Sizing a HVAC

Status
Not open for further replies.
kingpb said:
Where is the #3 AWG cable coming from, I come up with #6AWG.
Click to expand...

Thanks,

Accounting for 3% voltage drop.

The MCA is 15 amps, so I multiplied by 125% to get 18.75amps. So I size the wire to withstand 18.75amps for a distance of 1500, gives me #3 using mike holt spreadsheet.

But just reading from other posters, i don't need to add additional 125%, just the 15amps cause thats straight from UL nameplate data.

So just using 15amps, I get #4 size wire.

My fuses will be 15amps based on the MOCP at my MCC bucket. Looks like I am done! I hope so:?
 
kingpb said:
But isn't that 2% off of the rated equipment voltage of 460V, and not system voltage of 480V?

2% drop from 460V is approx 451V. You can have over 6% drop from the 480V and still be above the minimum of 451V.
Click to expand...


This is a good question.

I am sizing wire based off 3% voltage drop of the system voltage 480V.

I am using Mike Holt spreadsheet. it just asks for voltage.
 
goodoboy said:
This is a good question.

I am sizing wire based off 3% voltage drop of the system voltage 480V.

I am using Mike Holt spreadsheet. it just asks for voltage.
Click to expand...

I would double check the calc (by hand even) because 1500 feet of cable is a lot and the cost is going to measurable. I believe you can use a #6 but you have to be comfortable with your design.
 
goodoboy said:
This is a good question.

I am sizing wire based off 3% voltage drop of the system voltage 480V.

I am using Mike Holt spreadsheet. it just asks for voltage.
Click to expand...

The selected branch circuit wire size must be capable of handling the name plate"minimum branch circuit ampacity" with a voltage drop of no more than 2% for the length of the run.

A # 6 will give you 460.9. also remember to figure in ambient temperature if running across roof if required.
 
Gregg Harris said:
The selected branch circuit wire size must be capable of handling the name plate"minimum branch circuit ampacity" with a voltage drop of no more than 2% for the length of the run.

A # 6 will give you 460.9. also remember to figure in ambient temperature if running across roof if required.
Click to expand...

Thanks

How are you getting #6?

Distance 1500
load 15amps
480V 3 phase
3% minium voltage drop.

I calculate #4 per the mike holts spreadsheet :?
 
goodoboy said:
So size the wire just off the MCA? No need to add another 125%?

Thanks
Click to expand...

If you take individual loads within the unit and did your own calculation you will find that the 125% of largest motor is included in the MCA. So use MCA marked on such equipment and do not adjust it in any way.

Gregg Harris said:
#3 would be correct for 2% loss which is what the HVAC manufactures require.
Click to expand...
2% of what? Unless they say otherwise the unit is probably marked with a rating of 460 volts, one kind of has to assume everything about that unit is based on 460 volts. If source voltage is near 500 volts (pretty common around here) you can probably afford a lot more than 2% drop before you drop below 460. But when doing calculations most of us generally will use 480 as the base voltage unless we have other reason to be more specific with the base.
 
It would be 2% of 460V, which is 451V, minimum voltage.

VD = (1.732 x 1500ft x .49 ohms x 15A)/1000ft

VD = 19.1 Volts using a #6 conductor

480V - 19.1 = 460.9V which is a 4% drop from 480V but it provides 100%, i.e. no voltage drop, off the rated voltage of 460V.
 
kwired said:
If you take individual loads within the unit and did your own calculation you will find that the 125% of largest motor is included in the MCA. So use MCA marked on such equipment and do not adjust it in any way.

2% of what? Unless they say otherwise the unit is probably marked with a rating of 460 volts, one kind of has to assume everything about that unit is based on 460 volts. If source voltage is near 500 volts (pretty common around here) you can probably afford a lot more than 2% drop before you drop below 460. But when doing calculations most of us generally will use 480 as the base voltage unless we have other reason to be more specific with the base.
Click to expand...

2% of the equipment label, in the OP's scenario it is 460
 
kingpb said:
It would be 2% of 460V, which is 451V, minimum voltage.

VD = (1.732 x 1500ft x .49 ohms x 15A)/1000ft

VD = 19.1 Volts using a #6 conductor

480V - 19.1 = 460.9V which is a 4% drop from 480V but it provides 100%, i.e. no voltage drop, off the rated voltage of 460V.
Click to expand...

Good to go.
 
goodoboy said:
Using a #4, allows for 12.51 voltage drop and maintain 467.40V at the load and is within 3% requirement.
Click to expand...

Why are you so concerned with the 3%? It is not a code requirement, it is only a recommendation. The 2% requirement is much more important since it is an equipment need, and it says you need at least 451V at the unit. A 4% drop from 480V more than adequately meets the equipment requirement and it will save you a bunch of money.

But, like I said before, you have to be comfortable with the design, they won't be calling me if there's a problem. :happyno::D

Heck, I'd see if you can get them to tap the transformer ahead of the MCC and get the voltage up 2.5% to nominal 492V and then use a #8AWG; you could then drop conduit to a 3/4" and save even more. But hey, I'm cheap. :angel:
 
kingpb said:
Heck, I'd see if you can get them to tap the transformer ahead of the MCC and get the voltage up 2.5% to nominal 492V and then use a #8AWG; you could then drop conduit to a 3/4" and save even more. But hey, I'm cheap. :angel:
Click to expand...
Only problem with that approach is that the manufacturer's recommendation is probably based on knowing that if the voltage drop at nominal current is OK (<2%) the voltage drop during starting surge will be OK too (say <10%).
If you just go by their minimum operating voltage specs and ignore the voltage drop requirement, you can end up with a severe undervoltage during starting.
 
GoldDigger said:
Only problem with that approach is that the manufacturer's recommendation is probably based on knowing that if the voltage drop at nominal current is OK (<2%) the voltage drop during starting surge will be OK too (say <10%).
If you just go by their minimum operating voltage specs and ignore the voltage drop requirement, you can end up with a severe undervoltage during starting.
Click to expand...

Excellent point to make, you have to check both because sometimes the starting current with force you to increase wire size.
 
kingpb said:
Why are you so concerned with the 3%? It is not a code requirement, it is only a recommendation. The 2% requirement is much more important since it is an equipment need, and it says you need at least 451V at the unit. A 4% drop from 480V more than adequately meets the equipment requirement and it will save you a bunch of money.

But, like I said before, you have to be comfortable with the design, they won't be calling me if there's a problem. :happyno::D

Heck, I'd see if you can get them to tap the transformer ahead of the MCC and get the voltage up 2.5% to nominal 492V and then use a #8AWG; you could then drop conduit to a 3/4" and save even more. But hey, I'm cheap. :angel:
Click to expand...

:? Sorry, I am still learning. Are you stating that the voltage drop from my MCC bucket to where I plug in the unit should not exceed 5% per NEC code?

Help me understand please

Whats confusing me is this statement:
1. Branch Circuits ? This FPN recommends that branch circuit conductors be sized to prevent a maximum voltage drop of 3%. The maximum total voltage drop for a combination of both branch circuit and feeder should not exceed 5%. [210-19(a) FPN No. 4], Figure 2.

2. Feeders ? This FPN recommends that feeder conductors be sized to prevent a maximum voltage drop of 3%. The maximum total voltage drop for a combination of both branch circuit and feeder should not exceed 5%. [215-2(d) FPN No. 2], Figure 2.
 
Last edited:
Status
Not open for further replies.
Top