Feeder load calculations

[kman67rsss]

I am trying to figure out what size neutral i need to run for my sub-panel. I went to school for this and went to work for a utility and don't use this any more. Its in a detached residential garage. I am running a 100 amp sub with #1 AL, and a #6 CU grnd. My confusion comes into sizing the neutral. I hve been looking at annex D, and 220.*** and have come up with a #6 CU/4 AL for a neutral. I was hoping someone that deal with this stuff on a regular basis could double check my work. I used the residential dwelling standard for 3va per sq ft. I have 2160sq ft X 3VA=6480
50 amp welder 12kva
30 amp compressor 5760
1 required 20 amp circuit 1920

That gives me a total of 26160 va

1st 3k @100% = 3000
rest at 35% 23160 X .35 = 8106

8106+3000=11106

11106/240=46.275
Next biggest size is 50 amp #6/#4


Does this look right? Am i missing anything? Thanks for the help

 

[gatchel]

Doesn't the smaller neutral apply only to the main panel feeder, not any subs?

Not an electrician, I remember reading it somewhere. Could be wrong. I'll see if I can search it for you.

 

[kman67rsss]

It very well may be that way. i didn't see any exceptions and the guy whose brain ive been picking didn't think it mattered.

 

[BioHazard]

I don't see any reason why you wouldn't use the same size as your main power wires. My shop has a 100A (3 phase) subpanel and it's fed underground with four 1/0 feeders (3 hots and neutral) and a much smaller, un-determined ground wire.

 

[hidollartoys]

Derating the neutral is only allowed for service conductors. It does not apply to sub-panel applications.

 

[Ch3No2]

I don't see any reason why you wouldn't use the same size as your main power wires. My shop has a 100A (3 phase) subpanel and it's fed underground with four 1/0 feeders (3 hots and neutral) and a much smaller, un-determined ground wire.

Wouldn't your Neutral be considered a "smaller" wire if it's serving 3 hot legs?

 

[Aceman]

Derating the neutral is only allowed for service conductors. It does not apply to sub-panel applications.

That's not true, otherwise 220.61 wouldn't exist.

 

[hidollartoys]

That's not true, otherwise 220.61 wouldn't exist.

220.61 pertains to the unbalanced loads of HOUSEHOLD electric ranges, wall mounted ovens, counter mounted cooking units and electric dryers based on the derating parameters in 220.55 and 220.54. and for our discussion.... "any portion of the load over 200 amps". This is based on the total dwelling calculated load procedure which recoginizes "service" as the conductors from the utility co connection to the meter base and/or service disconnect and "feeder' as the conductors from the meter/service disconnect and the main panel. "Feeder" is also used to descride the branch circuit to these HOUSEHOLD appliances especially with regards to multiple cooking units such as double ovens, oven/microwave combos and any cooking appliance that has mulitple components, each with its own pigtail that all terminate at the "feeder" connection.

This is my understanding. Therefore my comment stating that neutral derating does not pertain to sub-panel installations.

 

[jvitez]

Are you allowed to use the residence percentage calculations for a subpanel in a detached garage? I thought residential load calculations were under the assumption that everything will not be turned on at the same time. In a garage, are you not supposed to look at the actual loads?

Eg. all the lights on, electric heater on, compressor on equals X load, so base your feeder calculations on that sum. You probably would not need to run the compressor while the welder is used, so take the higher load of the two. This is my understanding, so someone please correct me if I am wrong.

 

[hidollartoys]

Are you allowed to use the residence percentage calculations for a subpanel in a detached garage? I thought residential load calculations were under the assumption that everything will not be turned on at the same time. In a garage, are you not supposed to look at the actual loads?

Eg. all the lights on, electric heater on, compressor on equals X load, so base your feeder calculations on that sum. You probably would not need to run the compressor while the welder is used, so take the higher load of the two. This is my understanding, so someone please correct me if I am wrong.

The short answer is "no". The code only allows derating "service loads" of dwelling units, hospital rooms, hotel/motel rooms and warehouses. Garages are only included in dwelling calcs if they are attached and are supplied from the main panel and not a sub-panel. Your detatched garage is a seperate structure. While the code allows for a "load calc' to size the "branch circuit" or service, it does not allow for "derating". This is based on Article 220 and tables 220.12 and 220.42 (2005 code).

 

[BioHazard]

Wouldn't your Neutral be considered a "smaller" wire if it's serving 3 hot legs?

No...to put it basically you can use 1 neutral for 3 hots with 3 phase service exactly the same as using 1 neutral for 2 hots with normal 240v service. In a normal single phase system the two hots cycle back and forth, each line only using 1/2 of the neutral. In a 3 phase system, the cycle is like a circle between each of the three hots and the neutral only gets 1/3 of it.

Kinda-sorta.

 

[jvitez]

The short answer is "no". The code only allows derating "service loads" of dwelling units, hospital rooms, hotel/motel rooms and warehouses. Garages are only included in dwelling calcs if they are attached and are supplied from the main panel and not a sub-panel. Your detatched garage is a seperate structure. While the code allows for a "load calc' to size the "branch circuit" or service, it does not allow for "derating". This is based on Article 220 and tables 220.12 and 220.42 (2005 code).

