When does voltage drop impact any ampacity calculation?

[ramsy]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by hardworkingstiff:
I find this quote interesting. {abridged} Many inspector members of the IAEI, ASHI, and NAHI are comfortable with voltage drops as high as 8 to 10 percent..
mikeholt.com/documents/calculations/formulas/5-PercentVoltageDrop.pdf

That was a very interesting read, which points out another misapplication of voltage-drop measuremtents.

A 15A & 20A load tests is unnecessary for UL listed receptacles, and the resulting v-drop will be overstated for 12A & 16A branch-circuits, code compliant for 80% maximums.

 

[iwire]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by ramsy:
A 15A & 20A load tests is unnecessary for UL listed receptacles, and the resulting v-drop will be overstated for 12A & 16A branch-circuits, code compliant for 80% maximums.

That would depend on the circuit, not all receptacle circuits are limited to 80%.

 

[ramsy]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by iwire: ..not all receptacle circuits are limited to 80%

Granted, occupants can overload circuits with cord & plug-connected loads, but IMHO AHJ's checking for locally enforced v-drop limits, with a 20A load test are more likely to fail good installs with overstated results.

 

[iwire]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by ramsy:
Granted, occupants can overload circuits with cord & plug-connected loads, but IMHO AHJ's checking for locally enforced v-drop limits, with a 20A load test are more likely to fail good installs with overstated results.

I am not talking about overloaded circuits.

Let me ask this.

What code section prohibits a circuit supplying a receptacle from being loaded to 100%?

 

[ramsy]

Re: When does voltage drop impact any ampacity calculation?

iwire wrote: What code section prohibits a circuit supplying a receptacle from being loaded to 100%?

Yes, except for individual circuits dedicated to a single receptacle, my comments concerned 5-PercentVoltageDrop.pdf, originally posted by hardworkingstiff.

For multi-outlet circuits, this Doc. accurately states,
"NEC Table 210-21(b)(2) allows a maximum total cord and plug connected load in amperes of 12 for a 15 ampere rated receptacle and 16 for a 20 ampere rated receptacle". (ie) Tbl 210-24

Then the Doc. adjusts v-drop test results, which AHJ's rely on, as follows:
"Inasmuch as the SureTest? imposes a full 15 ampere load at the last receptacle, a reading of 6.25 percent voltage drop would be equivalent to 5 percent at 12 amperes."

Industry-diverse NEC panel members have historically confined load-dependant, voltage-drop complexity to FPN's, perhaps wisely anticipating misapplication by a well represented fraction of authorities having jurisdiction (AHJ).

Industry interests may influence AHJ preference for a Gen.Branch test instrument, with set load values for v-drop, which complicates results that are further suspect by being neither measureable nor calibratable with external equipment.

The NEC's preference to enforce voltage listings for equipment, and eventually use ieee definitions for "fault clearing" impedences(Z), may be simpler and more reliable than a v-drop rule.

--------------------
Roger R. From: Anaheim, CA.

[ February 06, 2006, 07:55 AM: Message edited by: ramsy ]

 

[Mike03a3]

Re: When does voltage drop impact any ampacity calculation?

Since I was just playing with one of these gadgets, I suppose I'll chime in.

This seems like a classic case of a handy tool being misused. As a diagnostic tool the SureTest? is a good way to quickly and easily check a circuit for anomalous conditions. I just used one to check my daughter's house, expecting that I would see steadily increasing voltage drops as I worked my way through a chain of outlets moving away from the panel. Happily, that's pretty much what I found.

However, there are too many variables and complexities for this to be a valid enforcement tool in my opinion. In addition, while excessive voltage drop is undesirable, let's not overlook the resulting voltage. In my daughter's case, the voltage is 123.0 at the can, so a test result of 2.6% drop under 20amp load for the worst receptacle on a small appliance circuit results in 119.8 volts at the receptacle. Not exactly a major problem. Of course, all this is somewhat theoretical, since this is not a calibrated item of test equipment.

It did come up with a few interesting things that could bear a littler more understanding.

1. The biggest voltage drop seemed to be between the line and load sides of GFCIs. Possibly reflecting a voltage drop from the electronics in the GFCI itself, since it seemed to be generally true on all the circuits that had load side receptacles.

2. Several of the receptacles had a difference between the two sockets of the duplex receptacle. I want to go back and try to understand what that could result from, since they are all pigtailed to the side screws, so the difference should only be the drop on the straps.

I'll post some more info when I've had a chance to gather more data.

 

[iwire]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by ramsy:

iwire wrote: What code section prohibits a circuit supplying a receptacle from being loaded to 100%?

Yes, except for individual circuits dedicated to a single receptacle, my comments concerned 5-PercentVoltageDrop.pdf, originally posted by hardworkingstiff.

For multi-outlet circuits, this Doc. accurately states,
"NEC Table 210-21(b)(2) allows a maximum total cord and plug connected load in amperes of 12 for a 15 ampere rated receptacle and 16 for a 20 ampere rated receptacle".

Roger I am really not advocating the sure test.

That has nothing to do with what I was trying to point out.

You had said.

A 15A & 20A load tests is unnecessary for UL listed receptacles, and the resulting v-drop will be overstated for 12A & 16A branch-circuits, code compliant for 80% maximums.

The circuits are not at all limited to 80%, the NEC limits the load on a single receptacle of a multi-outlet branch circuit to 80%.

So while the 20 amp test is a bit over the top for a single receptacle on a multi-outlet circuit there is no reason not to expect two 10 amp loads applied to one duplex outlet on the circuit.

Bob

 

[ramsy]

Re: When does voltage drop impact any ampacity calculation?

Originally posted by iwire:..there is no reason not to expect two 10 amp loads applied to one duplex outlet on the circuit.

True, and if needed with 20A+, hopefully the breaker can trip, provided impedance(Z) is low enough.

Whether we use max loads or not, Z doesn't vary, so circuits meeting min. nameplates of 108vac computer, printer, copier, or fax follows Nominal - Z*I, using "what-if" values for (I).

Which load fails target voltage depends on install choices, and measuring actual Z demands a load test, but with any linear load and with anyone's DVM & cl/amp meter.

Originally posted by Mike03a3:..The biggest (ShureTest) voltage drop seemed to be between the line and load sides of GFCIs. Possibly reflecting a voltage drop from the electronics in the GFCI itself

Never really looked for this B4, will try again and post back.

My ShureTest ST-1THD fixed menu indicates "15=" (for v-drop load), which can't move for power-pulse losses over time. The ~9 amp equivelant, rather than 15A shown, is penciled on the face of my unit. But, after 10 years, my ShureTest impedance(Z) sums remain accurate for all 3 conductors, along with the false-ground indicator, GFCI trip values, nominal, & common-mode votages.

Can't check harmonic accuracy, but I won't toss it, because Z's & cmv are convienent, and relative v-drop between receptacles still helps troubleshoot circuits.

--------------------
Roger R. From: Anaheim, CA.

[ February 08, 2006, 05:55 PM: Message edited by: ramsy ]