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Q: 3 Phase power calculation ( Answered 5 out of 5 stars,   0 Comments )
Subject: 3 Phase power calculation
Category: Science > Physics
Asked by: thehamlet17-ga
List Price: $20.00
Posted: 21 May 2006 20:08 PDT
Expires: 20 Jun 2006 20:08 PDT
Question ID: 731172
I am working on a project and I need some help.

We have a 3 phase 4 wire 400 Amp. 120/208 Volt panel. We  already have
a 3 phase 4 wire 200 Amp sub panel for general lighting and outlets
Now we want to power up  24 X 2000 Watt 120 volt Tungsten bulbs. The
configuration is 8 bulbs on each phase installed star with neutral
wire.It means 3 balance load of 16000 watt on each line installed star
with nuteral wire.
My questions are:
1- Is the 400 Amp main panel enough for whole project?
2- What  size breaker do I need for the 24 bulbs installed star?

Subject: Re: 3 Phase power calculation
Answered By: livioflores-ga on 23 May 2006 02:51 PDT
Rated:5 out of 5 stars

When you have a 3 phase 4 wire 400 Amp. 120/208 Volt panel, you have a
panel that supports currents until 400A per phase.
In the same way when you have a 3 phase 4 wire 200 Amp sub panel, you
have a sub panel that supports 200A per phase.

Since each phase circuit is equal to the others, the 3 phases will be
balanced and doing calculations for only one phase will be enough.

You want to power up  24 X 2000 Watt 120 volt Tungsten bulbs, with 8
bulbs on each phase; this means that the current on each phase will
Ip = 8 * 2000W / 120V = 133.33A

So the current load for each phase will be 133.33A.  
Circuit breakers are intended to protect mostly the conductors rather
than the devices, therefore you need to estimate the ampacity of the
conductors; as a general rule, they are commonly selected at 125% of
the continuous load current, in this case this value is 166.66 Amp.
Remember that the ampere rating of the circuit breaker must not exceed
the ampacity of the conductors.
The ampere rating for the circuit breaker must be between 110% and
125% of the current load, that is between 146.66A and 166.66A.
The National Electrical Code, in cases like this (for overcurrent
protective devices of less than 800A), allows the next higher standard
overcurrent protective device rating (above the ampacity of the
conductors being protected) to be used provided the conductor ampacity
does not already correspond to a standard overcurrent protective
device size.
The more closer standard ampere ratings for circuit breakers are 150A
and 175A, and the more closer conductor ampacities are for the 1 or 0
or 00 AWG cables, depending on models and material.
My suggestion here is to use conductors with an ampacity between 150A
and 170A and a circuit breaker of 175A.

Now I can answer your questions:
1- Is the 400 Amp main panel enough for whole project?

Yes, it supports 400A per phase (confirm this with the vendor, in my
country this statement is valid) and you will draw 133.33A per phase.
This means that the 200A sub-panel will be enough for this project.

2- What size breaker do I need for the 24 bulbs installed star?
For each phase you will need conductors with an ampacity of 150A to
175A (1 or 0 or 00 AWG depending on models and material) and a circuit
breaker of 175A.

"A. W. G. wire table":

"National Electrical CodeŽ - 2005":
To access this document just on the below link, a pop-up window will
open with the required document.

"ELECTRICAL PLAN REVIEW - Overcurrent Protection and Devices,
Short-Circuit Calculations, Component Protection, Selective
Coordination, and Other Considerations":
See "Overcurrent Protective Device Ratings" section, pages 4 and 5.

"Copper wire figures":

More tables for Cable's Ampacity:
"Okonite Ampacity Tables: NFPA 70, National Electrical Code, 600 Volt
Ampacity Tables":

"Wire Gauge - Ampacities":

"Wiring Conductor Ampacity to Temperature Rating":

See also:
"How Big?":
A brief explanation on how to calculate conductors' ampacity according
to NEC codes.

Search strategy:
awg ampacity table
awg ampacity
nec code
circuit breaker calculation

I hope this helps you. Feel free to use the clarification feature if
you find something unclear or incomplete. I will be glad to respond
your requests for further assistance on this question.

thehamlet17-ga rated this answer:5 out of 5 stars
Thanks. I got the same result

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