Amplifer Watts vs. Amps question

George Kimery · From: Limestone, TN, USA

If I understand it correctly, to get amps based on wattage, you divide wattage by 120. So a Nashville 112 Amp, at 85 Watts, would pull .708 amps whearas an amp of 400 watts would pull 3.33 amps. So on a 20 amp circuit, you could run 28 Nashville 112's or 6 400 watt amps. What I don't know, are the amps pulling the same wattage just being plugged in or only if they are being played and driven.

The reason for me wanting to know all this, is the upcoming Steel In on July 21st. We will have a lot of amps around the room plugged in and turned on, all at the same time. I don't want to kick a circuit breaker at the Holiday Inn. A good idea of how many amps can be on one circuit, as long as only one is played at a time, would be a very valuable thing to know.

Hope some of you knowledgable in electronics and / or electricity will give me some input.

Thanks,

Earnest Bovine · From: Los Angeles CA USA

David Doggett · Posted 5 Jul 2007 10:05 am

George you are looking at the wrong kind of wattage. The output watts are a measure of the sound volume. What you want are the power draw watts. This is sometimes printed where the power chord goes into the back of the amp. It's a totally different kind of watts. The Peavey web site is down for maintenance, so I couldn't look up any specs there. But over on the Fender site I looked up the Steel King. It has 200 watts output, but looking at a picture of the power chord receptacle, it says 540 watts. Generally the power draw watts bear no obvious relationship to the sound output watts.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

If all the power an amp pulled out of the wall socket was converted into audio energy, that wouldn't be far off for a "maximum" power rating. But that is not what happens. Amps aren't perfectly efficient, so the actual audio power is only part of the picture.

A better way to get a max power draw estimate would be to look at the back of the amp or at the amp's spec sheet. Another approach would be to sum up the fuse current ratings.

The Peavey Nashville 112 manual says max power consumption is around 200 watts, so roughly 2 amps is about right. The NV 1000 spec sheet says 360 watts power consumption, so I'd figure for roughly 3-4 amps max. Session 400 Limited says 600 watts for 200 watts average audio power, so clearly these efficiencies vary.

As a rough rule of thumb, I guesstimate a max wall power draw factor of 2 times the max audio power for a solid-state amp. So I would typically estimate roughly 1 amp for a 50-watt or smaller amp, 2 amps for an approximately 100-watt amp, 4 amps for a roughly 200-watt amp, and so on. But as you can see, those efficiencies vary some. I also think tube amps tend to be a bit less efficient.

Of course, those rated power draws are maximums. I imagine they would not be getting pushed, and certainly not all at once in a jam like this. But I like a margin of safety - I'm an American-trained engineer. I also play with rock and rollers, and they do push amps pretty hard. It depends on the what percent of capacity the ensemble of amps are using.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Here's a (conservative) quick-take...

Big amps - 4 or 5 per 20 amp circuit

Small amps - 10 to 12 per 20 amp circuit

Also, keep in mind that, usually, the sound of solid-state amps will degrade (from voltage drop) long before you trip a breaker.

Paul Arntson · From: Washington, USA

What Dave and Donny said.

Just a data point:
My 1990's solid state Fender Deluxe 90 is rated to draw 360 watts from the wall (=3 amps) and is rated to put out 90 watts to the speaker. The circuit really looks like that makes sense, too. That is a 25% efficiency. And that is rated at ear-splitting maximum volume. Modern amps like the Peavy 112 and 1000 are a lot more efficient than that. Old tube amps can be a lot LESS efficient than that.

So you could get 5 of my Deluxes running at max loudness continually (ouch!!) on one 20A circuit indefinitely, with one amp to spare. Or at realistic non-shred-punk levels, probably 10 of them. As long as you didn't flick the switches on all at the same instant. The inrush while the transformers magnetize on the first cycle can be considerable.

Side note: circuit breakers are designed to deliver 80% of their rated load on a continuous basis. If you exceed 80% continuous loading, the breaker will heat up and some might trip after 3-4 hours.

Alan Brookes · From: Brummy living in Southern California

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