when the three phase are balance

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Volta

Senior Member
Location
Columbus, Ohio
I assume you mean the current per leg. The NEC does not specify any limit. It is typically a job specification, often 10% maximum deviation. There are good reasons to limit the differences, but are less of an outright safety concern and are left to the design of the system.

Welcome to the forum.
 

e57

Senior Member
I don't think there is one - branch circuit and feeder neutrals should be able to handle the unbalanced load. (See 220.61)

I think the closest the code comes to addressing it 210.11(B) and it's not saying much...

Transformers and generator spec's on the other hand may require a certain level of balance... Otherwise it is just good practice...
 

e57

Senior Member
I have seen many Scope of Works which is based on NEC and allows only 5% diviation.
You have a specific code to point to for that? IMO there is no way to get, or more importantly maintain a balance that close on a system supplying single leg loads - or 2 leg loads on a 3 phase system that are in any way intermittent or end user switched.
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
You have a specific code to point to for that? IMO there is no way to get, or more importantly maintain a balance that close on a system supplying single leg loads - or 2 leg loads on a 3 phase system that are in any way intermittent or end user switched.


We had in most of Scope Work as Below

"Contractor shall provide (design and install) circuits for all mechanical equipment and any other equipment that requires power and make the final connections.
All loads shall be coordinated to provide balanced loading. Phase imbalance at each panel shall not exceed 5%.
Voltage Drop for branch circuits shall be limited to no more than 3%; voltage drop for branch and feeder circuits combined shall be limited to no more than 5%.
All circuit breakers shall use down-stream coordination to ensure the breaker nearest a fault or overload is the first to trip."

We were only allowed maximium for 5%

Here is one exmaple of calculation.

For exampel a technicia measures the voltage on a nominal 460 V system to be:

Phase 1 to Phase 2 475 V

Phase 1 to Phase 3 448 V

Phase 2 to Phase 3 461 V

The average voltage is:

(475 V + 448 V + 461 V )/ 3 = 461.3 V

The maximum deviation from average is:

475 V - 461.3 V = 13.7 V

The voltage unbalance is

13.7 V / 461.3 V = 0.0297 or 2.97%
 

e57

Senior Member
We had in most of Scope Work as Below

~~~~~~
But you understand there is no code to point to in that scope or spec.

One could balance branch panels and feeders to be relitively close or even completely equal if basing that upon connected load - but could not assume that load be in use to maintain it. Otherwise we would be required to keep everything "on" in order to do so - and that would not be very realistic.... ;)
 
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