ECN Forum Forums General Topics of the Trade General Discussion Area Voltage dop

In most European countries the rule is rather simple. You're allowed a total of 4.5% voltage drop from the service fuses but not more than 1.5% at the meter, that leaves you with 3% to the very end of the longest circuit. Your grid operator is responsible for supplying a voltage that keeps your appliances within limits if you stick to the 3% rule. VOltage drop is always calculated at full nominal circuit load, i.e. the size of the supplying fuse/breaker. The only exception to this rule are circuits that only supply one hard-wired load, usually a motor, which requires a larger fuse/breaker to account for startup inrush and the motor is protected by a thermal cutout.

The reason I stress actual voltage measurements is because you simply need to.

I'll use my own house as an example. It was pretty common to get measurements of 118 volts - well within the ratings of normal equipment. From NEMA standards, a nameplate rating of "120" would mean the appliance could be expected to operate properly anywhere between 132 and 108 volts.

For a simple CFL light bulb (maybe 13 watts), no voltage drop would be seen. There was nothing wrong with THAT circuit. Yet, the lights often dimmed.

By golly, when the water heater came on, the voltage dropped to as low as 92 volts - low enough to make the microwave oven simply not heat, motors to experience very short lives, etc.

There was nothing wrong with the water heater, or the house wiring. The problem proved to be damaged power company wires serving the neighborhood.

A similar issue arose when a restaurant had its' new exhaust fan burn out after a few months. When a new starter was installed, one with electronic overloads, the overloads kept tripping.

The cause of that intermittent low voltage problem proved to be overloaded ("saturated") transformers. Over the years, increased air conditioning and refrigeration loads at a neighboring grocery store, combined with summer heat and the lunchtime 'rush' at the restaurant created a demand the transformers simply could not provide.

You won't know these things unless you check the supply voltage regularly. That 3% calculation is a nice reference, but really tells you very little.

In residential settings, it's common for one home to receive considerably fewer volts than another, simply because one house is closer to the transformer than the other. The guy getting the 125 volts feed can afford to lose a lot more than the guy at the end of the street, who might be receiving only 110 volts - especially if every house before his has the air conditioner running!

Both homeowners need to start worrying at 108 volts- which means the last guy in line might be in trouble from the start. That's why the voltage needs to be checked under operating conditions .... the supply won't be under much stress when its' installed, if it's installed on an April morning.

I have had a Weston 901 meter plugged in here for a quarter century. my line voltage is pretty stable at 123-124 so I can afford a little voltage drop if I need to. That makes me feel better if I have 50' or 100' of extension cord strung out but that is actually rare because I have power wired everywhere.
I have one of those 1/2 ton A/Cs in the kids play house (a manufactured shed) and the voltage is still good there (120 or so) with it running. That is at the end of a pretty long 12ga branch circuit (100' or so) and then a 14ga extension cord going to the play house.

Things are getting interesting when someone's out in the sticks, say almost 1 km from the transformer (not all that unusual in Europe) and then needs power somewhere outside - achieved by plugging in two 50-m extension leads, 1.5 mm2. That's perfectly legal because in Austria and Germany voltage drop and short circuit current calculations only apply to fixed wiring but I doubt a short at the end of those 100 m (around 330 ft.) would do anything to a 16 A breaker but eventually trip the thermal overload. Since there are no mandatory maximum trip times for short/overload many even consider it relatively harmless to exceed the maximum VD in fixed wiring.

We ran the numbers on this a long time ago and I think it takes about a half a KM of 16 ga wire (typical cheap extension cord here) to dissipate 1800w without tripping a 15a breaker, assuming a bolted fault on the far end. Of course that assumes all of the receptacles and plug caps connect perfectly. It takes about 8 ohms end to end. so 250 meters would do it.

I seem to remember a few 'hot' cheap extension cords.

I remember the numbers you refer to Greg. I did a class project back when I was an instructor at Vo Tech on voltage drop. It kept the guys thinking for two (2) nights.

That's assuming a decent loop impedance at the meter though. I've heard of supplies where instant tripping a 16 A breaker (B type, i.e. instantaneous trip at 3-5 times nominal current) was already problematic. So assuming a lot of power is already dissipated before the meter you could get there. Keep in mind that it's much more common to have larger transformers with extended LV distribution even in rural areas in Europe! 1 km of LV from the transformer isn't at all unusual, supplying some ten houses.