Power requirements of studio strobes

In consideration of a pair of AlienBees B1600 flash units, I note the following specifications:
640 true wattseconds
recycle time to 100% - 2 seconds
power requirement - 120v AC, 8 amps
I also note that their lower powered units require the same 8 amps, but recycle proportionately faster.

Based on my limited knowledge of electricity, I make the following assumptions:
1) They would draw the full 8 amps of current only when recycling, and draw a relatively small amount of current after the capacitor is recharged (for the modeling lights and other electronics).
2) Since they would fire and recycle simultaneously, two units would draw a total of 16 amps, and three units would exceed the capacity of a 20 amp circuit (even three AB400 units would draw 24 amps).
3) Any strobe with similar power could recycle faster only by drawing more current. Hypothetically, a 640Ws strobe that recycles in 1 second would need approximately 16 amps to do this.

If the above assumptions are correct, then it would be impossible to power two 640Ws lights that recycle in 1 second (if such lights exist) on a single 20 amp circuit.

So - I have two questions:
a) Are the above assumptions correct with respect to monolights in general?
b) Do "pack & head" type systems have the same limitation?

Thanks,

Curtis,

Your assumptions are right on target. But, I would grind on the edges just a bit.

If a device is rated at 8 Amps, it is very likely to actually draw a bit less, or almost all units produced would draw less (normal manufacturing variation). When dealing with one device it is realistic to just go with the stated rating, but when looking at more than one device it is reasonable to assume the pair would require a bit less than twice the actual rating (it is very unlikely that you would end up with two units that both draw the 8 Amp maximum current).

So, it is very reasonable to think that two Alien Bee units rated at 8 Amps total would very likely work just fine on 15 Amp circuits common in households in the US, so the also very common 20 Amp circuits are likely to be more than adequate. It is also very likely (repeat likely) that a 20 Amp circuit would accommodate three of the Alien Bee units rated at 8 Amps each.

If you were using more than one power pack, on systems with a power pack and flash heads, they would work exactly the same way. But, the number of flash heads plugged into the power pack system would not affect the Amp requirements. Power pack/head systems either give all the power to one head, or divide the same amount of power with several heads. They are not like incandescent bulbs where increasing the number of bulbs increases the amount of light output.

Some larger capacity flash gear (like many Speedotron Blackline 1200, 2400, and 4800 W-s units) that tax some power circuits, actually have switches to slow down the recycling (limits maximum current) so they can adapt to marginal AC power. You recycle slower, but you do not trip circuit breakers or blow fuses. In most cases the slower recycling is not required, but in some old electrical service situations, the slower recycling is necessary.

I find it nice to think of the power capacitors in these units as buckets, and the power supply components as water pumps. All of the Alien Bee flash units seem to share the same pump. The B1600 has four buckets (power capacitors), the B800 has two buckets, and the B400 one bucket. It just takes longer for a pump with a fixed pumping capacity to fill four buckets than one. But, the maximum amount it can pump is very like the Amp rating, so it is the same for the B400, B800, and B1600.

I suspect there will be some on the forum that will predictably assume the bottom line of always being intrinsically safe and never using two 8 Amp devices like this on a 15 Amp circuit, but I also know you are a a pragmatic human where reasonableness transcends anal fastidiousness.

As a trivial aside, and just as an annoyance, you seem to be giving James Watt is due (W), but slight André-Marie Ampère (a).

Enjoy! Lon

My house is OLD, and the wiring is 80 years old this year. It was converted from fuses to breakers 13 years ago, but no change in wiring. All breakers are 15A. I frequently will run my Photogenic Pack with 2 400WS lights, then use two monolights at 150WS, and a background light at, say 30WS, running the modelling lights, a laptop computer, and some other items, all off of one power strip, plugged into the outlet, which is also running a PC. I've never had a problem with popping that circuit, and I know I'm on the edge even before turning on the strobes.

