Sizing a neutral

Status
Not open for further replies.
J

jpd24

Member
I am doing a job for a co- worker at our plant. He bought a kit for building a wood drying system.I calculated that a 100 amp 230 volt feed would handle everything.They are 230 volt motors. I want to pull a 115/230 v feed to the system to have 115 volts for lights and convenience outlets. My question is how would you size the neutral?

Thank You
 
Re: Sizing a neutral

This is design build so the bottom line would be to look at your expected 120 volt loads (lights and receptacles) and simply size for this. You will probably (should) balance these loads across the legs so if you wanted to, you could pull a real small neutral. :) I'd use a conductor for at least 30 amps worth of neutral connected loads if it were me.

No set in stone reason though

Roger

[ January 13, 2004, 08:20 PM: Message edited by: roger ]
 
Re: Sizing a neutral

Roger, your answer could put him in violation of 250.24(B)(1). If this is a 100 ampere service, the grounded conductor will be too small. However, if this is a down stream panelboard, you are correct. :D
 
Re: Sizing a neutral

Good morning Charlie, I thought about that too, but in the wording of the question
I want to pull a 115/230 v feed
Click to expand...
it appears as though we are indeed talking about a feeder. :)

Roger
 
Re: Sizing a neutral

The feed to this system is coming from an existing 200 amp panel which was recently upgraded probably with this system in mind.The only 120 volts on this system will be four 100 watt fixtures, a convenience outlet, and a small boiler.
 
Re: Sizing a neutral

Jpd24, take your known loads, the 100 watt fixtures, the boiler name plate rating, and add a value for the receptacle say 10 amps.

Then give yourself a 100% increase over this number and go with it.

What you are doing has no "set" rule of thumb.

Roger
 
Re: Sizing a neutral

I havent seen the boiler yet, but I know it is a propane fired and small circulator probably no larger than 1/3 hp.
Thanks for the replies.
 
Re: Sizing a neutral

If you were to assume the lighting load was continuous that would give you about 5 amps. The motor at 1/3 hp calculated at 125% of the table rating of 7.2 gives you another 9 amps. The receptacle load is figured at 180 va but I would use 15 amps for load calc purposes. This results in 29 ampscalculated to be on the neutral worst case. As a practical matter however I would use #2 copper for the phases(115A) and even though the calc is 29 amps and you could use a #10 wire for this you never know when someone would want to add load on the 120 volt potion. Therefore I would use a neutral good for at least 50 amps. The cost to do so is negligible when comparing using #8 against #10. Also If you are runnining this in emt you will have to use 1" regardless so you have plenty of room for the larger wire iszes. Remeber the code is a minimum requirement and it never hurtds to bump the wire size up for future loads. hope this helps.
 
Re: Sizing a neutral

Originally posted by jpd24:
I am doing a job for a co- worker at our plant. He bought a kit for building a wood drying system.I calculated that a 100 amp 230 volt feed would handle everything.They are 230 volt motors. I want to pull a 115/230 v feed to the system to have 115 volts for lights and convenience outlets. My question is how would you size the neutral?

Thank You
Click to expand...
Can I be a stickler, unless you are sure what the neutral load is, don't take any chances. Because, if your loads are not balanced, that neutral will be subject to caring that unbalaced portion, but God forbid you have someone decide to plug computers into those outlets. Are the lights fluorescent? I've seen things overheat in the field, and even cause a fire. If I were thoroughly sure of the neutral load, and could size it correctly then I would. Or....and I know you'll all rag on me :)

[ January 21, 2004, 10:44 AM: Message edited by: lady sparks lover ]
 
Status
Not open for further replies.
Top