Reduced Neutral

Status
Not open for further replies.
J

jpmccleery

Member
Location
Florida
Does Article 220.22 prohibit reduction of neutral conductor to a multifamily dwelling unit that is being served with 120/208 Volt single phase service?
 
Re: Reduced Neutral

If what you are saying is true , then why in Annex D, Examples D5(a) and (b) a calculation for reducing the neutral is provided?
 
Re: Reduced Neutral

highkvoltage,
120/208 Single phase??????
Click to expand...
In some areas it is common for a dwelling unit service to be fed with two phases and the grounded conductor from a 208/120 volt 4 wire wye system.
Don
 
Re: Reduced Neutral

Yes, the service to each dwelling unit is 120/208V, single phase, distributed from a 600 Amp 120/208 V. 3 phase 4 wire meter bank. I am computing the dwelling feeder using 220.30 (Optional Calculation - Dwelling Unit) which permits the use of 220.22 to calculate the neutral. Is it correct to size the neutral by multiplying the maximum unbalance between the neutral and the load on the largest ungrounded conductor by 140%?
 
Re: Reduced Neutral

posted July 14, 2004 01:25 PM
--------------------------------------------------------------------------------
OK, Here's my situation: I have 1248 SF apartment with the following appliances:
(2) appliance Circuits
(1) 8800W Range
(1) 744W Dishwasher
(1) 3928W Stacked Washer/Dryer (single outlet)
(1) 1200W AHU blower
(1) 5625W Heating Element

Calculating the Service according to 220.30 I came up with a 95 Amp service. What is the min. neutral conductor allowed by 220.22?
 
Re: Reduced Neutral

Originally posted by jpmccleery:
posted July 14, 2004 01:25 PM
--------------------------------------------------------------------------------
OK, Here's my situation: I have 1248 SF apartment with the following appliances:
(2) appliance Circuits
(1) 8800W Range
(1) 744W Dishwasher
(1) 3928W Stacked Washer/Dryer (single outlet)
(1) 1200W AHU blower
(1) 5625W Heating Element

Calculating the Service according to 220.30 I came up with a 95 Amp service. What is the min. neutral conductor allowed by 220.22?
Click to expand...
Just a question, but what happened to the other loads in the house...ie refrig, convience outlets, computers, lighting, and other miscellaneous loads?

Just a question...


Lady :)
 
Re: Reduced Neutral

:confused: Have you ever sized a residential service? Try 3 watts x the area or 3744 VA. Now tell me what the min. size neutral you can provide?
 
Re: Reduced Neutral

Lady,
when you have those loads calculated, then you use 250.66 and size your neutral, based on you other phase conductors.
Click to expand...
You have to use the larger of what is required by 220.22 or 250.24(B)(1).
The 3VA per foot would just be lighting.
Click to expand...
See 220.3(B)(10).
Don
 
Re: Reduced Neutral

Originally posted by don_resqcapt19:
Lady,
when you have those loads calculated, then you use 250.66 and size your neutral, based on you other phase conductors.
Click to expand...
You have to use the larger of what is required by 220.22 or 250.24(B)(1).
The 3VA per foot would just be lighting.
Click to expand...
See 220.3(B)(10).
Don
Click to expand...
I'm not surprised by a dispute from Don, once again. I seem to always be his target. :)
 
Re: Reduced Neutral

Back to the original question really quick. Remember that on a three wire circuit supplied by a four wire system, the nuetral carries a lot of current. For example, if you have 100 amps on phase "A", 100 amps on phase "B" and zero amps on phase "C", you will have 100 amps on the nuetral. This can be proven by the following formula:

Total nuetral current= The sqaure root of the following: (A squared+ B sqaured+ C sqaured)-(AB+BC+CA).

In my example, you would take the sqare root of: (10,000 + 10,000 + 0) - (10,000 + 0 + 0), which eqauls the sqaure root of 10,000= 100 amps on the nuetral...that is why no reduction is permitted.

