Why 108 Volts for Watts Ratings?

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2oldman wrote:

JesLookin wrote:
1620 watts / 124 volts = 13.06 amps

That's the obvious answer, but OP must be getting at something else.

for a pure resistive load yes. but not much is a pure resistive load. your toaster, your water heater element etc.

But NOT your ac unit, your microwave, your converter. For them watts may go up, may go down,may stay the same. It all depends on what kind of switch supply is used and what kind of load the supply is hooked to.

it all goes back to UL and NEC and the 120V "nominal" input that basically expects those devices to operate from 108V to around 130.

in many cases max current is at min voltage for reactive load devices. and induction motors also draw higher current as the input voltage goes down partially due to the reduced back emf generated by the motor.

BFL13 wrote:



Perhaps they chose 108 as being at the low end of the operating 105-130v range to allow for how the amps go up when the voltage goes down, so to get max amps you would take the lowest voltage. It is all probably laid out in the NEC somewhere?
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I think the Iota guy meant that input current went down with output voltage going down, not with the input voltage going down. They use their output voltages as their "stages" The converter will draw less VA from the generator when in 14.2v Absorption voltage, than at 14.8 Boost voltage.

I was specifically asking the guy about the AC input side and I asked at least a couple of times, same for the 935 watts. I wouldn't put a lot of credence to what he said.

ANSI sets the standards for utility supply voltage, ANSI C84.1 info. here from P&G Then the NEC allows max. 3% or 5% drop to the point of utilization in addition to the min. ANSI supply voltage. There will be some kind of standard for voltage rating of portable equipment NEMA I think - tried to find it once and didn't. The P&G info. says according to NEMA standards, appliances and motors should be able to operate at +/- 10% of the nameplate voltage which would make the min. 108V if nominal/nameplate is 120V. Maybe that's where 108 comes from?

There's no phase shift. Everything is in phase.

Gotta remember .7 pf = phase shift. Current surge.

Reminds me to pick up an El Supremo grade motor run capacitor. Beeg. Thanks for the unintentional reminder ;)

Mixing up threads here but I don't care at this stage, so looking at that info from the Iota guy and trying to make sense of it---

He said 13.4 x 108 is correct = 1447 watts or VA
He said at 120 watts would be 935, but 935/120 is 8a, too low.

So IMO the 935 is without the PF of 0.7ish so now VA is 1335 so that 1447 must be VA.

At 120 and 1336, current is 11 amps, which makes sense being lower than 13.4 and in the ballpark.

So IMO the Iota tech is ok and you don't have to phone PD to get a better answer :)

BTW, PD says their 60 amper is 1000w and the 55 amp Iota would be (from above numbers) 1447 x 7 = 1013w

smkettner wrote:
What voltage would you use to list maximum amps required on the safety label?

I think the max amps rating is a guide to what gauge wire and amps of circuit breaker is needed to run the converter.

So it is either a 15 or a 20a circuit needed, and that should be indicated. Also of course, the deck mount ones should have a 20a plug in their input wires if they need to be on a 20a circuit.

The non-PF corrected PD9280 needs a 20a circuit, but the PF corrected PM3-75 only needs a 15a circuit.

Perhaps they chose 108 as being at the low end of the operating 105-130v range to allow for how the amps go up when the voltage goes down, so to get max amps you would take the lowest voltage. It is all probably laid out in the NEC somewhere?
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I think the Iota guy meant that input current went down with output voltage going down, not with the input voltage going down. They use their output voltages as their "stages" The converter will draw less VA from the generator when in 14.2v Absorption voltage, than at 14.8 Boost voltage.

You get maximum input amps at minimum input voltage.

From Itoa 55A spec:

Vout = 13.6V
Iout = 55A
Pout_max = 750W

Max input power is:

Pin_max = Pout_max / (efficiency * PF)
Pin_max = 750W / (0.8 * .65)
Pin_max = 1442W

Input current at 108V = Pin_max / 108V = 13.35A
Input current at 120V = 12.0A

We know the converter can do more than 750W.

Pout = 14.8V * 55A = 814W

Got wondering enough that I called Iota to see if I could answers. I supposedly got their tech. guy - trying to get clarification was like trying to pull teeth. Not exactly the kinda guy to chin wag with.

The input current does indeed go down with voltage, not up and their listed 13.4 amps is correct at 108V. He agreed the voltage would normally be at or around 120V and the current would be less. At 120V he said the input wattage would be 935. As far as selecting a generator he said the inrush current shouldn't cause any problems because it's so short a duration and a little Honda generator should be just fine. He also said the 30 amps max. inrush is correct and also depends on the load on the DC side. Just didn't get the answer I was looking for on how to size a gen. I ended up kinda giving up on getting more anwers...

If you have an EMS that cuts out at 102 or 104 volts, what's going to happen to the input current? Would be nice to see a graph.

If it were me, think I'd rather talk tech. with someone like Progressive Dynamics. Or maybe just go buy a 'lil Honda gennie.

The voltage that would be -overwhelmingly- encountered in everyday use. Refer to UL and ETL testing parameters. NEMA standards.