Voltage Drop for Spa Installation

[catchtwentytwo]

Bob,

I missed that one.

"..... increased in size proportionately according to the circular mil area of the ungrounded conductors" would make an interesting toobox calculation.

Regards,
John

 

[iwire]

"..... increased in size proportionately according to the circular mil area of the ungrounded conductors" would make an interesting toobox calculation.

I don't think you will find many that will know how of the top of their heads.

For anyone interested here is the method.

Example
A 240-volt, single-phase, 250-ampere load is supplied from a 300-ampere breaker located in a panelboard 500 ft away. The conductors are 250 kcmil copper, installed in rigid nonmetallic conduit, with a 4 AWG copper equipment grounding conductor. If the conductors are increased to 350 kcmil, to what size must the equipment grounding conductor be increased?

Solution

Step 1.
Calculate the size ratio of the new conductors to the existing conductors:

Size Ratio= new size circuit conductor / old size circuit conductor.

350,000 / 250,000 = 1.4

Step 2.
Calculate the cross-sectional area of the new equipment grounding conductor. According to Chapter 9, Table 8, 4 AWG, the size of the existing grounding conductor has a cross-sectional area of 41,740 circular mils.

Step 3.
Determine the size of the new equipment grounding conductor. Again, referring to Chapter 9, Table 8, we find that 58,436 circular mils is larger than 3 AWG. The next larger size is 66,360 circular mils, which converts to a 2 AWG copper equipment grounding conductor.

 

[George Stolz]

Spa motor plate does read 230 volts, 1 hp, but heart element plate reads 240 volts at 5.5 amps. Do you and Trevor agree that #6 is still sufficient?

Stop me where I'm mistaken here, but we've got

230V 1 HP motor = 8.0 Amps
240V Heating element = 5.5 Amps
Subtotal.............= 13.5 Amps

Where is the rest of the load? (I've never spent much time looking at hot tubs. )

Sean, your heating element won't heat as quickly at 234V as it would at 240V. But it will still operate just fine, don't sweat it.

 

[infinity]

A 13.5 amp load over 190' with #6 conductors would have a voltage drop of about 2.2 volts.

 

[ramsy]

Spa motor plate does read 230 volts, 1 hp, but heart element plate reads 240 volts at 5.5 amps. Do you and Trevor agree that #6 is still sufficient?

If you can signal or tone trace each phase feeding the heart element, and are sure they are not the same phase at the panel, and also verify opposite phases feed the 230vac motor, then yes I agree #6 is still sufficient? (ie) your 40A line-line loads must be phased A, B.

I figure your line-line is 232vac unbalanced, if affected by 120vac spa lights not divided evenly between those A & B phases, and 236vac if those L-N lights are evenly balanced, and no bulbs are burned out.

 

[winnie]

I don't think you will find many that will know how of the top of their heads.

For anyone interested here is the method.

[method for calculating increase in EGC size when increasing the size of ungrounded conductors]

If you limit yourself to wire gauge sizes, the calculation is much simpler. Wire gauges are selected to be a geometric series, with the proportional circular mil area between any given number of wire gauges a constant.

In other words, if you calculate the proportional area of #4 wire versus #8 wire, you should get the same result as #12 wire verus #16 wire. Small errors will creap in because of permissable manufacturing tolerance and rounding.

This means that if you increase the ungrounded conductors by 4 gauges, simply increase the EGC by 4 gauges.

This falls apart once you get to the 'kcmil' sizes, because these wire sizes are selected to be nice round numbers of circular mils, rather than the geometric sizes associated with wire gauge.

-Jon

 

[iwire]

If you limit yourself to wire gauge sizes, the calculation is much simpler.

Kind of but that falls apart quickly as well.

The EGCs ratio to circuit conductor size does not remain constant.

We can forget about 16 AWG and smaller altogether.

15, 20 and 30 amp circuits require a 1 to 1 ratio of EGC to circuit conductors.

31 to 60 amp circuits get a 10 AWG EGC minimum.

61 to 100 amp circuits get a 8 AWG minimum.

Of course I may be missing something, Jon is sharp and I am on thin ice going against him.

 

[sean1]

Spa Relocation Additional Information

Spa Relocation Additional Information

Thank you everybody for your responses. After reading through the abundant material you have supplied to me, I think this is an acceptable solution to my problem. Please let me know if I have misinterpreted any of the information you have supplied.
This is how I am going to do my installation:
At the existing disconnect, which is 80 feet from the panel, which is supplied by 2 #6 power conductors, 1 #8 neutral and 1 #10 ground, I will put a junction box and continue my additional 110 foot run with 2 #6 power conductors, 1 #8 neutral and 1 #10 ground, reinstall spa disconnect and run from spa disconnect to sap control panel with the same size wires. If you see any problems with this installation, please notify me.
Thank-You
Sean1

 

[ramsy]

As George pointed out, unless you have 25 amps of L-N receptical loads & lights you don't have a 40A load. Further, smaller breakers or fuses may be required to seperately protect that 8A motor, 6A hart element, the lights, or receptacles.

My V-drop spreadsheet has some usefull values for this.

1) Max Derating. I/MaxC#6cu
If you do have 40A total load. 40/75 = 0.53. So, 9 hot wires can share a conduit, NEC Tbl.310-15(b)2a, with room to spare for Ambient adjustments. That means your sap control panel has room to match 4 different OCPD's between various equipment ratings if neccessary.

2) ASCC E/Z
Regardless of load, your ASCC is 240/0.093 = 2578 Amps. This helps you match the proper ASCC rated OCPD, GFCI's, and other equipment & devices for your installation, NEC 110-10.

 

[ramsy]

For anyone interested here is the method.

Thanks iwire, I will use this to add a EGC calc to my V-drop spreadsheet.

 

[ramsy]

Re: Jon's AWG method.

Kind of but that falls apart quickly as well. The EGCs ratio to circuit conductor size does not remain constant.

Jon's original idea is similar to one of my own.

My clossest approximation for circular mils, in the field, without NEC Tbl.8 handy is to remember #10 only. 10380 CM * 1.59 = #8CM * 1.59 = #6CM and so on. The next size down is 10380 / 1.59 = #12CM / 1.59 = #14CM and so on. The only hitch is 1.59 changes to 1.26 for #3 AWG to 4/0.

So, circular mils is always handy without Tbl.8, if at least one AWG# CM is known, and if the ratio is changed from 1.59 to 1.26 beyond #2 AWG.