Lady: Don is right.
220.3(B)(10)(10) Dwelling Occupancies. In one-family, two-family, and multifamily dwellings and in guest rooms of hotels and motels, the outlets specified in (1), (2), and (3) are included in the general lighting load calculations of 220.3(A). No additional load calculations shall be required for such outlets.
(1) All general-use receptacle outlets of 20-ampere rating or less, including receptacles connected to the circuits in 210.11(C)(3)
(2) The receptacle outlets specified in 210.52(E) and (G)
(3) The lighting outlets specified in 210.70(A) and (B)
Click to expand...
Notice that items 1, 2 and 3 are for the receptacle outlets being discussed. :) Also, what value would you assign these receptacles? The 180VA provision is pretty specific when it says that it does not apply to dwelling units. :)
 
Re: Reduced Neutral

Originally posted by ryan_618:
Back to the original question really quick. Remember that on a three wire circuit supplied by a four wire system, the nuetral carries a lot of current. For example, if you have 100 amps on phase "A", 100 amps on phase "B" and zero amps on phase "C", you will have 100 amps on the nuetral. This can be proven by the following formula:

Total nuetral current= The sqaure root of the following: (A squared+ B sqaured+ C sqaured)-(AB+BC+CA).

In my example, you would take the sqare root of: (10,000 + 10,000 + 0) - (10,000 + 0 + 0), which eqauls the sqaure root of 10,000= 100 amps on the nuetral...that is why no reduction is permitted.

Lady: Don is right.
220.3(B)(10)(10) Dwelling Occupancies. In one-family, two-family, and multifamily dwellings and in guest rooms of hotels and motels, the outlets specified in (1), (2), and (3) are included in the general lighting load calculations of 220.3(A). No additional load calculations shall be required for such outlets.
(1) All general-use receptacle outlets of 20-ampere rating or less, including receptacles connected to the circuits in 210.11(C)(3)
(2) The receptacle outlets specified in 210.52(E) and (G)
(3) The lighting outlets specified in 210.70(A) and (B)
Click to expand...
Notice that items 1, 2 and 3 are for the receptacle outlets being discussed. :) Also, what value would you assign these receptacles? The 180VA provision is pretty specific when it says that it does not apply to dwelling units. :)
Click to expand...
Ok, I see...thanks Ryan.


Lady :)
 
Re: Reduced Neutral

Originally posted by wirebender:
If what you are saying is true , then why in Annex D, Examples D5(a) and (b) a calculation for reducing the neutral is provided?
Click to expand...
Ryan, can you address this question, please?
Click to expand...
From what I can see in the examples, the only nuetral reduction used is for the range. This is specifically allowed in 220.22 for all ranges. I beleive this is because, as you would agree, the nuetral doesn't do much on a range. In fact, there are probably ranges out there that don't even use a nuetral. The way I read 220.22 is this: Ranges are subject to nuetral reduction...always. I think that section is poorly written and needs the verbage re-arranged in the section. I think that, other than for ranges and dryers, you cannot reduce the nuetral capacity on a three wire feeder supplied by a four wire service.

I think if you read it like this it might help: I think that after the words....[table 220.18 for dryers.] a new paragraph should begin addressing the rest of the section, permitting no reduction other than nuetrals of ranges and dryers.

Just my opinion, and it is certainly subject to argument!!! But if you think about it, it really does make sense.
 
Re: Reduced Neutral

Remember that on a three wire circuit supplied by a four wire system, the nuetral carries a lot of current. For example, if you have 100 amps on phase "A", 100 amps on phase "B" and zero amps on phase "C", you will have 100 amps on the nuetral
Click to expand...
:D :confused:
 
Re: Reduced Neutral

Originally posted by wirebender:
:D :confused:
Click to expand...
On the range you probably have about 3 amps of 120-V load. So, doing the formula, you have the sqaure root of: (9+0+0)-(0+0+0)=3 amps.

Think of it this way: If you have a single phase 208-V water heater that doesn't require a nuetral, you could run a nuetral to it, but that doesn't mean there will be a load on it. That is because the equipment doesn't require a nuetral to function. Same with a range, other than the clock and the light.

So, the way I would do the math is this: Phase "A" carries 29 amps, phase "B" carries 26 amps, the nuetral carries three amps from the 120-v portion of the load.

I'm not an electrical engineer, so don't take my thoughts here as law. Does anyone else have any insight?
 
Re: Reduced Neutral

Okay, I understand what you're saying but it seems like that would allow the neutral to be sized for just the 120v load plus 70% of the range and dryer. You shouldn't have to include any of your other 208v loads. But then that wouldn't jibe with what you (and others) have said about not reducing the neutral on 3-wire 208 services. Of course I don't see the need to downsize unless it is an extremely long run, I'm just curious as to what the rule is. Thanks, I gotta go to work. :( (99 degrees and rising)
 
Status
Not open for further replies.
Top