208Y/120 - 3P - 4W Load Calculations

[load_n_code]

Let's say the panel schedule load in VA is as follows: Location Commercial Strip Mall

PH-A Total Load = 19,276VA
PH-B Total Load = 19,085VA
PH-C Total Load = 17,781VA

PH-A: 19,276VA/208 = 92.673 = 93 Amperes
PH-B: 19,085VA/208 = 91.754 = 92 Amperes
PH-C: 17,781VA/208 = 85.486 = 85 Amperes

Next Panel Size Up = 100 Ampere Panel

Assume that any (LCL) Continuous Loads have been factored into the example.

Why are loads calculated by dividing the load by 208V when OCPD sizing is calculated by dividing the branch circuit load by 120V?

Correct me if I have made any mistakes, I have lots to learn.

Thanks,
--Mario

[ January 09, 2005, 02:31 AM: Message edited by: load_n_code ]

 

[coulter]

Re: 208Y/120 - 3P - 4W Load Calculations

Mario -

Quote: Why are loads calculated by dividing the load by 208V when OCPD sizing is calculated by dividing the branch circuit load by 120V?

Well, you shouldn't. You listed the loads as phase to neutral, so your calculations should be phase to neutral. 120V is what you would use.

carl

 

[petersonra]

Re: 208Y/120 - 3P - 4W Load Calculations

You actually can have two different types of loads with 208/120 systems - 120V single phase, and 208V 3 phase.

120V single phase loads are supplied from one phase to neutral.

3 phase loads use all 3 phases (and occasionally the neutral).

the current used in a single phase circuit is VA/120 (where VA is the volt-amps required).

ex: 1200VA/120V = 10 amps one pahse

the current in a 3 phase circuit is VA/(208*sqr(3)) and is the same on all 3 phases (or at least very close).

ex: 1200VA/(208V*1.73)=3.33 amps EACH phase

(edited to add examples)

[ January 09, 2005, 10:26 AM: Message edited by: petersonra ]

 

[charlie b]

Re: 208Y/120 - 3P - 4W Load Calculations

Originally posted by petersonra: You actually can have two different types of loads with 208/120 systems - 120V single phase, and 208V 3 phase.

There is a third type of circuit: 208 volt single phase. It has two wires, one connected to Phase A and the other connected to Phase B (just one example).

But keep in mind the nature of the question. If you are asking about the load on the panel, you have to add up everything in terms of VA. Take the single phase 120V currents times 120, the single phase 208V currents times 208, and the three phase 208V currents times 208 times 1.732, and add these three numbers. Divide the result by 208, then divide the result by 1.732, to get the total load (in terms of current) on the panel.

Originally posted by load_n_code:
PH-A: 19,276VA/208 = 92.673 = 93 Amperes
PH-B: 19,085VA/208 = 91.754 = 92 Amperes
PH-C: 17,781VA/208 = 85.486 = 85 Amperes

Next Panel Size Up = 100 Ampere Panel

Correct me if I have made any mistakes, I have lots to learn.

Your math is not correct. As I mention earlier, you add up all the VA numbers first. 19,276 plus 19,085 plus 17,781 equals 56,142 VA. To get current, you divide this by 208, and divide again by 1.732 (i.e., the square root of 3). The result is 155.8 amps. For this load, you need a panel rated higher than your 100 amp result. I would use a 225 amp panel.

 

[load_n_code]

Re: 208Y/120 - 3P - 4W Load Calculations

Carl,
Bob, &
Charlie

Thanks for the replies.

Do the calculations change because of Power Factor if the figures for the panel are listed in Watts instead of Volt-Amperes?

--Mario

 

[charlie b]

Re: 208Y/120 - 3P - 4W Load Calculations

Yes. You have to do the math in units of VA (or KVA). You did not tell us where the original VA numbers came from. But if you are given the information in terms of watts, you need to convert to VA, by dividing by the power factor.

 

[load_n_code]

Re: 208Y/120 - 3P - 4W Load Calculations

Charlie,
Thank You.
--Mario

 

[W6SJK]

Re: 208Y/120 - 3P - 4W Load Calculations

I can't resist...

It's actually more complicated than that as you are adding loads of varying power factor (represented as different vectors) so you cannot add them algebraicly. But nobody bothers with that. They just assume a power factor of 1.

 

[charlie b]

Re: 208Y/120 - 3P - 4W Load Calculations

Sparkie, here I must beg to disagree. The original question expressed the loads in terms of VA. Now someone may have had to go through a power factor calculation to come up with those numbers. That is seldom necessary, but it might have been done. However, once those numbers had been determined, all future math is done without the need to ever look at power factor again. VA is, in fact, a scalar value, and is added algebraically.

 

[kiloamp7]

Re: 208Y/120 - 3P - 4W Load Calculations

VA is VA is VA

It is easier to use VA (not watts & power factor) when doing alternating current load calculations. It is especially helpful when mixed voltage loads are involved. Also, technically, it is also more correct to speak of AC loads in VA rather than in watts.

Several years ago, the terminology "watts" was changed to "VA" in the NEC, and for good reason.