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I have a 240 volt 3-phase motor that I want to connect to my three phase panel. Someone told me that I have
to be careful when hooking up the high leg. Can you tell me what that’s about?
Three phase power comes in two different
configurations- delta or wye. The configuration is determined by the utility company transformer that supplies your building.
In a low voltage single phase application, the utility supplies you with 120 volts single pole or 240 volts two-pole. In a
three phase application, your higher voltage depends on the configuration: 240 volts in a delta, or 208 volts in a wye. While
most newer three phase devices will operate on a range of 208-240 volts and can be used on either a delta or wye system, some
older equipment is rated for 230 to 240 volts and thus, can only be used in a three phase delta configuration system. When
ordering 3-phase power, it’s important to check the power requirements of the equipment you intend to use before ordering
the electric service.
Now, with regards to your high leg
question, this has to do with the voltage measured between each single pole of a three phase system and
ground. In a wye system, you will measure 120 volts to ground on each pole, but 208 volts will be measured between any two
poles. In a delta system, you will measure 120
volts to ground on two of the poles. The third pole will measure 240 volts to ground. But, you will measure 240 volts between
any two poles.
The concern over the high leg
only comes into play if you are connecting single pole loads to a three phase delta panel. In this case, it is important NOT
to connect a single pole circuit breaker to the phase that measures 240 volts to ground or you will damage any devices connected
to this circuit. For three-phase equipment, the high leg is not a concern since you are utilizing power over all three poles
simultaneously. The same rules apply for higher voltage
configurations (i.e. 277/480 volt systems, etc.) as with the low voltage example cited here.
I need a 220 volt circuit on the opposite end of my house
from the panel. Do I have to run a new line, or can I use two existing 110 circuits somehow?
You would have to install a new circuit. It's not legal or safe to derive 220 volts
from two separate and existing 110 volt circuits.
I need to add a GFCI outlet into an existing circuit but
because of the location it would be difficult to add the additional outlet "in series." Rather, I'd like to tap
into the existing circuit at a midpoint and wire the outlet in parallel with the existing wiring. Is there any problem with
this?
Technically, all AC receptacles are wired in parallel. They may be 'daisy-chained'
from one to the next, but there is no rule that says you have to add a new one at the end of the circuit - any point would
do. Just be careful not to overload the capacity of the circuit you're connecting to.
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