#### piercho

##### New member

In AC, V X A = VA (volt-amps), not watts. V X A X PF = W. AC amps and volts are vector quantities and their magnitudes only multiply directly if they are in perfect phase. The value used to get from VA to W is PF, or power factor.

A common example: the HX-NPF 250W H37 ballast recommended in the link "MH for less than $100", and sold by PFO in their 250WEYE ballast. Described on this Advance data sheet: http://www.advancetransformer.com/ecom_PDFS/out/9071568311.pdf

The ballast draws 285W, so it's amp draw is (by your formula) 2.4A, right? Not correct. Check out the power factor on the data sheet. 0.5. The worst case (running) amp draw is 285/(.5 X 120V) =4.75A. Typical running amps is 4.20, according to the data sheet. For this particular ballast circuit, starting amps can be even worse at 6.85A.

Lets say I was budgeting a 15A house electrical circuit. If I took the 2.4A value, I'd believe I could fit 6 of these ballasts on a single circuit. I reality, if I was starting the ballasts off of the same timer, I could only fit 2. For amp budgeting purposes, in this case, A=W/V is off by 185%.

Cap and coil (CWA) magnetic ballasts have a PF of .9, and some electronic ballasts have a PF approaching unity. So, in those cases, using VA for W does not produce such large errors. But there is still an error. Most ballasts have a max amp draw printed on them, and that is the value that should be used to calculate amp draw.

There are devices that draw LOTS more current than the watts that they consume reveal. IMO, these HX-NPF ballasts are probably the worst of them for the devices that are in typical use for the reef aquarium. Also, you only pay for (as a residential customer) W, not VA, so the V X A = W is wrong there, too.

You've given me a lot of help over the last few months so I am very reluctant to criticize. But I've been fighting this issue since I came to the board and putting it in print as fact just makes this erroneous notion harder to correct.