use of proper equations and voltage values - 3 phase or single phase

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ZMIKE123

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hi everyone
this is such a great site
i have 2 questions and wonder if someone could help me out please
if i have a 208/120 voltage source, / 3 phase / 42 ckt. / 200amp panel
on one circuit i have lights / 20a breaker /
to compute the power on this circuit
p=vi = 120v x 20a x .8 correct ? 1920 watts allowed on this circuit
1. why do i use the lower voltage value in this formula ? 120v not 208v
2. do i use the 3 phase formula or single phase formula for this ltg. circuit ? i dont use the 1.73 factor here correct ?
even though this is three phase power ? why ?
i am confused a bit
 
If you are running a 120V circuit, (hot and neutral) you use 120.
If you are using a 208V circuit comprised of two of the phase wires, use 208.
If you are using all three of the phase wires, use 208 and 1.732.

oh, and the .80 is used to derate a circuit that is considered continuous...loaded nonstop for 3 hrs. or more.
A lighting circuit could be either. Certainly all night exterior lighting is continuous but a closet light on a motion sensor may not.
 
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If its a 20A 1-pole breaker, your circuit is connected 120V line to neutral, making it a single phase circuit. Its a 3? source, but you can still connect single-phase circuits to it. Connecting a 2-wire circuit to a 2-pole breaker would be 208V 1?.
 
THANK YOU !!!!!!!!!!!!

THANK YOU !!!!!!!!!!!!

conmgt said:
If you are running a 120V circuit, (hot and neutral) you use 120.
If you are using a 208V circuit comprised of two of the phase wires, use 208.
If you are using all three of the phase wires, use 208 and 1.732.

oh, and the .80 is used to derate a circuit that is considered continuous...loaded nonstop for 3 hrs. or more.
A lighting circuit could be either. Certainly all night exterior lighting is continuous but a closet light on a motion sensor may not.
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Thank you so much for explaining
but it is a bit confusing
you have two voltage values 208/120 and the formula doesnt tell which one to use
they just tell you use "E" but which one ????
so it depends on that particular circuit and what is on it
also when it comes to the three phase formula you dont always use it even though the voltage is 3 phase
again it depends on what is on that particular circuit
 
ZMIKE123 said:
Thank you so much for explaining
but it is a bit confusing
you have two voltage values 208/120 and the formula doesnt tell which one to use
they just tell you use "E" but which one ????
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It's always the voltage applied to the load(s)

ZMIKE123 said:
so it depends on that particular circuit and what is on it
Click to expand...
Correct

ZMIKE123 said:
also when it comes to the three phase formula you dont always use it even though the voltage is 3 phase
again it depends on what is on that particular circuit
Click to expand...
Correct again but I would add that it is more correct to say "it depends on whether the circuit is a single phase circuit or a three phase circuit", see Smart's post.

Roger
 
THANK YOU

THANK YOU

Smart $ said:
If its a 20A 1-pole breaker, your circuit is connected 120V line to neutral, making it a single phase circuit. Its a 3? source, but you can still connect single-phase circuits to it. Connecting a 2-wire circuit to a 2-pole breaker would be 208V 1?.
Click to expand...

Thank you for your help !!!!!!!!!!!!
 
480/277 vots

480/277 vots

roger said:
It's always the voltage applied to the load(s)

Correct

Correct again but I would add that it is more correct to say "it depends on whether the circuit is a single phase circuit or a three phase circuit", see Smart's post.

Roger
Click to expand...

Thanks again
so if i have a 480/277 voltage supply / 3 ph./ 20a breaker and a ltg circuit
the load would be 277v x 20a x .8 many more lights i can put on this breaker
4432 watts on this circuit
 
ZMIKE123 said:
Thanks again
so if i have a 480/277 voltage supply / 3 ph./ 20a breaker and a ltg circuit
the load would be 277v x 20a x .8 many more lights i can put on this breaker
4432 watts on this circuit
Click to expand...

Correct but, you wouldn't necessarily have to use the continuous load derating if the fixture usage was not in fact continuous.

Roger
 
receptacles off 480/277

receptacles off 480/277

roger said:
Correct but, you wouldn't necessarily have to use the continuous load derating if the fixture usage was not in fact continuous.

Roger
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thank you roger
one other question
if you have a 480/277v supply
can you run general receptacles off the 277 voltage
also and how many
 
ZMIKE123 said:
is the voltage too big for them ?
why can lights be run off 480/277 but not receptacles ?
this is a bit confusing to me
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Ok, I don't mean to sound condenscending but, I think you're in over your head. What is the extent of your training and experience of US electrical systems?


Do you have a copy of the NEC?


Roger
 
If you do have a copy of the NEC please read through article section 210.6, it won't answer all of your questions but it will be a start.

Roger
 
roger said:
If you do have a copy of the NEC please read through article section 210.6

Roger
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im a cad operator
learning electrical design on my own
but different people you talk too give you different answers
or just say put 6 or 8 outlets on a 20a breaker or 12 - 100watt lights
not explaining why and what the constraints are
speaking in a very general format but they are still correct
 
ZMIKE123 said:
im a cad operator
learning electrical design on my own
but different people you talk too give you different answers
or just say put 6 or 8 outlets on a 20a breaker or 12 - 100watt lights
not explaining why and what the constraints are
speaking in a very general format but they are still correct
Click to expand...

thanks roger i will and i do have copy
but most professionals i know only know very small parts of the code
contractors and even engineers
the code can be very confusing also and mind boggling to know it all
i think it was written by lawyers more so then engineers
the wording alone
 
ZMIKE123 said:
im a cad operator
learning electrical design on my own
but different people you talk too give you different answers
or just say put 6 or 8 outlets on a 20a breaker or 12 - 100watt lights
not explaining why and what the constraints are
speaking in a very general format but they are still correct
Click to expand...

Ok, the fact is, you will not be able to get very far just talking to people who may or may not be giving you correct information.

You need to look into purchasing some basic training materials or classes and go from there.

This trade at all levels from Electricians to Engineers can be confussing even to those with years of training and hands on experience.

Roger
 
ZMIKE123 said:
thanks roger i will and i do have copy
but most professionals i know only know very small parts of the code
contractors and even engineers
the code can be very confusing also and mind boggling to know it all
i think it was written by lawyers more so then engineers
the wording alone
Click to expand...

Well, you and I are on the same page. :)


Roger
 
ZMIKE123 said:
is the voltage too big for them ?
why can lights be run off 480/277 but not receptacles ?
this is a bit confusing to me
Click to expand...
You would not want a receptacle that provides 277V to look just like one that is connected to 120V! You don't want be able to plug in a 120V appliance to a 277V source.
 
THANK YOU EVERYONE

THANK YOU EVERYONE

thanks everyone for all your help
i know i have alot to study and learn
but i am picking up alot here and by self study
but from what see this is an immense field which would take years of study to master every concept
im just interested in a very small slice of it to get the job done
my boss is a PE and he gave me three charts from the NEC
and said this is all you need for most of the work here
cable and conduit sizes
grounding wire sizes
 
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