# Math Time

Discussion in 'Lighting and Electrics' started by cruiser, Dec 20, 2003.

1. ### cruiserActive Member

Messages:
243
2
Location:
Melbourne
Orite.... Let's see here....

I think ive stuffed something up.

Ive got 32Amps..... and all my wattage from fixtures added up is equal too 13,050W. ive never done it this way before, but this venue and whatever requires that it be added up to match the amps and whatever out.
It has quite confuesed me, so Im hoping someone can steer me in the right direction.... =)

2. ### wolf825Senior Team EmeritusPremium Member

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1,155
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Location:
Eastcoast USA
Hiya,

General rule of thumb when calculating amps for lights (at least for me and my thumb) its safe to say its 1 amp per 100 watts. Its truely more along the lines of 0.877-something-something amps per 100 watts..so 1 amp per 100 watts, and 10 amps per 1000 watts, always seemed to me to be a safe over-calculation. So..with that said, your 13,050w is along the lines of 130 amps needed...while the real calculated amperage is around 110.448-something-something amps. Is your 32 amps "per leg / phase" or total amps available???

I don't believe the amperage draw changes whether its 120v/60hz or 208v/50hz...unless physics is different down under too.

hope this helps ya... (did you ever get the GAM gel swatch you ordered?)

-wolf
"...its too early on a saturday morn to do math"

3. ### DMXtoolsActive Member

Messages:
355
3
Location:
Elgin, IL, USA
John says (putting on his Electrical Engineer's hat):" Watts is amps times volts - it's as simple as that."

If I remember correctly, single-phase power in Australia is 240 volts. 240 X 32 is 7680 - a little over half the wattage you need... if you turn everything on at once.

If you feel like doing the math for it, it's probably okay to have 13000+ watts of fixtures total, as long as you plan all your scenes, including any "grand finale", so you never have more than 6500 watts-worth on at the same time. The 1280 watt margin covers the "keep-warm" current your dimmers supply to fixtures that are supposedly "off." When you're adding up the fixtures, remember that most movers don't turn the lamp off immediately when you command a "black-out" - they just close a shutter and delay turning the lamp off for about a minute to see if you're going to turn it back on again.

A safer way, though, would be to talk them into giving you an additional 32-amp circuit, if at all possible, and try to split your load between them.

Failing that, I'd try to eliminate some fixtures or start replacing lamps with lower-wattage equivalents and get down to about 8000 watts total, then plan my scenes to have the 6500 watt-per-scene limit (the idea there is to avoid having to explain, repeatedly, why you don't turn everything on during the "grand finale").

Good luck!

John

4. ### cruiserActive Member

Messages:
243
2
Location:
Melbourne
Ok then... I think i understand.... the volts in the amps = watts / volts equation is 240V cuz that is our standard single phase. or first do I have to multiply the single phase by 3 to make it 3phase? or multiply it by 32?

And no, I never did get a Gam swatch book

5. ### wolf825Senior Team EmeritusPremium Member

Messages:
1,155
93
Location:
Eastcoast USA
Well if you want a GAM swatchbook...just PM me and I can snail mail ya one....

-wolf

6. ### DMXtoolsActive Member

Messages:
355
3
Location:
Elgin, IL, USA
My calculations assumed single-phase 240 volts.

If they're giving you 32 amps on a three-phase 240-volt circuit, and if you balance the load so it's spread out evenly across the three phases, you should be pretty safe with the full 13,050 watts. 3-phase 240 volts would be a delta connection - the voltage across any two of the three wires is the actual voltage of one phase. the current through any one wire is the vector sum of the currents from two phases, 120 degrees apart. With 32-amp breakers on the 3-phase, it will allow you to draw about 20 amps (4800 watts) per phase.

If it's three phase at some other voltage, there's a good chance it's a wye system. The 208-volt 3-phase circuits we have over here are three 120-volt circuits in a wye configuration. For a wye configuration, each wire carries the current for a single phase, but the voltage between any two wires is the vector sum of the voltages of two phases, 120 degrees apart. 32 amps of 208-volt 3-phase would resolve to three 32-amp 120-volt circuits - 3840 watts per phase or 11520 watts total - you'd have to lose a few fixtures.

A wye connection at 240 volts per phase would yield a voltage around 416 between any two of the wires, but would resolve to three 32-amp 240-volt circuits, 7480 watts per phase and 22400 watts total available - more than enough, though you'd still have to try to keep the load balanced, split evenly across the three phases.

Confusing enough?

John