General power tolerances/requirements

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New member
I am an engineer installing several new pieces of equipment. I have to verify the actual power supplies and grounding against the available supplies. What does the code say about tolerances around voltage and grounding? For example, one machine requires 115VAC and our actual supply at the time of testing was 119VAC. Does the code specify that +/- 10% is acceptable?

Similarly with grounding, 0 Ohms is the most desirable, but does the code give any requirements for acceptable grounding?


Staff member
Plano, TX
Re: General power tolerances/requirements

These are not code issues per sey. Service voltage ranges are gennerally dictated by state utility agencies or by the utility themnselves I believe.

There is no specific value for impedance for grounding. The code does state that a single rod ground electrode which measures higher than 25 ohm's shall be supplemented by an additional rod. If after adding the second rod and the impedance measures 100 ohm's you are done and meet requirements. That is why most electricians just drive two-rods and do not bother with any measurements.

Code basis GEC and EGC sizes from tables 250.66 and 250.122. 250.66 sizes the GEC based on the size of service conductors, and 250.122 is sized per the OCPD.


Re: General power tolerances/requirements


I believe that I can provide some information to the ground resistance portion of your question.

More than likely you are interested in the ground resistance (AC impedance(Z)) value at the receptacle, called the equipment grounding conductor (EGC), rather than the DC resistance of the earthing system or made electrodes such as ground rods.

EGC impedance maximums can be found in Table 6-1 of IEEE standard 1100-1999, "Recommended Practice for Powering and Grounding Electronic Equipment".
Here are the numbers based upon a 120V system:

For a 15 amp overcurrent device: 1.0 Z ohms
For a 20 amp overcurrent device: 0.8 Z ohms
(If your voltage or overcurrent devices are of different values, let me know and I'll post the correct Z values.)

You now have the values, but here is possibly some bad news: most electricians do not have the correct meter to measure AC impedance. (Multimeters can only measure DC resistance, not AC impedance.)

The impedance meter we use cost about $1200.00, and some used in hospitals run over $6000.00- an expensive tool to be banging around in a tool box.

Check your yellow pages for a power quality consultant- most have this type of meter and can easily take the measurement for you.

Good luck!



Re: General power tolerances/requirements

Voltage Range
The American National Standard ANSI C84.1(1) establishes nominal voltage ratings and tolerances for 60-hertz (alternating current, AC) electric power systems above 100 volts and through 230, 000 volts. Voltage operating ranges are recommended for two voltage categories:

1) The service voltage, typically the point of connection between utility and customer.

2) The utilization voltage, typically the termination point to equipment.

The utilization voltage range takes into account a voltage drop within the end user?s distribution circuits.

ANSI C84.1 expects equipment to operate at service voltages between 95% to 105% with a utilization voltage range of 87% to 106% (120V to 600V)

Refer to ANSI C84.1 for additional operating voltage ranges.

Service Voltage allowable ranges.

114 to 126 for 120

198 to 218 for 208

228 to 252 for 240

263 to 291 for 277

456 to 504 for 480


Senior Member
Re: General power tolerances/requirements

Gregory, despite what the ANSI Standard says, it is the local utility commission that establishes what the tolerance is for the electric utility. For us, it is 5% above and below nominal in cities and 6% for rural areas. The chart that you produced is what we normally adhere to for our tolerances. UL specifies that all equipment must work all right if the voltage is within 10%. The difference is the 5% that the FPNs in the NEC are telling you that you can work with. :D
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