Feeder Calculation

[Dennis Alwon]

I got a pm from a new member and I thought it best to post his question here. Posted by Jgonzalez

I'm having difficulties with a question. I was given a question and didnt know how to go about it. The question is this,
Determine the feeder capacity needed for a 120/240v. Then given a list to determine feeder capacity for each one then add it all up to determine the feeder load for all of it combined.
List:
water heater 4000w.240v =
Kitchen disposal 1/4hp.240v =
Diswisher 1200w.120v =
Furnace motor 1/4hp.120v
Attic fan 1/2hp.120v =
water pump 1/2hp.240v =

Here are my answers.
1. 4000w÷240v =16.6
16.6 amps x 1.5 = 24.9 rounded up 25amps
2. 2.9amps
3. 1200w÷120v= 10amps
4. 5.8amps
5. 9.8amps
6. 4.9amps

 

[infinity]

I got a pm from a new member and I thought it best to post his question here. Posted by Jgonzalez

I'm having difficulties with a question. I was given a question and didnt know how to go about it. The question is this,
Determine the feeder capacity needed for a 120/240v. Then given a list to determine feeder capacity for each one then add it all up to determine the feeder load for all of it combined.
List:
water heater 4000w.240v =
Kitchen disposal 1/4hp.240v =
Diswisher 1200w.120v =
Furnace motor 1/4hp.120v
Attic fan 1/2hp.120v =
water pump 1/2hp.240v =

Here are my answers.
1. 4000w÷240v =16.6
16.6 amps x 1.5 = 24.9 rounded up 25amps
2. 2.9amps
3. 1200w÷120v= 10amps
4. 5.8amps
5. 9.8amps
6. 4.9amps

Did you mean 1.25?

 

[Dennis Alwon]

Sorry to dump this but I got it late at night so I posted and went to bed. I hope the op responds.

I agree that for a water heater we use 1.25 but for overcurrent protective device one would use a max at 150%

422.11(E) Single Non–Motor-Operated Appliance. If the branch
circuit supplies a single non–motor-operated appliance,
the rating of overcurrent protection shall comply with
the following:
(1) Not exceed that marked on the appliance.
(2) Not exceed 20 amperes if the overcurrent protection rating
is not marked and the appliance is rated 13.3 amperes
or less; or
(3) Not exceed 150 percent of the appliance rated current if
the overcurrent protection rating is not marked and the
appliance is rated over 13.3 amperes. Where 150 percent
of the appliance rating does not correspond to a
standard overcurrent device ampere rating, the next
higher standard rating shall be permitted.

 

[augie47]

Real world you would need to watch which phase had the 120v loads but this sounds more like a test question so since you are dealing with both 240 and 120v loads I think the more appropriate method would be to convert the loads to kw.
In regard to the water heater, if I recall correctly the Code requires the branch circuit to be calculated at 125% but not the feeder.
You would, however, need to add 25% of the largest motor load per 430.24

Bit rushed this morning but I;'ll play with the calculations alter if no one else does.

 

[Smart $]

...so I posted and went to bed.

I agree that for a water heater we use 1.25 but for overcurrent protective device one would use a max at 150%

Are you sleep posting again, Dennis? :blink:

 

[augie47]

FWIW:
My calculations:

WH 4000
Disp 696
DW 1200
Furn 696
Fan 1176
WP 1176
25% 294

​ 9238 w @ 250 = 38.5 amps

 

[Dennis Alwon]

Are you sleep posting again, Dennis? :blink:

Could be-- I don't remember posting that....hahaha

 

[augie47]

Could be-- I don't remember posting that....hahaha

I believe the current excuse du-jour is "Russian hackers"

 

[adamscb]

On any of those pieces of equipment does it list VA's? Or power factor? I would calculate the currents based off of total power, not just real power. The cables have to carry both the real and reactive power. Since these loads are small I would think you could get away with sizing based off of Watts, but I would size based off of VA or kVA just to be sure.

And just my two cents, I would size your cables based on the feeder breaker. When the NEC says 1.25x the biggest load and the summation of the remaining loads, that completely ignores the fact you may put more things in this panel in the future.

