277 volt power at home
Posted: Sat Jun 09, 2018 5:20 pm
Just took another step toward what might become a real fusor power supply. Have long wanted 277 volt power for some nice new 60 mA NST's, bought cheaply on ebay 'cause of their primary voltage. Then last month, the 277 volt parking lot lights at work were changed from sodium to LED. Facility manager had the old luminaires stacked by the dumpsters. Pile gradually dwindled to nothing, but not before I took a couple home to play with. (Each unit is about 12 x 40 inches, with a 32 inch long lamp.)
Today's story might sound familiar on the weldingweb forum. My house has two 240V receptacles, one in the kitchen and one in garage for clothes dryer. The dryer has a now-deprecated 3 wire cord with NEMA 10-30 plug (hot-hot-neutral, no separate grounding conductor). But the wire run is entirely enclosed in steel conduits and boxes.
I spliced a spare dryer cord onto a long 10-AWG 4-conductor cord. The fourth wire is extended along the dryer cord and terminated with a ring lug, for a removable bond to the electrical box. Time to verify that the steel enclosure could safely serve as an "intentional low resistance groundING conductor", with no local connection to the groundED conductor (white wire). Today's code doesn't permit my new copper water pipes to be used for grounding. Any wire connected to a water pipe is there to divert electricity out of the pipe, to protect plumbing users. Not to divert electricity into the pipe, to protect electricity users. And ground rod connections aren't sufficient -- in the event of a hot to ground short, the earth resistance is too high to keep voltage at a safe level, and to draw enough current to immediately trip the overcurrent protection device.
This called for a milliohmeter -- the opposite of a megger. With the dryer circuit turned off, the groundED and groundING wires at end of new cord were connected to a low-voltage, high-current transformer controlled by a variac. It took 2.7 volts to get 15 amps. Outside of the picture, circuit runs through the neutral wire back to ground bar at the breaker panel, then returns through the steel conduit and cord wire #4.
Voltages were measured with respect to breaker panel ground, accessible via the neutral of a nearby 120V circuit that was turned off.
Then we could compute the resistances, including the effects of any temperature rise from carrying 15 A.
Flexible cord dropped 320 mV in each wire (21 milliohms).
Steel pipe dropped 122 mV (8 milliohms).
Neutral wire inside the pipe dropped 1700 mV (122 milliohms).
Neutral contact from plug blade to fixed wire dropped 219 mV (14 milliohms).
The neutral wire resistance was much higher than expected. Then I found that it's only 12 AWG, unlike the black and red wires.
Still higher than expected. I found that the clip-on ammeter was indicating 15 A when actual current was 18 A.
Voltage drops posted above are based on 15.2 A measured with a current shunt resistor.
Next step: connect the big variac or a boost transformer, get 277 V, light up some sodium vapor.
Today's story might sound familiar on the weldingweb forum. My house has two 240V receptacles, one in the kitchen and one in garage for clothes dryer. The dryer has a now-deprecated 3 wire cord with NEMA 10-30 plug (hot-hot-neutral, no separate grounding conductor). But the wire run is entirely enclosed in steel conduits and boxes.
I spliced a spare dryer cord onto a long 10-AWG 4-conductor cord. The fourth wire is extended along the dryer cord and terminated with a ring lug, for a removable bond to the electrical box. Time to verify that the steel enclosure could safely serve as an "intentional low resistance groundING conductor", with no local connection to the groundED conductor (white wire). Today's code doesn't permit my new copper water pipes to be used for grounding. Any wire connected to a water pipe is there to divert electricity out of the pipe, to protect plumbing users. Not to divert electricity into the pipe, to protect electricity users. And ground rod connections aren't sufficient -- in the event of a hot to ground short, the earth resistance is too high to keep voltage at a safe level, and to draw enough current to immediately trip the overcurrent protection device.
This called for a milliohmeter -- the opposite of a megger. With the dryer circuit turned off, the groundED and groundING wires at end of new cord were connected to a low-voltage, high-current transformer controlled by a variac. It took 2.7 volts to get 15 amps. Outside of the picture, circuit runs through the neutral wire back to ground bar at the breaker panel, then returns through the steel conduit and cord wire #4.
Voltages were measured with respect to breaker panel ground, accessible via the neutral of a nearby 120V circuit that was turned off.
Then we could compute the resistances, including the effects of any temperature rise from carrying 15 A.
Flexible cord dropped 320 mV in each wire (21 milliohms).
Steel pipe dropped 122 mV (8 milliohms).
Neutral wire inside the pipe dropped 1700 mV (122 milliohms).
Neutral contact from plug blade to fixed wire dropped 219 mV (14 milliohms).
The neutral wire resistance was much higher than expected. Then I found that it's only 12 AWG, unlike the black and red wires.
Still higher than expected. I found that the clip-on ammeter was indicating 15 A when actual current was 18 A.
Voltage drops posted above are based on 15.2 A measured with a current shunt resistor.
Next step: connect the big variac or a boost transformer, get 277 V, light up some sodium vapor.