I've also never had a problem running my lights anywhere, even with long extension cords, So I can't imagine that they ever pull a full 8amps from a single monolight, cause i just don't think it takes that much to re-load the capacitor.

Thanks, Lon & Kev.

I was hoping for a panacea of power - to use 1500 or 2000Ws worth of lights and recycle them in 1/2 second. I want to be able to shoot sports teams and other large groups on location, where it might be difficult to find more than one circuit to plug into.

In theory, charging a 640Ws capacitor would require 640 watts for one second, or 320 watts for 2 seconds. 320 watts at 120 volts would be only 2.67 amps, so I don't know why the AlienBees units would require anything approaching 8 amps, especially since the modeling lights turn off while they recycle. But there's a lot about this general topic that is beyond my electrical education.

True, yet I have somehow managed to not burn the house down so far. But I think electrical fuses are no longer made in cylindrical shape because of guys like me who used to replace them with sawed-off pieces of bolts.

Wasn't there someone named Voltaire involved too?

Curtis N wrote in post #2598725
Thanks, Lon & Kev.
In theory, charging a 640Ws capacitor would require 640 watts for one second, or 320 watts for 2 seconds. 320 watts at 120 volts would be only 2.67 amps, so I don't know why the AlienBees units would require anything approaching 8 amps, especially since the modeling lights turn off while they recycle. But there's a lot about this general topic that is beyond my electrical education.True, yet I have somehow managed to not burn the house down so far. But I think electrical fuses are no longer made in cylindrical shape because of guys like me who used to replace them with sawed-off pieces of bolts.

But the flow of electrical current is not constant as your evaluation assumes.

When the capacitor starts to be recharged (after discharging) it gobbles up a huge amount of current (the 8 Amps), but as time passes and the capacitor is partially filled, the propensity for more current is placated and the charging rate (current) goes way, way, way down. When there are no electrons in the capacitor, they are all keen on getting in there, but as time passes the electrons have less and less inclination to join the party as there are so many other electrons already there (and they basically do not like each others company).

So, as the power capacitors start to charge the current is very high, but levels off to a much smaller amount very quickly. I suspect that actually for some very small amount of time (milliseconds or shorter time periods) the current might be higher than 8 Amps, but this level is so quick, the circuit breakers do not have time to react, or a fuse does not allow enough heat buildup to melt the weak link (or do the same thing to your wiring). Or, possibly the power supply circuit limits the current to at or near the units Amp rating.

You can see this effect in the capacitor charging graph about half way down this page:

http://www.kpsec.freeu​k.com/capacit.htm

Curtis N wrote in post #2598725
Wasn't there someone named Voltaire involved too?

Voltaire? The French writer/philosopher? How about the Italian Volta.

See:

http://en.wikipedia.or​g/wiki/Alessandro_Volt​a

Enjoy! Lon

Thanks for the insights, Lon. If strobes consume their rated current for only a fraction of a second, that may explain why KevBailey is able to power them as he described. The current doesn't exceed the circuit breaker's rating long enough to trip it.

Curtis N wrote in post #2599014
Thanks for the insights, Lon. If strobes consume their rated current for only a fraction of a second, that may explain why KevBailey is able to power them as he described. The current doesn't exceed the circuit breaker's rating long enough to trip it.

That might be the case, but also Kev's gear may not be intrinsically stressing the circuit.

He stated the W-s ratings of his units, but not the current ratings (Amps). The actually total Amp rating of his gear may not be stressful (or it might). But, there is not enough data in his post to codify this.

Many flash units that have relatively hight power rating (high W-s rating) still have relatively small power supplys that just collect current over a very long time.

I have two Speedotron Brownline 1600 W-s power packs that take forever to recycle. I do not recall the Amp rating on the units (and cannot check it as I am out of town for an extended period of time). So just looking at W-s ratings and assuming all other factors are equal on the flash unit can mislead. The Amp rating of a power pack is likely to have more to do with the ability of the circuit's power supply to provide the power, than the capacitors used to store it (the AB units reveal this).

