Rated and Utilization Voltages for NEC/CEC's newcomers from IEC

[vagojr]

Good morning everyone,

This thread intends to be useful for those people more used to working in IEC standards but NEC/CEC newcomers.
Hence I think that it would be useful if you could confirm my approach and please provide the correct NEC/CEC points which defines the matters the best.
On the other hand, please correct whatever you consider necessary.


In IEC the rated voltage at secondary side of transformer is related to the non-load voltage and the rest of equipment downstream (even motors) are voltage rated with the nominal voltage (if you are going to measure with a tester, the measurement should match closely with the rated voltage of the panel).
Voltage drops in % are referred to the nominal voltage or utilization voltage.

In CEC/NEC, transformers and downstream panels are rated with the non-load voltage (if you are going to measure with a tester in a panel, the measurement will be less as it is the utilization voltage, i.e. 575V but panel is 600V).
Motors are rated based on this utilization voltage.
Voltage drops in % are referred to the rated voltage of the system.

Note: the voltage tolerances for system and motors are different in IEC vs NEC/CEC (NEMA/CSA); this is logical since it is related to the differences (IEC vs NEC/CEC) of rated voltage of the systems against their utilization voltages.


Summarizing, please find below example charts of Rated Voltage of the NEC and CEC but "translated" into a IEC understanding.
it is referred to a case of LV but it can be extrapolated.

Rated Voltages:	NEC	PoV IEC
TRAFOS	4160/480V	4160/480V
SYSTEM	480V	460V
PANELS	480V	460V
LOADS	480V	460V
MOTORS	460V	460V
CABLES	600V	1000V
VD%	VD/480V	VD/460V
TESTER	±460V	±460V

Rated Voltages:	CEC	PoV IEC
TRAFOS	4160/600V	4160/600V
SYSTEM	600V	575V
PANELS	600V	575V
LOADS	600V	575V
MOTORS	575V	575V
CABLES	600V	1000V
VD%	VD/600V	VD/575V
TESTER	±575V	±575V

PoV IEC: From Point of View of IEC, means that if the project had developed in IEC std. Attention: this does not mean compatibility between equipment designed in different standards; only useful to understand the voltage matter.


Hope this can be useful.
Best regards

 

[paulengr]

Close. The North American standard is an IEEE standard. Unlike IEC adherence is not legally required. There is some equipment out there running on European voltage standards. Neither IEEE nor IEC addresses voltage stability due to transformer load directly...you assume stuff bus. All North American equipment gives a voltage range or a tolerance even if it is rather vague and just confirms to UL/cUL or CSA standards. Most motors (but not all) fall under NEMA which has tighter tolerances than UL. And CBEMA addresses voltage droop but is rarely followed explicitly.

The reason for utilization vs distribution voltages is to allow for losses in the conductors between the transformer and the load. We aim for 5% total voltage drop or less, 2% in the feeders/service and 3% in the branch circuits. Voltage stability is generally 1% with current transformers if you don’t overload them. Customers rarely do this but utilities often do. Up to 25% overload rarely has any significant impact on voltage or longevity. Most of the limitations are in the conductors. Industrial plants often tap up their transformers to the highest tap setting to allow for greater losses as the plant naturally expands over time. It’s not unusual to see voltages out of tolerance on the high side during outages when the plant is unloaded which frequently triggers UPS equipment to go on battery and lose power several hours later (APC brand, most others don’t do this).

There really is no significant difference between IEC and North American standards except that with tighter (and enforced) requirements IEC transformers tend to have a lot more taps and a wider range than just 0, +/-2.5%, and +/-5%. Utilization voltages explicitly allow for conductor voltage drop so you will see a 460 V motor and a 480 V transformer. Consumer products are 110 V fed by a 120 V transformer. But with UL allowing -15% / +10% and often manufacturers going even beyond this (relays and contractors generally drop out at 85 V) for CBEMA the UL tolerances in North America are very conservative. IEC specs and operation in comparison tends to be more rigid leading to larger conductors and transformers. That’s not a negative either way, just very different. But don’t let yourself get into the attitude that voltages in North America don’t matter...they do. And IEC equipment which may already be designed for 440 V is going to have problems in an industrial plant where they jacked the transformer up to 500 V.

Also it would seem like voltages are very uniform here (but sloppy) and they are but Canadians for instance like to run 600 V in some areas and some Southern US plants are on the 550 V (cotton gin) standard where the rest run 480 V mostly with 460 V equipment and the occasional 440 V.