Confirmation of Wire Size and Fuse Confirmation

[goodoboy]

Hello,

Can I have an extra check on my wire and fuse size calculations please?

480V 3 phase, 50amp load, 1000 ft from power supply to load.

3% allowable voltage drop per code.

Results

AWG 1 for the 3 phases conductors and Ground sized to AWG 10 (Table 250.122)

Fuse size: 125% of 50 = 62.5 amp fuse. I choose 70. (210-20(a), and 215-3)


Thanks for the help. I hope I did well.


Thanks for your help in advance

 

[augie47]

Mike's Voltage Drop Calculator shows a #1 Cu so you are probably O.K. on that part.
Assuming this is not a motor load, your 70 amp fuse would be fine, possibly not necessary depending on the nature of the load, but acceptable.
The flaw in the system is the #10 equipment ground. 250.122(B) requires a proportional increase in the equipment ground when the phase conductors are increased in size.

 

[david luchini]

AWG 1 for the 3 phases conductors and Ground sized to AWG 10 (Table 250.122)

You will need to increase your equipment ground size proportionally to the increase in the size of your ungrounded conductors. See 250.122(B).

Edit: Or what Gus said.

 

[goodoboy]

You will need to increase your equipment ground size proportionally to the increase in the size of your ungrounded conductors. See 250.122(B).

Edit: Or what Gus said.

Without any voltage drop considerations my conductors wire size would be AWG 8 ( 310.16) for the 40 amp load. 40 x 1.25 = 50amp corrected. And ground would be AWG 10 (250.122). correct?

But due to voltage drop I need to increase my conductor wires to AWG 1/0.

What shall I increase my ground wire size to?

 

[goodoboy]

Mike's Voltage Drop Calculator shows a #1 Cu so you are probably O.K. on that part.
Assuming this is not a motor load, your 70 amp fuse would be fine, possibly not necessary depending on the nature of the load, but acceptable.
The flaw in the system is the #10 equipment ground. 250.122(B) requires a proportional increase in the equipment ground when the phase conductors are increased in size.

The motor is a A/C condensor unit.

I am scratching my head on this one "250.122(B) requires a proportional increase in the equipment ground when the phase conductors are increased in size.":?

how do I know how much to increase the ground wire to.

 

[GoldDigger]

The motor is a A/C condensor unit.

I am scratching my head on this one "250.122(B) requires a proportional increase in the equipment ground when the phase conductors are increased in size.":?

how do I know how much to increase the ground wire to.

If it were not for the derating, what size ground would have been required?
If you increase the current carrying conductors by, for example, two wire sizes from the minimum without derating, you need to also increase the EGC by two wire sizes from whatever the original required value would have been.

 

[augie47]

side note: You might check you MOCP on you air conditioner, your 70 amp may not handle the inrush.
(I'm not too sure about your #1 taking the type load into account. You may need to add a softstart)

All that aside...

250.122 tells you that if your initial wire size is increased then so shall the equipment ground.
In your case, let us assume a #8 (You did not provider the MCA on the A/C)
You increase from an #8 to #1. Table 8 tells us a #8 is 16510 cmil, a #1 is 83690 cmil,
whcih would call for an increase of 5.06 on your eq ground.
Assuming a #10, which has an area of 10380, you would need to increase to 52616 cmil or a #3.

 

[david luchini]

250.122 tells you that if your initial wire size is increased then so shall the equipment ground.
In your case, let us assume a #8 (You did not provider the MCA on the A/C)
You increase from an #8 to #1. Table 8 tells us a #8 is 16510 cmil, a #1 is 83690 cmil,
whcih would call for an increase of 5.06 on your eq ground.
Assuming a #10, which has an area of 10380, you would need to increase to 52616 cmil or a #3.

This is the right approach for increasing the EGC proportionally according to circular mil area.

But, didn't catch this before...

AWG 1 for the 3 phases conductors and Ground sized to AWG 10 (Table 250.122)

Fuse size: 125% of 50 = 62.5 amp fuse. I choose 70. (210-20(a), and 215-3)

The starting EGC size for a circuit protected by a 70A OCPD would be #8, not #10.

 

[goodoboy]

If it were not for the derating, what size ground would have been required?

Thanks, without derating (not considering voltage drop or temperature factor) my ungrounded conductors will be 8 AWG (40 amp load x 1.25 = 50amp) (310.16). So my grounded cable would be 8 AWG cause I am using 70 amp fuse. Am I correct in chosing 8AWG and 8AWG for ungrounded conductors and grouneded condcutor, respectiviely.

Thanks

 

[goodoboy]

side note: You might check you MOCP on you air conditioner, your 70 amp may not handle the inrush.
(I'm not too sure about your #1 taking the type load into account. You may need to add a softstart)

All that aside...