Enjoy! Lon

My understanding is that a 15a circuit really supports 13.5a of constant drain and up to 15a of peak, momentary drain. Manufacturers recommend a current to which each of their fuses may be loaded. This steady-state current level is the current rating.
What probably saves your bacon is that circuit breakers do not trip instantaneously, but have some lag time. Circuit breakers thermal element takes the form of a spring. The spring expands with heat and thereby trips open the circuit. The slow-trip response helps discriminate between safe temporary surges and prolonged overloads. (There are fast acting circuit breakers availalble.)

Wilt wrote in post #2599231
15a of peak, momentary drain.

Yes, there is peak and there is peak. Some peaks are so transitory they deify peak ratings.

Go figure.

Another aspect of the mix (if I remember correctly).

Theoretically uncharged capacitors initially (for a very short period of time) act much like a short circuit (a short) to an applied charge. And theoretically initially inductors (for a very short period of time) act much like an open circuit to an applied charge.

So the capacitors in a flash unit initially can accommodate huge peak current, but the power supplies in the same flash units, that typically contain inductors, cannot be so accommodating. Inductors are like shock absorbers and resist rapid change.

Enjoy! Lon

I do not know a thing about electricity and how volts & amps work together. I do know that putting a couple of knifes in the outlet was probably not my smartest moment LOL

Kevin

londuck wrote in post #2599080
Many flash units that have relatively hight power rating (high W-s rating) still have relatively small power supplys that just collect current over a very long time.

To add to londuck's points in illustration, I have a 2000ws Dynalite power pack. It draws 18A but I can put it into a slower recycle mode to reduce it down under 15A draw.

Curtis N wrote in post #2597997
power requirement - 120v AC, 8 amps

Confirming what has been said their on-line manual states:

"The units are rated at 6 amps average current, and may briefly peak at nearly 18 amps input current at the very beginning of the recycle period. The AlienBees units will draw less average current if the flashpower is set for lower output and the modeling lamp is off. Depending on the model lamp rating and the model lamp power setting, the AlienBees require approximately 1 to 2 amperes to illuminate the model lamp and maintain the flash."

Thanks, ToyTrains. That is consistent with what Lon has provided.
I guess I'm wondering how many of these things it would take to trip a 20 amp breaker when they recycle together.

Curtis N wrote in post #2604964
Thanks, ToyTrains. That is consistent with what Lon has provided.
I guess I'm wondering how many of these things it would take to trip a 20 amp breaker when they recycle together.

My best guess is that a 20A circuit will accomidate three, but this is just a reasonable guess and I could be very wrong.

Hey folks, all you many fans with AB flash gear. Plug in two (then three) Alien Bee flash heads (the largest power ones you have) on one 20 Amp circuit in your house and let them rip at the highest power setting. Please let us know is the dam holds and which mix of Alien Bee flash heads you actually used.

Help someone out that is always at hand to help everyone else (namely Curtis).

Enjoy! Lon

I have had two AB1600 and two AB800 on one 15 or 20AMP circuit at full power before, no problem so far. I can't remember if I had in my living room plug or the downstairs bedroom plug (via extension cord. However, I generally don't do that except when I am lazy and in a hurry. I usually split the lights between the two circuits and usually I don't blast away at full power either (although not sure if that makes any real difference).

But I usually have my computer, 1 AB800, 1 AB400 off my 15AMP circuit, Along with occasionally my Home Depot Work light when shooting video.
And I have 1 AB800, 1 Ab400, and occasionally 1 AB1600 or a hot light (when shooting video) off the 20AMP circuit. All are operating at the same time.

If you are worried about it with AB lights, get the Vagabond (plan on not using modeling lights and taking longer to charge) and run your ABs through the Vagabond.

Just my experience.