 

[Smart $]

FWIW:
My calculations:

WH 4000
Disp 696
DW 1200
Furn 696
Fan 1176
WP 1176
25% 294

9238 w @ 250 = 38.5 amps

Where's the 220.53 75% demand allowance for four or more fixed-in-place appliances? :angel:

 

[augie47]

Where's the 220.53 75% demand allowance for four or more fixed-in-place appliances? :angel:

I'd say that would be a legitimate application. I originally gave it some thought but dismissed the idea for no good reason

 

[Jgonzalez]

feeder capcity

feeder capcity

FWIW:
My calculations:

WH 4000
Disp 696
DW 1200
Furn 696
Fan 1176
WP 1176
25% 294

​ 9238 w @ 250 = 38.5 amps

Thank you guys so much for the answers but it would be a bigger help to me if you could tell me how and why you got the answers you did.. That way not only would I understand but would be helpful to me.

 

[Jgonzalez]

feeder capcity

feeder capcity

Hello everyone and thanks so much for the answers but if you could not only give the answers but explain how you got them that way I can go through the materials that I have and understand a lot better. And again this would be for that feeder capacity question. I do have a learning disability so it takes me a little longer to learn it and understand it but I do wont to learn it. So thanks everyone so much and being patient with me.

 

[Jgonzalez]

Three Phase

Three Phase

Hello everyone its me again have another good one for yall and of course its three phase he it is. And again if everyone wouldn't mind please not only give the answer but show how you got the answer. And sorry for being a pain I just really wont to learn this and for me this is the best way.

If three three-phase motors are connected to a 120/240 single to three-phase converter each motor has a full-load current of 8 amps ,what is the min amperage of the conductors feeding the phase converter.

My answer was I just added 8ampsX3=24amps

 

[Jeff Crandall]

Reply submitted

I got a pm from a new member and I thought it best to post his question here. Posted by Jgonzalez

I'm having difficulties with a question. I was given a question and didnt know how to go about it. The question is this,
Determine the feeder capacity needed for a 120/240v. Then given a list to determine feeder capacity for each one then add it all up to determine the feeder load for all of it combined.
List:
water heater 4000w.240v =
Kitchen disposal 1/4hp.240v =
Diswisher 1200w.120v =
Furnace motor 1/4hp.120v
Attic fan 1/2hp.120v =
water pump 1/2hp.240v =

Here are my answers.
1. 4000w÷240v =16.6
16.6 amps x 1.5 = 24.9 rounded up 25amps
2. 2.9amps
3. 1200w÷120v= 10amps
4. 5.8amps
5. 9.8amps
6. 4.9amps

 

[Dennis Alwon]

I am bumping this thread because the op has written some info that was still moderated till now

 

[Smart $]

Hello everyone its me again have another good one for yall and of course its three phase he it is. And again if everyone wouldn't mind please not only give the answer but show how you got the answer. And sorry for being a pain I just really wont to learn this and for me this is the best way.

If three three-phase motors are connected to a 120/240 single to three-phase converter each motor has a full-load current of 8 amps ,what is the min amperage of the conductors feeding the phase converter.

My answer was I just added 8ampsX3=24amps

I realize this may be a practice or test question, but does it provide the converter's output voltage, i.e. the motors' operating voltage?

• Determine the kVA of each motor (voltage times 8A times the square root of three because it is three phase).
• Multiply that value by three
• Divide that value by converter input voltage (no root of three because it is single phase).
• Multiply that value by the appropriate factor as determined by 455.6(A).

 

[Jgonzalez]

Three Phase

Three Phase

I realize this may be a practice or test question, but does it provide the converter's output voltage, i.e. the motors' operating voltage?

• Determine the kVA of each motor (voltage times 8A times the square root of three because it is three phase).
• Multiply that value by three
• Divide that value by converter input voltage (no root of three because it is single phase).
• Multiply that value by the appropriate factor as determined by 455.6(A).

And no converter output its just as you see on the question

So when I do this I'm doing this for both voltages
120v X 8 amps X 1.73 =1660.8 X 3=4982.4
240v X 8 amps X 1.73 =3321.6 X 3=9964.8

 

[Jgonzalez]

Three Phase

Three Phase

And no converter input its just as you see on the question

So when I do this I'm doing this for both voltages
120v X 8 amps X 1.73 =1660.8 X 3=4982.4
240v X 8 amps X 1.73 =3321.6 X 3=9964.8

All I have is the volts and amps on that question.
Volts X Amps X 1.73 I get my answer and multiply by 3 times for the three motors.
But I did that for both voltages and kinda suck here or really dont understand from here.
I did look that up about 455.6 A and says to 125%

 

[Dennis Alwon]

You don't use 1.73 for single phase only for 3 phase motors