Voltage Drop - Real World Appication

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Jerrodlk

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I have a project to extend a few circuits. The existing circuits are 120v 1/p feed from a 480/277; 10 amp load on a 20 amp breaker; #10 wire; 350ft from panel to outlet.

Our client wants us to extend the circuit to 700ft total and just tie on to the existing #10 wire. This is a commercial warehouse; all THHN in EMT conduit.

#1) Could the existing circuit be working correctly? They say it works great !!

#2) What size wire could we actually use?

Please let me know what you all would do here. Our client does not like what we actually did.
Thanks,
 
1) Very roughly #10 is 1 ohm per 1k feet, so you have 700 feet of wire (there and back again), giving 0.7 ohm. With a 10A load you get 7V of drop, or about 6%. This would be considered excessive but most loads will work just fine.

2) If you extend the same circuit to 1050 feet total (2100 feet 'there and back again') then you have 2.1 ohm of resistance and 21V of drop at 10A. This is _huge_ and many loads would have difficulty functioning; other loads would work just fine.

As to what you should actually do, that depends on the characteristics of the load (will it tolerate huge voltage drop?), how long it runs (if it runs 24x7 then the power loss in the wire costs lots of $$), and what energy codes (if any) apply.

If this were a load that only runs now and then, and demonstrated a capability of tolerating a huge voltage drop, then I might just let the voltage drop be huge.

-Jon
 
One of the loads is a resistive heat load for a shrink wrap machine. Roughly 8 of the 10 amps is associated with the heater and blower for the shrink wrap machine. The other loads are computers and printers.

I was not aware that we can allow more than 3% voltage drop on any circuit.
 
I am confused; are you looking at adding another circuit or tying on to this existing circuit?

A resistive heat load will 'tolerate' a large voltage drop just fine...it simply produces less heat. If the heater is thermostat controlled, then the 'on' time will be longer, and if the voltage drop is too great then the system won't reach target temperature. But if the unit reaches target temperature than as far as the user is concerned, it works just fine.

Computers and _printers_ will be a problem. The computer has a switching power supply that will compensate for voltage drop to a point; but by 'printers' you probably mean laser printers, and these often draw significant surge current when a print cycle is started. I bet that if you did the 1050 foot run with 10awg for a 120V circuit for some computers and printers, everything would work fine until someone decided to print something.

The 3% voltage drop spec is 'fine print note' in the NEC and not enforceable; rather it is a good design practise. However it may also be in an enforceable part of an energy code.

-Jon
 
Jerrodlk said:
I have a project to extend a few circuits. The existing circuits are 120v 1/p feed from a 480/277; 10 amp load on a 20 amp breaker; #10 wire; 350ft from panel to outlet.

Our client wants us to extend the circuit to 700ft total and just tie on to the existing #10 wire. This is a commercial warehouse; all THHN in EMT conduit.

#1) Could the existing circuit be working correctly? They say it works great !!

#2) What size wire could we actually use?

Please let me know what you all would do here. Our client does not like what we actually did.
Thanks,
Click to expand...

You appear to be missing a transformer in your description, and it's location makes all the difference to questions about voltage drop.
 
I was asked to extend four (4) existing circuits to be exact. The distances I mentioned are form the breaker for the circuit to the outlet. The transformer for that panel is within 60ft Of the panel.
I was asked to put 4 computers, 4 shipping label printers, and 2 shrink wrap machines on One (1) circuit, #10 wire, 20amp breaker, 700ft.
The shrink wrap machines themselves would pull 4-6 amps each.
Would this have worked?? I don’t think it would have been a good idea?
 
FWIW Figuring a 15 amp load, #10 at 700 ft gives you about a 26 volt drop.. normally unacceptable

Note: You can go to MikeHolt.com, Free Stuff, Voltage calculator and plug in the appropriate numbers and see the results for yourself.
 
what Winnie said, except many present day switching power supplies onprinters and such run just fine down to 85Vrms.
heaters will draw less current but the power supplies higher current.
 
Anyhow, it is safe to figure that if all the magical bits aligned perfectly, the installation that the customer wanted could work but would be marginal at best, and very likely would have worked poorly.

What did you actually end up installing?

-Jon
 
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