Specifications

MedWeld 3000 Catalog Revision 8
July 2002 Page 64 of 72
Sizing Circuit Breaker and Isolation Contactors
RULES FOR SELECTING CIRCUIT BREAKER FRAME SIZES
By Jack Farrow, WTC R&D Center July 7, 1993 rev. 11/3/95, 8/6/97 and 2/7/01
For WTC Resistance Welding Controls
DISCLAIMER:
The purpose of this document is to assist in selecting the proper size circuit breaker
for a given welding application. This document does not constitute any
recommendation, warranty or obligation on the part of WTC or any of its employees.
The user of this information assumes all responsibility for proper use of the
information. The information in this document is presented as-is; no warranty of
correctness or suitability for any application is given or implied.
ASSUMPTIONS:
The first part of this selection guide relates to the computation of the minimum
required circuit breaker frame size based upon the considerations of average current.
The second part relates to the computation of the required magnetic trip current
range. The circuit breaker can then be selected so that it meets both requirements.
This selection guide assumes the following things:
The circuit breakers are magnetic-trip only (no thermal trip) and the magnetic trip
current range is adjustable from at least 4 times nominal circuit breaker rating up to
at least 8 times nominal circuit breaker rating. For a 250-amp circuit breaker, this
means a magnetic trip adjustment range of 1000 to 2000 amps.
The KVA ratings on the nameplates of the welding transformers are in accordance
with the RWMA standards for resistance welding transformers.
The power line voltage is between 440 and 500 VAC nominal.
When two or more welding transformers are connected to the same welding control
and are fired simultaneously, the effective transformer KVA is the sum of all the
transformers that are fired simultaneously.
In multiple-transformer cascading applications, the effective transformer KVA is the
KVA rating of the largest transformer, NOT the sum of all of them.
All calculations here apply to SINGLE-PHASE welding controls only.