250.122 tells you that if your initial wire size is increased then so shall the equipment ground.
In your case, let us assume a #8 (You did not provider the MCA on the A/C)
You increase from an #8 to #1. Table 8 tells us a #8 is 16510 cmil, a #1 is 83690 cmil,
whcih would call for an increase of 5.06 on your eq ground.
Assuming a #10, which has an area of 10380, you would need to increase to 52616 cmil or a #3.

Thank you Augie47,

Can you please explain how you calculate the numbers 5.06 and 52616? Thanks

 

[goodoboy]

But, didn't catch this before...



The starting EGC size for a circuit protected by a 70A OCPD would be #8, not #10.

Thank you, that is correct # for 70A

 

[augie47]

Thank you Augie47,

Can you please explain how you calculate the numbers 5.06 and 52616? Thanks

''


I can, but as the others pointed out, I also missed the #10 on a 70 amp OCP so my actual number is incorrect. Your guide is Table 8 (Pg 681 in '08).

Since it's an air conditioning unit and you have not advised the MinCktAmps or MaxOvercurrent for the unit, the numbers we use are somewhat of a guess, but let's say the unit without an increases requires a #8 wire and a 70 amp breaker.
Using a THHN or THWN you would use a #8 conductor. As noted the normal equipment grounding conductor would be a #8 also.
If you increase your phase conductors from a #8 to a #1 you would look at Table 8 and compare the two. #8 is 16510 c/m, #1 is 83690 for a ratio of 1/5.06 so your required #8 would need to increase by the same ratio, in this case a #1 also. IF you had a 60 amp OCP and a #10 would have been a sufficient EGC you could use #10's cm of 10380 x 5.06 for 52616 cm or #3

 

[goodoboy]

side note: You might check you MOCP on you air conditioner, your 70 amp may not handle the inrush.
(I'm not too sure about your #1 taking the type load into account. You may need to add a softstart)

Thank you for the response. Can you explain your statement please? "You might check you MOCP on you air conditioner, your 70 amp may not handle the inrush.
(I'm not too sure about your #1 taking the type load into account."

My data for condensor is:

MOCP = 75amp
MCA = 54amp
LRA = 161amp
RLA = 26.3

Thank you

 

[augie47]

Thank you for the response. Can you explain your statement please? "You might check you MOCP on you air conditioner, your 70 amp may not handle the inrush.
(I'm not too sure about your #1 taking the type load into account."

My data for condensor is:

MOCP = 75amp
MCA = 54amp
LRA = 161amp
RLA = 26.3

Thank you

That kinda changes the whole ballgame a bit.
The 70 amp breaker is fine, but with a MCA of 54 amps, your "normal" conductor (assuming 75? terminations are in play) would be a #6 THHN (or NM or SE).
With the new numbers, I will honestly have to take the back seat and see what others say about the increase in wire size. Your actual load is 26.3 amps and if we use that number, a #4 would be sufficient for a 3% drop at 1000 fr., however, considering the LRA, etc. I would be hesitant to stay that small.
Motor/air conditioning loads are a place where I would be more comfortable with some engineering input.

 

[goodoboy]

With the new numbers, I will honestly have to take the back seat and see what others say about the increase in wire size. Your actual load is 26.3 amps and if we use that number, a #4 would be sufficient for a 3% drop at 1000 fr., however, considering the LRA, etc. I would be hesitant to stay that small.

Thanks for your input.

Can you help me understand the concern? This is my second time wiring up a load from our MCC so any extra input helps me.

Thanks,

 

[augie47]

My concern is simple: 161 amps LRA
Your 26 amp RLA with a #4 keeps you at around 3% VDrop
161 amps with a #4 gives you a voltage drop of 140 volts.
Even with your #1 gives you a drop of 43 volts.

It takes folks a lot smarter than me to tell you how your A/C will react to that.

 

[goodoboy]

My concern is simple: 161 amps LRA
Your 26 amp RLA with a #4 keeps you at around 3% VDrop
161 amps with a #4 gives you a voltage drop of 140 volts.
Even with your #1 gives you a drop of 43 volts.

It takes folks a lot smarter than me to tell you how your A/C will react to that.

Thank you for responding.

Should I not size the wire for the MCA (minimum circuit capacity) 54amps? Correct me if I am wrong please. Maybe I need to discuss this with the vendor supplying the A/C unit and ask them about the 161 LRA.

My concern is with at LRA 161 and a 70amp fuse at the MCC bucket, this will cause trip correct when the motor trys to start up correct?

The A/C unit will run off and on, so there will be lots of starting up.

Just searching, I think I found something here that may help me http://highperformancehvac.com/tag/locked-rotor-amps/

Thanks for any support and I be sure to post the outcome of the design back here incase in future someone needs help wiring up a A/C condensor unit.