That's what I thought, in a fuzzy kind of way. Thank you for the clear explanation.

 

[W-Cummins]

No...to put it basically you can use 1 neutral for 3 hots with 3 phase service exactly the same as using 1 neutral for 2 hots with normal 240v service.

So far so good!

In a normal single phase system the two hots cycle back and forth,

Yes on a 240V center tapped transformer system.

each line only using 1/2 of the neutral.

not the case here..... on a 240v system ( hot to hot) the grounded conductor, (the "neutral") gets NONE of the load. When you operate at 120v to ground the system can get from 0-100% of the load depending on how it's distributed between the hot conductors.

So for an example

(1.)
100 amps of 120 volts to ground on conductor "A" and none on conductor "B" puts 100 amps on the grounded service conductor.

(2.)
100 amps of 120V to ground on each hot conductor, 0 amps on the grounded service conductor.

(3.)
150 amps of 120 volts to ground on conductor "A" and 100 amps on conductor "B" puts 50 amps on the grounded service conductor.

This is also seen on a multiwire branch circuit ( an Edison circuit). So you can run a 12 gauge 3 wire with equipment ground cable, and the hots "Share" the grounded conductor and allow 2 20amp circuits to load the one grounded conductor from 0-20 amps.

In a 3 phase system, the cycle is like a circle between each of the three hots and the neutral only gets 1/3 of it.
Kinda-sorta.

Well not really, in the 3 phase system there is no "need" for any grounded conductor. Now if you want to get single phase out of a 3 phase wye connected service, (120v and 277v are common in the US), you will not get the same reduction on the grounded service conductor as you do with the 240V single phase system. The reason is the phases are 120 degree apart in a 3 phase system and will not completely cancel out.

William....

 

[Aceman]

220.61 pertains to the unbalanced loads of HOUSEHOLD electric ranges, wall mounted ovens, counter mounted cooking units and electric dryers based on the derating parameters in 220.55 and 220.54. and for our discussion.... "any portion of the load over 200 amps". This is based on the total dwelling calculated load procedure which recoginizes "service" as the conductors from the utility co connection to the meter base and/or service disconnect and "feeder' as the conductors from the meter/service disconnect and the main panel. "Feeder" is also used to descride the branch circuit to these HOUSEHOLD appliances especially with regards to multiple cooking units such as double ovens, oven/microwave combos and any cooking appliance that has mulitple components, each with its own pigtail that all terminate at the "feeder" connection.

This is my understanding. Therefore my comment stating that neutral derating does not pertain to sub-panel installations.

Let's try this again:

Start with 215.2(A)1. The feeder conductors(including the neutral) are calculated from Art. 220 parts III, IV, V. The neutral shall not be smaller than the minimum EGC size as determined from 250.122.

220.61(A) states you must determine the neutral load as the maximum unbalanced load as determined from this article. That being Article 220.

220.61(B) states that an additional 70% derate is allowed on feeders supplying ovens, dryers, etc.

Outside feeders would start with 225.5 and end up in Art. 220.

 

[BioHazard]

not the case here..... on a 240v system ( hot to hot) the grounded conductor, (the "neutral") gets NONE of the load.

Good point...I was only thinking about 120 stuff. I tend to shy away from 240v things since the "208v" I have in my shop is kinda lame. It's like trying to hook something up through a long extension cord...unless it was designed for 208.

 

[hidollartoys]

Let's try this again:

Start with 215.2(A)1. The feeder conductors(including the neutral) are calculated from Art. 220 parts III, IV, V. The neutral shall not be smaller than the minimum EGC size as determined from 250.122.

220.61(A) states you must determine the neutral load as the maximum unbalanced load as determined from this article. That being Article 220.

220.61(B) states that an additional 70% derate is allowed on feeders supplying ovens, dryers, etc.

Outside feeders would start with 225.5 and end up in Art. 220.

I agree with all you have stated but do not understand how this applies to a sid-panel application. Again my understanding of "feeder" in this instance (and the way it has been explained to me by individuals far more versed in the code) is the conductors that are between the meter base/ service disconnect and the main panel. The code uses terms differently in different sections. If the "feeder" between a main panel and subpanel was allowed to be derated just as service entrance conductors, then why is not allowed to use table 310.15(B)(6) to size the conductors?

 

[Aceman]

I agree with all you have stated but do not understand how this applies to a sid-panel application. Again my understanding of "feeder" in this instance (and the way it has been explained to me by individuals far more versed in the code) is the conductors that are between the meter base/ service disconnect and the main panel. The code uses terms differently in different sections.

Read the definition of feeder in Art. 100. It applies to any circuit conductors between a power supply source and the final branch circuit overcurrent device.

If the "feeder" between a main panel and subpanel was allowed to be derated just as service entrance conductors, then why is not allowed to use table 310.15(B)(6) to size the conductors?

I have no idea how the code panel determines it.

 

[hidollartoys]

OK aceman after a more through search I understand and also defend YOUR argument.

There are situations in sub-panel installations where derating the neutral conductor would have application, albiet only a small potential reduction in overall cost. Since most of what we discuss here are garages and most dont have cooking appliances(even though some have ovens) and electric dryers, I doubt that we can actually take advantage of it.