has a PF of 1, right? I'm used to converting everything to KW, not KVa because I usually work with only single phase units. That said, the KVA and KW should be the same here given a PF of 1?

...same meaning not insert a PF or 1.732 anywhere. Or should I not convert any of the appliances to KW individually, yet add up all of the amps for the house then divide by 208 and 1.732?

Everything in a load calculation is in VA units. Doesn't matter whether it's 1? or 3?. Code specifically states some items wattage is equivalent to VA (e.g. ovens and ranges). Motors rated in watts must be converted to HP then FLA is determined per Article 430. Those few items that have a wattage and not an ampere rating on the nameplate (rare) have to be converted to VA (i.e. watts ? pf).

For 208/120V 3? 4W, take the total VA ? 360 (= 208 ? 1.732), and that's assuming your loads will be balanced. It gets a bit more complex where total load is unbalanced.

 

[Iron_Ben]

If the load is 100% single phase as you say, you can do it on a per phase basis. Let's say you determined it's 6 kva on each of the three phases, perfectly balanced (ha!). Current for each phase is 6000 volt amps divided by 120 volts, or 50 amps on each phase. 6 kva per phase is 18 kva total of course. And a lot for a residence. The three phase calculation would be (50*208*1.73) = 18,000 volt amps or 18 kva. So the square root of 3 is in there on the three phase calc, but not if you figure each phase individually. In the utility world we assumed balanced three phase currents in most cases and just multiplied line to line voltage by current by 1.732 to get three phase kva. But we knew from experience that not all 3 ph circuits and not all 3 ph customers draw a balanced load, and as Smart$ noted, that's when it gets a bit more complex and can bite you in the ass.

And again as far as residential I would be comfortable assuming pf of 1, so kw = kva. Even though it's more likely to be 0.95 or some such. But with an industrial customer such an assumption would be a big mistake.