Sharing neutral

[allinill]

If I am designing fluorescent lighting 277V on phase A,B and C and I want to share the neutral between all 3 circuits. What calculations do I need to do to ensure that the neutral is sized correctly.

 

[Mr. Bill]

Just size it the same as the phase A, B, & C conductors. You don't need any math for the neutral sizing for lighting.

 

[76nemo]

You wouldn't bother upsizing for flourescent lighting and MWBC's???

 

[LarryFine]

Welcome to the zoo, All! :smile:

You wouldn't bother upsizing for flourescent lighting and MWBC's???

They're a bit harmonic-rich, but not enough to overload a conductor that would otherwise have (presuming balanced loading) no normal current.

 

[iwire]

You wouldn't bother upsizing for flourescent lighting and MWBC's???

I do not, but I also design the circuit with only 10 to 12 amps of load on a 20 amp circuit.

 

[chris kennedy]

I do not, but I also design the circuit with only 10 to 12 amps of load on a 20 amp circuit.

Agreed....

 

[jrannis]

Agreed....

Chris,
When you are done with the installation. How about taking some amperage readings and let us know what you ended up with.

 

[chris kennedy]

Chris,
When you are done with the installation. How about taking some amperage readings and let us know what you ended up with.

My pleasure. System is 208y/120. All loads on these lighting circuits are fluorescent electronic ballast with a few EM lights peppered in. Readings taken with a Greenlee 850 (brand new out of the box) and Ideal 61-774.

PNL G1

Cir 20 = 10.1A
Cir 22 = 10.7A
Cir 24 = 10.4A

All on, grounded circuit conductor = 2.8A
Circuits 22 and 24 only = 10.7A
Circuits 20 and 24 only =10.2A
Circuits 20 and 22 only = 10.2A

I also measured 2 more MWBC's and the results are similar. I'll do the math here in a minute to see how these jive.

 

[chris kennedy]

Just ran the math. Looks like I have almost 3A of harmonics with all 3 circuits on and about 1A with any 2 on. Does this make sense?

 

[jrannis]

Thanks Chris,
It would also be interesting to see the neutral load when the circuit is not that expertly balanced. Are you still on that job? Can you turn off circuit #20 and see what the neutral reads?

 

[Mr. Bill]

Can you turn off circuit #20 and see what the neutral reads?

I thought it was 10.7A.

 

[LarryFine]

Can you turn off circuit #20 and see what the neutral reads?

He already did:

Circuits 22 and 24 only = 10.7A

 

[allinill]

Why do you only load lighting circuits to 10 or 12 amps. What would be the downside of putting 15 or 16 amps on a 20A circuit.

 

[Smart $]

Why do you only load lighting circuits to 10 or 12 amps. What would be the downside of putting 15 or 16 amps on a 20A circuit.

Overloading on future changeouts (say replacement ballast/lamps
pulled slightly more current).

Less efficient in terms of I?R loss.

The inability to add future luminaires to a circuit, if and when desired.

 

[mull982]

Overloading on future changeouts (say replacement ballast/lamps
pulled slightly more current).

Less efficient in terms of I?R loss.

The inability to add future luminaires to a circuit, if and when desired.

Is 10-12A or 50-60% loading standard for new lighting circuits?

How about when adding lights to an existing ciruict. Can you load the circuit up to its full potential?

 

[Mr. Bill]

Why do you only load lighting circuits to 10 or 12 amps. What would be the downside of putting 15 or 16 amps on a 20A circuit.

With common last minute changes to the plans I don't want to worry about if the circuit can handle two more amps. Lighting is usually a continuous load so 16A is the max on a 20A circuit.

I also had an issue with T5HO lamps drawing more load than anticipated. The published data is based on 25?C. As the lamps warm up to 35?C the current draw increases about 10% with the increased light ouput. Took awhile to figure that out.

Engineers like margins of safety in their numbers. Reduces liability and upset customers when s*** happens. Helps to cover for unknows as well.

 

[chris kennedy]

It would also be interesting to see the neutral load when the circuit is not that expertly balanced. Are you still on that job?

This was the least balanced of the 3 I checked. Again all non-linear.

PNL G1

Cir 12 = 8.6A
Cir 14 = 7.5A
Cir 16 = 4.7A

All on, grounded circuit conductor = 4.2A
Circuits 14 and 16 only = 5.1A
Circuits 12 and 16 only = 8.9A
Circuits 12 and 14 only = 8.3A

 

[mull982]

With common last minute changes to the plans I don't want to worry about if the circuit can handle two more amps. Lighting is usually a continuous load so 16A is the max on a 20A circuit.

This would mean that 12A on a circuit would be about 80% of the maximum load allowed on the circut. (12/16A = .75%). 80% is the standard loading I see in most electrical designs to account for future 25% future spare capacity (transformers, MCC's, etc...)

 

[glene77is]

... (presuming balanced loading) no normal current.

Larry,
In that case, are you suggesting he could just forget the neutral?
The OP might get confused with this response.

D) D) D) D) D)

Well, that is a surprise. I thought I was typing ascii smiley faces,
and the editor subtitutes graphical smiley faces!

 

[don_resqcapt19]

This was the least balanced of the 3 I checked. Again all non-linear.

PNL G1

Cir 12 = 8.6A
Cir 14 = 7.5A
Cir 16 = 4.7A

All on, grounded circuit conductor = 4.2A
Circuits 14 and 16 only = 5.1A
Circuits 12 and 16 only = 8.9A
Circuits 12 and 14 only = 8.3A

And the calculated neutral currents would be 4.7, 6.6, 7.4 and 8.1 respectively. It is interesting for the circuit 14, 16 combination that the calculated current is greater than the measured current. I wonder why.